Philippine Fault System.docx

Philippine Fault System

The Philippine Fault System is a major inter-related system of faults throughout the whole of the Philippine Archipelago, primarily caused by tectonic forces compressing the Philippines into what geophysicists call the Philippine Mobile Belt. Philippine Mobile Belt

The Philippine Mobile Belt is composed of a large number of accretionary blocks and terranes. These terranes are long and narrow like the Zambales ophiolites which is at least 400 km long and 50 km wide. The strips generally run north-south and the zones of convergence are usually demarcated by fault lines. The Philippine Mobile Belt is compressed on the west by the Eurasian Plate and two arms of the Sunda Plate, and on the east by the Philippine Sea Plate. These tectonic plates have compressed and lifted parts of the Philippines causing extensive faulting, primarily on a north-south axis. All faults in the Philippines are inter-related by the tectonic forces of the Philippine Mobile Belt, or its tectonic induced volcanism. A more complete understanding can be gained by viewing the faults in the Philippines as an inter-related Philippine Fault System. Philippine Fault Zone

The Philippine Fault Zone (PFZ) extends 1200 km across the Philippine archipelago behind the convergent boundary of the Philippine Trench and the subduction of the Philippine Sea Plate.[3] This left-lateral strike-slip fault extends NW-SE (N30 – 40 W) accommodating the lateral oblique motion of the subducting Philippine Sea Plate with respect to the Philippine Trench.[4][3]It extends from Davao Gulf in the south, bisects the Caraga region at the Agusan River basin, crosses to Leyte and Masbate islands, and traverses Quezon province in eastern Luzon before passing through Nueva Ecija up to the Ilocos region in northwest Luzon. The northern and southern extensions of the PFZ are characterized by branching faults due to brittle terminations. These horsetail faults are indicative of the lateral propagation and further development of the PFZ. The fault’s current activity can be observed in Holocene sandstone outcrops on the Mati and Davao Oriental islands. [5] The fault experiences a slip rate of approximately 2-2.5 cm/year.[6] Formation

It is proposed that the Philippine Trench and PFZ represent a ‘shear partitioning’ mechanism, where the oblique physical motions of subduction at the convergent zone resulted in the development of the major strike-slip fault. In the Philippine Sea, the oblique motion of the subducting Philippine Sea Plate resulted in the formation of the Philippine trench and the PFZ back arc fault system. The oblique motion is accommodated by two vector components; one vector perpendicular to the converging Philippine Trench and one vector parallel to the PFZ. Approximately 30% of the oblique motion is accommodated by the PFZ while the remaining proportions are displaced along other regional tectonic features as the Philippine Sea Plate currently subducts below the Philippine archipelago at a rate of 6–8 cm/year. These two tectonic features thus correlate to a similar time of development. The formation of the PFZ was a result of two stages. The first stage began at ~10 Ma, when the northern segments of the PFZ developed due to the convergence of the China Sea Crust underneath the nearby Manila Trench. The lack of accretionary prism at the Philippine Trench is suggestive of young origin correlating to an early second stage of development (2-4 Ma) with the central PFZ proposed to have developed between 2.7 and 3.8 Ma.[3] Earthquakes

The central Philippine Fault Zone consisting of the Guinyangan, Masbate, and Central Leyte faults are the most seismically active regions transecting the islands of Bondoc to Leyte. The northern and southern extensions of the Philippine Fault Zone experience infrequent earthquakes and often described as locked segments which are capable of larger magnitude earthquakes. The largest (M7.0) and most destructive earthquakes are generated along the Guinyangan fault every 30 to 100 years with slip rates of 20–33 mm/year as determined by GPS and historical records. Moderate earthquakes (M3.0-5.0) are observed along the Masbate fault with frequent aftershocks indicative of continued displacement and regional slip of 5–35 mm/year. The northern and southern segments of the Central Leyte fault experiences different seismic activity dependent on regional geology. While the Southern Central Leyte fault experiences moderate seismic events, the Northern Central

Leyte fault creeps at approximately 25 mm/year. Historical data on the PFZ is limited due to the faults geographical location predominantly offshore, lack of complete paleoseismic data and lack of permanent Global Positioning System (GPS) that can trace movements over long periods of time. [4] List Of Active Fault Lines In The Philippines

1.

Marikina Valley Fault (Montalban, San Mateo, Marikina, Pasig, Taguig, Muntinlupa, San Pedro, Binan, Carmona, Santa Rosa, Calamba, Tagaytay, Oriental Mindoro) 2. Western Philippine Fault (Luzon Sea, Mindoro Strait, Panay Gulf, Sulu Sea) 3. Eastern Philippine Fault (Philippine Sea) 4. Southern of Mindanao Fault (Moro Gulf, Celebes Sea) 5. Central Philippine Fault (Entire Ilocos Norte, Aurora, Quezon, Masbate, Eastern Leyte, Southern Leyte, Agusan Del Norte, Agusan Del Sur, Davao del Norte) Of these, the MARIKINA VALLEY FAULT poses the greatest danger because it cuts through all the modern and progressive portions of Manila such as Eastwood, Rockwell, Ortigas Center, Bonifacio Global City, Ayala Center, and Alabang. Also, the PhiVolcs people have warned that this fault line can move anytime because it is already “11 years late” for its movement.

*The earthquake that destroyed Guinsaugon is the Central Philippine Fault *The 1990 earthquake that destroyed Central Luzon and Baguio is also the Central Philippine Fault.

Of these, the MARIKINA VALLEY FAULT poses the greatest danger because it custs through all the modern and progressive portions of Manila such as Eastwood, Rockwell, Ortigas Center, Bonifacio Global City, Ayala Center, and Alabang. Also, the PhiVolcs people have warned that this fault line can move anytime because it is already “11 years late” for its movement. Do analyze the figure provided below:

Figure A Tectonic setting of the Marikina Valley fault system (MVFS) in central Luzon, The Philippines. In A, subduction zone trenches are shown by white barbed lines and other faults with high rates of Quaternary activity by heavy black lines. White dots show locations of recent earthquakes on the Philippine fault in Luzon (magnitude 7.8; 1990) and the Aglubang River fault in Mindoro (magnitude 7.1; 1994). Figure B shows how the Marikina Valley pull-apart basin (MVPB) may have been formed through extension caused by clockwise rotation (dashed circle) and shearing of this part of central Luzon, which is caught between two active left-lateral strike-slip faults–the Philippine fault and the Lubang fault. A zone of extension and young volcanism has also influenced the structural development of the valley. Philippines are in the Pacific ring of fire,there are so many Major Faults in the country, and we have to trenches surrounding both sides of our country the first one is located at the South China Sea and the other one is called the Philippine trench which is located at the right side of our country, facing the pacific ocean. Source: http://blogphilippines.com/2009/10/list-of-active-fault-lines-in-the-philippines.html Destructive Earthquakes in the Philippines

Bohol Earthquake (○2013 Ms 7.2 ○1996 Ms 5.6 ○1990 Ms 6.8)

1990 February 08 Ms6.8 Bohol Earthquake Bohol Earthquake - February 08, 1990

This shallow seated tectonic earthquake with magnitude 6.8, struck the island of Bohol at 3:15 pm, caused panic to general public, damaged several houses and infrastructure and presented several geologic disturbances. Its epicenter was located about 17 kilometers east of Tagbilaran City with a maximum felt intensity of VIII, based on Rossi-Forel Intensity Scale, in the towns of Jagna, Duero and Guindulman all situated on the lower area of the NE quadrant of the island. It was felt at intensity VII in Garcia Hernandez, Loboc, Valencia and Anda, Intensity VI in Tagbilaran City, the rest among the 16 municipalities of Bohol and in the neighboring islands of Cebu and Camiguin. Intensity V was felt over areas of Cagayan de Oro in Mindanao, Dumaguete City in Negros, Intensity IV in the areas of Canlaon in Negros and Cotabato City in Mindanao. Reported felt intensities ranging from I to III was also felt as far as Palo in Leyte and Bislig in Surigao. Observed geologic phenomena related to this event include ground fissures, landslides, rockfalls, ground subsidence and collapse, sand/mud fountaining and sudden increase on the sea level. Most of the manifestations were particularly observed and experienced by the towns of Jagna, Valencia, Duero, Guindulman and Garcia Hernandez. The force of the incoming waves from the sea caused Alijuan River in Duero to flow inland immediately after the earthquake. Based on the orientation of the main fracture zones, focal mechanism solution and aftershock distribution, the earthquake may have represented subsurface rupture along segments of the NE-SW Alicia thrust fault. Studies by the Bureau of Mines (1986), however, point to the fact that in most portion of the fault is being overlained by Miocene to recent limestone which does not reflect any deformation suggesting that the fault has been inactive for quite a long time. This would pose a question as to whether the earthquake represented reactivation of an old fault or indicated new fault movement in the island.

Impact and damage documentation revealed that the worstly affected portion of the island was sustained by the eastern and southeastern coastal areas, observed to be mostly underlained by alluvial deposits which have tendency to amplify ground motions generated by an earthquake. Likewise, most of the damaged buildings were either old/poorly-built or lacked the necessary reinforcements to resist strong ground shaking. About 3,000 units of houses, buildings and churches were affected and damaged where a total of 182 were totally collapsed including two historical churches built centuries ago. Some 200,000 sq.m. of fishpond in the town of Guindulman sustained damage due to cracked and collapsed dikes. Mud eruptions on these fishpens contributed to the death of fishes and prawns. The bridge connecting the towns of Jagna and Duero collapsed. Roads to Anda sustained cracks and fissuring. Landslides and rockfalls blocked some portions of the roads that caused inaccessibility to some areas between Anda and Garcia Hernandez.

Six fatalities were reported and more than 200 were injured in the event. About 46,000 people were displaced by the event and at least 7,000 among them were rendered homeless. Estimated damage to properties is amounting to 154 million pesos. References: Umbal, J.V., Masigla L.M., Medrano R. N. and Diolata G.P. - Report of Investigation on the February 08, 1990 Earthquake in Bohol Province. PHIVOLCS 1996 May 27 Ms5.6 Bohol Earthquake

Bohol Earthquake - 27 May 1996 Preliminary reports indicated that there were damages in Clarin, Inabanga, Trinidad, Tubigon and Loay Municipalities all in Bohol, as well as in the cities of Cebu and Mandaue in Cebu Island. Due to these reports, a Quick Response Team was immediately dispatched to investigate the reported affected areas. The team consists of a personnel from the main office and two field station observers from Tagbilaran and Lapu-lapu Seismic Stations. Investigation commenced on the 29th of May at Cebu Island. Buildings and other structures at the reclamation areas of Mandaue City (Landmark Cinema, SM Shopping Mall) and in Cebu City (Skyways, City Hall and buildings along M.J. Cuenco Avenue) were inspected. After that the team proceeded to Bohol Island to verify reports of damages in the towns of Clarin, Inabanga, Tubigon, Loay and Trinidad. Date of Event May 27, 1996 Origin Time 1:45 p.m. (0545 GMT) Epicenter 09.98 N Latitude 124.10 E Longitude or In between Clarin and Inabanga, Bohol Magnitude 5.6Ms Depth approximately 4 km from the surface.

Intensity Report: Intensity VI Clarin-Bohol, Inabanga-Bohol Intensity V Cebu City, Lapu-lapu City, Trinidad-Bohol, Tubigon-Bohol Intensity IV Tagbilaran City , Loay-Bohol Intensity III Cadiz City, Negros Occidental Intensity II Iloilo City, Bacolod City Intensity I Palo-Leyte, Camiguin Island Note: Intensity scale used in these observation was the Rossi-Forel Earthquake Intensity Scale. Findings of the QRT Team: Cebu

The SM Mall located at the reclamation area in Mandaue City reportedly sustained cracks on the wall plaster and floor tiles due to the earthquake but Mall Manager Ranier Matubayas disproved the alleged report about the damages. He said that a press conference was being held at the Mall when the earthquake occurred. The media people attending the conference witnessed the panicking public that caused them to attribute the preexisting hairline cracks on the wall plaster and floor tiles to the earthquake. Likewise, the owner of Landmark Cinema disproved that the pre-existing cracks on the pebble washout plastering were related to the said earthquake. There were no damages found in buildings and houses along M.J. Cuenco Avenue as reported earlier. However, minor cracks were found at some non-structural parts of Cebu City Hall, which was rebuilt in 1946. These cracks developed on old masonry wall near the windows, below the beams, above the doors and other masonry parts of the building. Moreover, City Hall employees confirmed that some of these cracks can be attributed to already existed after the 08 February 1990 earthquake and was only aggravated by the ground sharing during the 27 May 1996 earthquake. Continuous cracks across the Skyway Bridge were seen under the two identical skyways situated at Fuente-Osmeña Boulevard and Avellano High School - Osmeña Boulevard. The cracks formed on plasters that were used to cover the space along shear gaps between two adjacent bridges. Cebu City is 38 kilometers away from the epicentral area. Loay, Bohol

Loay is about 40 kilometers south-southwest from the epicenter. Inspite of its closeness to the epicenter, only few residents felt the earthquake. Injured were Mr. Escolastico Borong and his wife who sustained minor

injuries caused by the collapse of their old wooden shanty at Barangay Bonbon. The posts of the shanty was not firmly fixed to the ground. Its collapse made the other residents aware of the occurrence of the earthquake. No other damages to buildings and infrastructues were observed elsewhere in Loay. Tubigon, Bohol

Tubigon is about 10 kilometers west-southwest from the epicenter. In general, ground shaking only produced minor cracks on the walls and plaster of the column, about 2 millimeters opening and 3 meters long, of the Tubigon Municipal Hall. However, it was observed that the reinforced concrete linings of the same walls were damaged by peeling-off from the piled limestone blocks normally used in the olden times as construction materials in Bohol. Slight cracks were also observed on one of the walls of the Priest's room of San Isidro Labrador Church. Trinidad, Bohol

The town of Trinidad is about 15 kilometers northeast from the epicentral area. The rail-guard and pathways of four wooden bridges were damaged. These bridges are very old that their use must be limited to light vehicles even before the earthquake. Pre-existing cracks on the floor and entrance steps of the Municipal Health Center grew and became more pronounced after the earthquake. Health workers confirmed that some of the cracks already existed before the earthquake but new minor cracks appeared on the walls of the center after the 27 May 1996 earthquake. Local officials were able to report the cited damages because of their available communication lines e.g. handheld radio. Inabanga, Bohol

During the visit to Inabanga Municipal Hall, Vice Mayor Victoriano M. Basilan reported that the arch at Barangay Kagawasan Chapel and one side of the walls of the buildings of Nabuad High School collapsed. Likewise, cracks on the walls of an abandoned Gabaldon-type building of Inabanga Elementary School were aggravated. This prompted the Municipal Office to act for demolition of said building. The Municipal Hall itself also suffered minor cracks of about 3 millimeters opening and at least a meter high length on the wall and other non-structural parts of the building. Employees narrated that some of the books from their shelved, as well as the tropies placed on top of the cabinets fell down the floor. Also, residents and storeowners reported falling objects from open shelved and racks such as bottled and canned goods, medicine etc. The earthquake caused general panic among the residents because of sudden jolt with both vertical and horizontal motions and accompanied by rumbling sound. Inabanga town is about 5 kilometers from the epicenter. Clarin, Bohol

One of the residents said that sleeping persons were awakened by the strong jolt accompanied by rumbling sound apparently originating from below the ground. Books from shelves fell down. One parked motorcycle and a refrigerator toppled during the earthquake. Concrete hollow blocks to be used for construction of a residential house was crushed. Masonry oven (pogon) owned by Mr. Ponciano Rama was damaged, an example of unreinforced masonry vulnerable to ground shaking. Bottled and canned goods, paints, groceries etc. fell down.

The rear masonry wall of Barangay Cumang Ricemill collapsed inward the building. That was due to the poor connection between the said wall and the wooden parts. The wall also served as load-bearing wall of earth materials and rice shells dumped at the read side of the building. Walls of Clarin Elemntary School suffered minor cracks. Pronounced damages were sustained in Sam Miguel Church particularly on the bell tower, which suffered cracks on the walls, across the columns, along the beams, the peeled-off decorative masonry works. The front door, which is under the bell tower, was closed to the public after the earthquake, avoiding

eminent danger from such damages. The church was built in 1924 with reinforced concrete and its oldness is one of the factors of suffering such damages during earthquake.

Three residents were injured. One was hit by falling object, another fell down from the stairs due to panic and the last, a fisherman, met an accident while on his pump boat. The latter was 15 meters away from the shore when he allegedly encountered two sea waves, about a meter high that damaged his boat and caused injuries to his right ankle and leg. The victim however was uncertain whether the incident was simultaneous with the earthquake occurrence or not. Summary: The 27 May 1996 earthquake did not cause major damages to properties. Damages were confined to poorly built structures and/or old wooden, masonry, limestone walls of houses and buildings, generally due to ground shaking. There were no reports and observations that can be attributed to other phenomena such as liquefaction, ground subsidence, landslide and any other geologic ground disturbances. Recommendation: 1. A public information forum for municipal and barangay officials should be organized by NDCC and RDCC with PHIVOLCS assisting to explain to the people the phenomena related to earthquake. This is to avoid misconception, exaggeration of the facts about earthquake and to allay the fear and panic. Earthquake disaster mitigation plan should be considered on these affected places. 2. Since the place is rarely visited by major earthquakes, information campaign should be conducted from time to time to keep the level of awareness of the people to various earthquake hazards.

3. There should be an immediate inspection and repair/replacement/retrofitting/demolition of the old and poorly-built structures and affected infrastructures such as roads and bridges. 2013 October 15 Ms7.2 Bohol Earthquake *INSERT PDF HERE* Negros Oriental Earthquake (2012 Ms 6.7)

2012 February 6 Ms6.7 Negros Oriental Earthquake *INSERT PDF HERE*

Masbate Earthquake (2003 Ms 6.2)

2003 February 15 Ms6.2 Masbate Earthquake Masbate Earthquake REPORT OF INVESTIGATION 15 FEBRUARY 2003 The Earthquake

A strong earthquake with Ms6.2 struck the province of Masbate at 7:01 in the evening of 15 February 2003. Preliminary determination of epicenter indicated that the event was generated along the Masbate Segment of the Philippine Fault Zone (PFZ) in central Philippines. The epicenter was located offshore of Magcaraguit Island (12.2ºN, 123.8ºE) and about 22 kilometers deep, which is approximately 28 km east of Masbate City. Initial reports from nearby stations implied that the earthquake was felt all over the island of Masbate including the nearby provinces of Bicol, Leyte, Panay, Cebu, Negros and Romblon (Figure 1).

The focal mechanism solution released by PHIVOLCS suggests a dominantly left-lateral slip along a vertical NW-SE fault (Figure 2). This left-lateral slip correlates well with the known structure in the area as observed in Dimasalang, Palanas and Cataingan along its mapped trace. Figure 3 shows the historical earthquakes along the Masbate Segment of the Philippine Fault Zone that dates back in the year 1800’s. Magnitues 6.5 and 7.0 earthquakes were reported on 16 August 1869 and on 19 October 1897 on the island respectively (Lanuza, 1994). Another reported earthquake struck the province of Masbate on 23 October 1877 between 1:00 and 2:00 in the afternoon (SEASEE Vol. IV). Other reported earthquakes for the island of Masbate are as follows: 1915 (Ms 6.6); 1970 (Ms 6.3); and 1971 (Ms 6.3). In 07 May 1998, a magnitude Ms 5.7 quake was felt at intensity VI all over the City of Masbate and the municipalities of Mobo and Uson. This event was preceded by a magnitude Ms 5.1 with reported intensity VI at Masbate City (Torrevillas et. al., 1998). A Quick Response Team (QRT) was deployed to Masbate on 18 February 2003 based on the extent of the preliminary reported damages submitted by the Masbate Seismic Station (Dela Cruz and Torrevillas, 2003, unpublished internal report). The QRT’s primary purpose was to evaluate the extent of damages due to the earthquakes in the province of Masbate especially those municipalities located near the epicenter. The team was also tasked to verify and map the reported ground rupture, conduct intensity survey, disseminate correct information regarding the event, and install additional seismographs in order to monitor aftershocks of the Ms 6.2 earthquake. This paper reports the overall findings of the QRT during its 10-day investigation in Masbate. All intensity reports in this paper are according to the Philippine Earthquake Intensity Scale (PEIS) as shown in Figure 4. Foreshock and Aftershock Analysis

The 7:01 February 15, 2003, Ms6.2 earthquake that hit Masbate has numerous foreshocks and aftershocks. The main shock was preceded by a moderately strong foreshock with a magnitude of Ms5.2 located at 12.2ºN and 123.7ºE with a depth of 30 km (Figure 5a). The moment tensor solution indicates strike-slip faulting with strike=55, dip=78, slip= -175 with Mo=5.3x 1017 Nm equivalent to a seismic moment magnitude Mw 5.8 (Figure 5b). This foreshock was felt over a wide region at the following PEIS intensities: Intensity VI

Dimasalang, Masbate Palanas, Masbate Magcaraguit Island, Dimasalang, Masbate Deagan Island, Dimasalang, Masbate Uson, Masbate

Intensity V

Masbate City, Masbate Batuan, Masbate Burgos, Masbate

Intensity IV

Baleno, Masbate Aroroy, Masbate Legazpi City, Bicol Talisay, Masbate

Intensity III

Milagros, Masbate San Jacinto, Masbate Borabantique, Masbate

Kumawit, Monreal, Masbate Palo, Leyte Bulusan, Sorsogon Mandaue City, Cebu Intensity II

New Washington, Aklan

Intensity I

Mandaon, Masbate

Intensity surveys revealed that this foreshock was notably strong and caused damages in the epicentral area. Some residents of affected structures straddling the fault in the municipality of Palanas, Masbate reported that their houses were damaged by the foreshock manifested by visible cracks on walls and floors and as fissures in fields and gardens. In Brgy. Sta. Cruz, Palanas, the road was reportedly fissured during this foreshock. According to interviewees, the same fissures later became wider and larger during the Ms6.2 7:01 pm earthquake. In the island of Magcaraguit, Dimasalang, the non-reinforced walls of a chapel collapsed. In Deagan, people and animals panicked during this earthquake. In Deagan island, Dimasalang, interviewees described the booming sound they heard preceding the quake to be very strong and "as if its origin is just there". In Uson, Masbate, many appliances like televisions and cabinets toppled down during the shaking that accompanied the foreshock, which prompted the residents to bolt their appliances to the wall in preparation of the eventuality of another earthquake.

Figure 5a. Map showing the location of the largest foreshock that occured 5 hours before the main event. Solid circles are the plotted foreshock events.

As of February 24, 2003, a total of 187 aftershocks were recorded in Masbate Seismic Station (MMP). Some of these aftershocks were felt at intensity II or III along the ground rupture area, in the islands Deagan and Magcaraguit, and as far as the southern part of Ticao Island. A strong aftershock was felt in Magcaraguit and Deagan islands and in Dimasalang and Palanas on February 18, 2003 at Intensity IV. Some of the moderately strong aftershocks were recorded in other permanent PHIVOLCS seismic stations and hypocentral parameters were located. Preliminary determination indicated that these aftershocks are located 15-40 km southeast of Masbate City. This implied the continuous activities along the Philippine Fault that ruptured in the southeast portion of the Masbate Island in the vicinity of Dimasalang and Palanas. Table 1 shows the list of aftershocks plotted in Figure 5. In Magcaraguit and Deagan Islands of the municipality of Dimasalang, residents reported an average of 2 felt aftershocks everyday. In these islands and in places near the ground rupture, aftershock events were observed to be always preceded by rumbling sounds that vary in intensity depending on the size of the event (e.g. louder booming sounds preceded bigger aftershocks). In Ticao Island, reports regarding felt aftershocks were in lesser frequency but were likewise preceded by booming or truck-like sounds. Please see Figure 5 for the location of aftershock events with resolved epicenters. In order to detect aftershocks that are very local to the Masbate region and beyond the detection capability of the existing nearby PHIVOLCS permanent stations, a total of two temporary short-period seismographs were installed in the area. One of these instruments was installed in Cataingan, Masbate (N12º00’10.1"; E123º59’53.9’’) that operated from l9-24 February 2003. The other station was placed in Batuan Poblacion, Batuan, Masbate in Ticao Island (N12º25.33’00"; E123º46.765') and

operated from 20-24 February 2003. Cataingan station (CAT) is near the southern termination of the rupture while Batuan station (BAT) is located near the projected extension of the fault southwest of Ticao Island (Figure 6). From 22-23 February, CAT station recorded 5 events while the BAT station recorded 20 quakes. During this very short period of observation, it was noted that more earthquakes were recorded to the north of the mapped rupture. The location of the plotted earthquakes and the results of the installation of the temporary network showed that possible adjustments of the fault were confined along the observed ground rupture and extend up to the possible offshore extension of the said ground rupture, west of Ticao Island.

Figure 6. Map showing the location of CAT and BAT stations. These were temporary stations installed during the QRT investigation. Considering the magnitude of the 15 February 2003 quake, the felt aftershocks were anticipated to be observed up to about a month after the mainshock. Thus, everyone was advised to take proper precautions especially when entering buildings and houses that suffered minor to severe damages. Results of damage assessment and intensity survey conducted by the team throughout the province of Masbate suggest that the Ms=6.2 earthquake at 7:01 in the evening was felt at Intensity VII and VIII within the epicentral area. Isoseismal distribution based on PEIS would be as follows:

Intensity VIII

Sta. Cruz, Palanas, Masbate* Suba, Dimasalang, Masbate*

Intensity VII

Dimasalang, Masbate Brgy. Poblacion, Palanas, Masbate Uson, Masbate Nabangig, Palanas, Masbate Nipa, Palanas, Masbate Deagan, Dimasalang, Masbate Magcaraguit, Dimasalang, Masbate Talisay, San Fernando, Masbate Nipa, San Fernando, Masbate Pinamughaan, San Fernando, Masbate Buyo, San Fernando, Masbate

Intensity VI

Batuan Poblacion, Batuan, Masbate Burgos (Lagundi), Batuan, Masbate Bacolod, Milagros, Masbate Cataingan, Masbate Dapdap, Uson, Masbate Armenia, Uson, Masbate Canvañez (Aricomo), Batuan, Masbate Poblacion, San Fernando, Masbate

Intensity V

Masbate City Sto. Niño (Borabantique), Monreal, Masbate Placer, Masbate Pio V. Corpuz, Masbate San Jacinto, Masbate Putoson, Baleno, Masbate Poblacion, Aroroy, Masbate Ambulong, Aroroy, Masbate Irosin, Sorsogon Legazpi City, Albay

Intensity IV

Palo, Leyte Naga City, Bicol Roxas City, Panay

Intensity III

Metro Cebu Lagta, Baleno, Masbate Cagara, Baleno, Masbate Poblacion, Baleno, Masbate

Poblacion, Mandaon, Masbate Intensity II

Luya, Aroroy, Masbate

Intensity I

Lezo, Aklan Kalibo, Aklan

*along the ground rupture

The above observations clearly indicated that intense ground shaking was concentrated along and near the epicenter and the ground rupture (Figure 7). Figure 8 shows few of the interviewees that gave valuable information regarding their observations during the earthquake. Felt and observed intensities decreased accordingly northeastward and southwestward relative to the epicentral location with apparent elongation along location of the PFZ-Masbate Segment (Figure 7).

Figure 8. Photos of some of the interviewees in Nabangig (upper photo) and Sta. Cruz (lower photo) in Palanas Municipality

Ground Rupture

The ground rupture was verified and mapped through field investigations. The total length of the rupture onland is approximately 18km transecting several barangays of Dimasalang, Palanas and Cataingan. The ground rupture was characterized mostly by right-stepping en echelon faults with a general trend of ~N3OW to ~N4OW and had a maximum opening of 20 cm (Figure 9). The maximum horizontal displacement along the fault was 47cm in Brgy. Sta. Cruz, Palanas (Figure 10) while the maximum vertical displacement (23cm)

was found in Brgy. Suba, Dimasalang (Figure 11). The average horizontal and vertical displacements mapped along the ground rupture were 15 cm and 5 cm, respectively.

Figure 9. The observed ground rupture in Brgy. Sta Cruz, Palanas typically manifested by an echelon fissures as shown in the left and above photos

Figure 10. Photo showing a pilapil (rice paddy dike) transected by the ground rupture in Sta. Cruz, Palanas. Photo is looking SW.

Figure 11. The observed ground rupture in Dimasalang displacing a line of vegetation (right, looking NW) and a residential lot (top, looking SE) horizontally and vertically. On the other hand, the average width of the fault zone measured was about 75 cm and the widest measurement (153 cm) was found in Brgy. Sta. Cruz, Palanas. The ground rupture mapped during the 10-day investigation was traced from Brgy. Suba, Dimasalang to Sitio Burabod, Brgy. Pawican, Cataingan. The February 2003 ground rupture, more or less, followed the old trace/location of the active fault with about 3m localized deviations in some areas. Maximum PEIS intensity of Intensity VIII was observed in some areas along the ground rapture wherein several houses were totally damaged due to significant horizontal and vertical displacements (Figure 12). A displaced coconut tree (Figure 13) and the ground rupture manifestation into the seashore (Figure 14) were observed in Matugnaw, Palanas and Suba, Dimasalang, respectively.

Figure 12. Damaged houses transected by the ground rupture in Bry. Suba, Dimasalang (above photo, looking SE), and in Brgy. Sta. Cruz, Palanas (right photo, looking SW)

Figure 13. Displaced coconut tree found in Bry. Matugnaw, Palanas (left photo, looking SE), Also manifested along the ground rupture in this area were mole tracks (right photo, looking NW).

Figure 14. The Masbate 2003 ground rupture as it entered the sea transecting the shorelines and coral reefs fronting Bry. Suba, Dimasalang (photo looking SW)

To monitor possible post-earthquake deformation along the February 2003 ground rupture, temporary benchmarks were established in Palanas municipality at barangays Sta. Cruz and Nabangig (Figure 15).

Figure 15. Temporary stakes installed across the ground rupture to monitor and measure possible postearthquake deformation (photo looking SW)

Considering the following facts and observations: 1. the Masbate Segment is a left-lateral fault, 2. its epicentral location is west of Magcaraguit Island, 3. the distribution of observed aftershocks up to Talisay in Ticao Island, 4. and the observed concentration of high intensities, the projected northern extension of the ground rupture associated to the February 15, 2003 earthquake is deduced to be up to the Brgy. Talisay in western Ticao Island. Thus, this would indicate a ground rupture with a total length of at least 50km (Figure 16). Damage on vertical and Horizontal Infrastructure Large magnitude earthquakes determine the structural integrity of solid structures built on earth. Based on the initial survey conducted by the QRT during its field investigations, some school buildings, roads, bridges and river flood control projects performed poorly. Moreover, they were deemed structurally unsafe and hazardous to life and property after experiencing the strong ground shaking of the 15 February 2003 quake. 5.

During this event, engineered structures proximal to the fault trace sustained the worst damage. A road section of the Masbate-Cataingan Road was intersected by the fault near the Dimasalang-Palanas boundary (Figure 17). In this area the road was damaged as lateral longitudinal cracks were formed along the fault producing buckled and cracked section in this road. On the other hand, the Nipa Bridge along the Masbate- Cataingan Road (km post 57+607), located less than 2km from the ground rupture suffered significant structural damages. In this bridge, at least one of its columns showed concrete spalling at mid-height with striking vertical misalignment on both horizontal directions (Figure 18). Displacement at the bridge deck with respect to the bridge approach was also noticeable along with the yawning deck joints. Moreover, the slope protection grouted riprap at Nabangig Bridge located along the Masbate-Cataingan Road (km 62+560) and the Cantil River Control in Brgy. Poblacion in Palanas, Masbate were also severely damaged. The riprap structures in these areas suffered numerous large cracks as their foundation failed most probably due to compaction and slumping.

6. 7. 8. 9.

10.

Figure 18. The Nipa Bridge in Dimasalang (left photo) and the riprap structures (right photo) that were damaged during the February quake.

Furthermore, several school buildings at Masbate National Comprehensive High School suffered severe shear cracks and column-wall joint failure (Figure 19). In the same structures, some longitudinal and transversal fractures along the length of the beam and of the column were likewise observed. The Provincial Health Office's Administration building's middle concrete roof beam reveals a possible longitudinal rupture. The same failure characteristic was observed on at least two school buildings in Jose Zurbito Sr. Elementary School (also known as Jose Masbate South Elem. School) in Masbate City (Figure 20).

Figure 19. School buildings at Masbate National Comprehensive High School that suffered severe shear cracks and column wall joint failure

Figure 20. A collapsed concrete wall of a Marcos-type school building in Dimasalang, Masbate.

11.

12. It was also noted during the investigation that very little attention was given to non-engineered houses or facilities (Figure 21). Based on ocular investigations, many concrete or semi-concrete houses in Palanas and Dimasalang suffered severe damages. Since the owners of these houses are quite unsure about the condition of their residence, the assessment of these houses is urgently needed. Local engineers both private and government practitioners are enjoined to take the necessary steps to help owners assess the condition of their damaged houses and facilities immediately after the quake. 13.

Figure 21. During the earthquake, this non-engineered fence of a residential house (upper-right, looking NW) dismembered and a parking garage (lower-right, looking South) collapsed causing severe damage on parked vehicle. Both photos were located in Bry. Poblacion, Dimasalang.

Other Previous Significant Events From the intensity surveys and ground rupture mapping activities, several significant earthquakes that affected the Masbate area were likewise obtained. These events were either experienced by the interviewees

to be weaker and/or stronger than the 7:01 15 February 2003 Ms 6.2 event and with observable ground rupture along the Dimasalang-Cataingan area.  1957 This event is very memorable for an interviewee in Batuan, Ticao Island and some other interviewees in Deagan Island and Palanas because it is stronger than the 7:01 February 15, 2003. The aftershocks of the 1954 event were observed to be more frequent and lasted for at least a month compared to the 2003 mainshock. In Magsaysay, Masbate people in the streets lost balance and fell due to intense ground shaking. People along the rupture area in Brgy. Matugnaw, Palanas, reported co-seismic ground fissures in the same zone that ruptured during the February 2003 earthquake.  1967 In San Fernando, Ticao Island, interviewees talked of a strong earthquake during the 1960s at the time of Mayor Romero. After strong ground shaking, water fountaining was observed in Sitio Magsasaka, San Fernando Poblacion, Masbate that reached a height of about 16m. The fountaining phenomena can be associated to either liquefaction or localized disturbance of an aquifer. During this 1967 quake, ground rupture was likewise observed in Palanas.  1973 This event was remembered by residents of Kumawit, Monreal of Ticao Island to be more intense than the 7pm February 15, 2003 earthquake. The 1973 event caused large fragments of the limestone cliffs to fall into the sea in the northern point of Ticao Island.  1979 An interviewee from Magcaraguit Island remembered the 1979 earthquake to have caused stronger ground shaking in the island compared to the 2003 event. The felt aftershocks were greater in number than the felt aftershocks during the 2003 event and were described to have occurred "incessantly night and day". There was no tsunami report in Magcaraguit Island.  1989 This earthquake caused many objects to fall in Sto. Niño (then Borabantique), Monreal in Ticao Island with associated ground shaking stronger than the February 2003 7pm earthquake.  1998 Remembered by Kumawit, Monreal inhabitants with stronger ground shaking compared to the February 15, 2003 7pm event with accompanying booming sound. An interviewee from Brgy. Nabangig, Palanas observed that during this earthquake, boulders crumbled down from the mountain. It also produced ground rupture in the same area but with much wider ground rupture compared to the ground rupture produced by the February 2003 earthquake. Almost all of the felt aftershocks in Palanas during this quake were accompanied by rumbling sounds. Considering where significant events were fairly remembered, the northern segment of the Masbate Fault: PFZ implied higher rates of activity (Fig. 22). Moreover,its apparent from the above observations that M6 event in this segment of PFZ produces a relatively large movement of ground rupture. Tsunami, Rockslides, Lanslides, and Liquefaction Tsunami is a wave or series of waves generated when a significant displacement occurred under water due to underwater faulting with large vertical component, underwater landslides and underwater volcanic eruption. Rock slides and landslides, on the other hand, are slope failures that occurred along mountain or hill slopes during intense ground shaking. Ground failure can likewise occur in flat regions especially when intense ground shaking is experienced by areas underlain by water saturated loose sediment (e.g. along shorelines, deltas and riverbanks). During the February 2003 event, the occurrence of these kinds of phenomena were very few or almost none. Reports of unusual wave height and activity were located only near and around the northwestern part of Ticao Island. Since observed displacement along the ground rupture showed very minimal and localized vertical displacement, the tsunami in northwest Ticao Island can be attributed most probably to local

underwater landslides. Onland landslides and rockslides were not observed. Minor rockfalls were observed in Sitio Kumawit, Monreal in the northern part of Ticao Island as well as in Hamoraon and Cudao islands located near Magcaraguit and Deagan Islands in Dimasalang. Fragments of limestone exposed along vertical cliffs bordering the shoreline collapsed in these above-listed islands. Liquefaction on the other hand, was also very minimal. Although there were some reports of ground failure along and near the shorelines, local compaction of the underlying basal foundation cannot be discounted as the main cause of such failure considering the prevailing construction practices in the region. Taking into account the intensity of the felt ground shaking near and along the ground rupture area, it is very notable that effects of liquefaction, landslides and rockslides were very minimal to almost non-existent. Such striking and fortunate incident can probably be directly attributed to the very short duration of the February 15, 2003 quake. Based from the interviews, the ground shaking during the 2pm and 7pm events on February 15, 2003 lasted only for about 4sec and 7sec, respectively. Active Faults in Uson Area Initial aerial photograph and topographic map analyses indicate the possible presence of an active fault about l km southeast of Uson Poblacion traversing west of Badling, Dungon, and Balucauc vicinities (Figure 11). However, compared to the Dimasalang-Cataingan trace, the evidence in this area indicates that the Uson trace is much older (Certainty IV) and thus, probably exhibits much lesser activity. Further analysis will be undertaken to ascertain its presence. If its presence is further resolved, more field investigations will be conducted 1. to establish its location, 2. to determine the level of its activity, and 3. to inform immediately all concerned institutions so that proper actions can be promptly undertaken. Information Dissemination Campaigns Information dissemination in the form of lectures and briefings were conducted all throughout the fieldwork. People, especially those living near the hardest hit areas, were very enthusiastic and interested to know more about earthquakes. Lectures were given in schools (Figure 23), government offices (Figure 24), and barangay halls but more often in the field whenever there are people who were willing to listen to the Quick Response Teams all over Masbate and adjoining islands (Figure 25). Briefings and updates were given to proper authorities everyday and immediately especially in areas with severely damaged structures (Figure 26).

Figure 23. QRT members conduct lectures on earthquakes for students and teachers on a school ground in Palanas

Figure 24. Lectures undertaken at a government office (DPWH District Office) in Dimasalang.

Figure 25. Earthquake Info-dissemination inside a Barangay Hall in Nabangig, Palanas.

Figure 26. Lectures undertaken in open fields.

During the lectures, it was noted that the usual cause of their fear was due to wrong information that they received either from other people, hearsay and/or exaggerated information. Another cause of panic or worry was the lack of knowledge of what proper things to do before, during and after an earthquake. To immediately address some of these problems, information about the earthquakes were likewise distributed through brochures, leaflets and posters (Figure 27). An information board entitled “The Liki Maker: The Masbate 2003 Quake” was likewise produced during the QRT investigation to provide instant information to walk-in inquirers at the MMP station. The information board was also used during official briefings and lectures.

Figure 27. The information board produced during QRT activities where results of field investigation were displayed for easy information access to the 15 February 2003 earthquake.

Conclusions The 15 February 2003 event was generated along the Masbate Segment of the PFZ. The Ms6.2 earthquake produced at least 18 km long ground rupture onland with a total length of about 50km. Significant displacements along the Masbate Segment of PFZ caused a maximum intensity of Intensity VIII wherein houses were totally damaged along the ground rupture. Intensity distribution showed that ground shaking and damages were concentrated near and along the epicentral area and the ground rupture, respectively. Severe damages were generally observed on structures like school buildings, bridges and irrigation canals. Minor to severe damages were likewise observed on un-engineered houses and facilities that were manifested as small fissures on floors and walls or even as total collapse of walls and houses. Phenomena like tsunami, landslides, rockslides and liquefaction were not observed except for the tsunami due most probably to underwater landslide and minor rockslides in western part of Ticao Island. Aftershocks were usually observed near and along epicentral area and are expected to last up to about a month after the mainshock. Based on PHIVOLCS historical data and other previous events gathered through the QRT field investigations, the return period of large magnitude earthquakes along the Masbate Segment of PFZ can be as short as five years or as long as thirty years with an average of about 10 years. Typical large earthquakes along this part of the PFZ usually have magnitudes from Ms 6.0 to Ms 7.0. Furthermore, it is very notable that even earthquakes with less than Ms 6.0 produced ground rupture along the Masbate Segment and caused significant damages near and along the location of the Masbate Segment. Considering the possible locations of the recent significant events along Masbate Segment and the scale of damage suffered by manmade structures, a closer study on these historical events is highly recommended. Moreover, information dissemination about earthquakes should be regularly undertaken together with the implementation of an annual earthquake drill especially in schools and government offices. At present, the Uson splay of the PFZ in Uson vicinities cannot yet be all together discounted. Further studies will be undertaken to ascertain its level of activity and its location. Recommendations and Future Activities To further understand the activity along the Masbate Segment of PFZ, the following activities will be forthwith undertaken.  Detailed mapping, if possible, of the whole February 2003 ground rupture and continuous measurements of horizontal and vertical displacements to monitor possible post-earthquake deformation using the temporary benchmarks established on some selected areas.  Installation of permanent benchmarks along the Masbate Segment wherein future movement will be measured more extensively.  Encourage the local populace to mark the location of the ground rupture as a reminder of the active fault location and to prevent future inhabitants in constructing any structures within the 5m-buffer zone from the active fault or from the deformation zone caused by ground rupture.

       



More intensive information dissemination campaign in Masbate Island and adjoining islands and provinces to allay fears of future large magnitude earthquakes. Production and distribution of posters, leaflets and brochures written in Tagalog and in local dialect. Production of visual aids for lectures that will be conducted in areas where modern equipment are not available. Establish links with local barangay officials in noting time and date of felt earthquakes in their localities, information that should be immediately relayed to the PHIVOLCS MMP station in Masbate City. Immediate and thourough study on the structural soundness of all structures, both engineered and non-engineered, by the concerned authorities. Immediate evacuation of severely damaged buildings and thus, eliminating further usage of damaged facilities and exposure of local inhabitants to possible future collapse. Active and practical integration of earth science to school curriculum and strict implementation of the annual earthquake drill especially in schools and government facilities. Active participation of the Disaster Coordinating Councils from all levels in terms of disaster preparedness prior to large events and rapid damage assessment after a large magnitude earthquake like the 15 February 2003 event. Identification of evacuation sites and evacuation routes so that local inhabitants would know where to go in case of major earthquakes or whenever the need arises that they have to evacuate their premises/residence. Further studies on the historical events along the Masbate Segment of PFZ.

References       

PHIVOLCS Earthquake Information # 3 for the 1:47 pm 15 February 2003 event USGS Earthquake Information Website http://neic.usgs.gov/neis/FM/previous_mom.html Lanuza, 1994 Dela Cruz and Torrevillas, 2003 Bautista, 1999 SEASEE Torrevillas and Dela Cruz, 1998 Active Faults Mapping Comp 2000

Acknowledgements We gratefully acknowledge the following whose help and support made our investigations possible:  NDCC-OCD Ret. Gen. Rosales  ASEP and Engr. Ronnie Izon  PICE Legaspi?  Masbate Governor  Provincial Administrator  Assist. Provincial Administrator  Masbate City Mayor  Masbate City Engineer  Uson Mayor and constituents  Dimasalang Mayor and contituents  Palanas Municipal Administrator  Mr. Enrico Camacho and family

   

Barangay Nabangig Kagawad and wife Cataingan Mayor Cataingan Police Chief Ms. Nilda of Masbate City Mayor’s office

Palimbang Earthquake (2002 Ms 6.8)

2002 March 06 Ms6.8 Palimbang Earthquake Summary of the event: Ma. Leonila P. Bautista and et al.

The earthquake occurred on March 6, 2002 at 05:15 am (local time). Its epicenter as located by PHIVOLCS is at 6.1 N; 124.0 E; 81 km or about 81 km SW of Isulan, Sultan Kudarat. PHIVOLCS computed its depth of focus at 15 km. Its surface magnitude was computed by the Pacific Tsunami Warning Center as 6.8 while its moment magnitude (Mw) and body wave magnitude (mB) were computed by the the U.S. Geological Survey (USGS) as 7.2 and 6.3, respectively. Based on the earthquake location and mechanism solutions, its source is attributed by PHIVOLCS to subduction along the Cotabato Trench.

Fig 1. Shows the PHIVOLCS earthquake bulletin for this event.

As of March 9, 2002, the Office of Civil Defense (OCD) records show that 8 people had died and 41 were injured due to the earthquake. It affected 7,684 families in the provinces of Sultan Kudarat, Sarangani, North Cotabato and South Cotabato including four cities and 17 municipalities (OCD Memorandum dated March 9, 2002). The quake damaged 4 road networks, 7 bridges, 36 school buildings, 29 business establishments, 1 megadike, 2 health centers and 17 public buildings. Damage amounted to 4.175 million pesos or about 80,000 US dollars.

The focal mechanism as determined by Harvard University (Figure 2a) is underthrusting along a NW-trending plane with minor strike-slip component. The strike conforms to the established trend of the trench. The USGS's solution for the event (Figure 2b) shows an almost similar mechanism with a slightly larger strike-slip component than the solution of Harvard University.

Fig 2a. Focal Mechanism of the earthquake as determined by Harvard University

Fig 2b. Focal Mechanism of the earthquake as determined by USGS. Historical Seismicity:

Based on the PHIVOLCS catalog, the area is characterized by moderate to high intensity (Figures 3a and 3b) as shown by seismicity plots for the last 100 years. The location of the 2002 event is very near the location of the 1976 Ms 7.9 event.

Fig 3a. Seismicity of Mindanao for the last 100 years. Arrows point to the locations of the 1976 and 2002 earthquakes (source: PHIVOLCS catalog)

Fig 3b. Earthquakes with Ms 6.5 and above and depths 0-100 km for the last 100 years (source: PHIVOLCS catalog)

The quake's source region, the Cotabato Trench, is an active N- to NE-dipping subduction zone system whose slab has been traced down to 150 km deep based on earthquake data (B. Bautista, 1996). Figure 4a shows a section called N-N’ that traces the trend of the trench based on seismicity data. The figure also shows the slab to be moderately-dipping while a cluster of hypocenters are found at 0-100 km depth. Figure 4b shows the location of section N-N’ where the seismic cross section (section N-N’) was cut. The trench's past movement had caused large magnitude earthquakes to occur such as the August 17, 1976 Moro Gulf Earthquake that killed more than 3000 people due to high tsunami waves that swept many towns facing the Moro Gulf including Cotabato and Pagadian Cities.

Fig 4a. Downgoing slab of the Cotabato Trench (labelled CT) as defined by seismicity data (after B. Bautista, 1996)

Fig 4b. Map showing the location of line N-N’, the cutting plane used for Figure 4a (after B. Bautista, 1996) Impacts:

The quake's ground shaking intensities at various places were estimated using the Fukushima and Tanaka's attenuation (1990) relation (Figure 5). The highest intensity is estimated to be Intensity IX in the epicentral region specifically in the town of Palimbang in the province of Sultan Kudarat. The quake was estimated to have been felt at Intensity VIII in the town of Maitum in the province of Sarangani and in the town of Lebak of Sultan Kudarat province. Meanwhile, the observed intensity reports are shown in Figure 6 and are based on actual reports of PHIVOLCS field stations, Office of Civil Defense and the mass media.

Fig 5. Predicted isoseismal map using Fukushima and Tanaka's attenuation model (1990) and Gutenberg and Richter's intensity-acceleration relation.

Fig 6. Isoseismal map of the event. Intensity reports based on actual accounts. Tsunami Model:

Dr. Yuchiro Tanioka, PHIVOLCS JICA Expert, modeled for the possible tsunami generated by the event. He computed for a possible 3-m high tsunami in the town of Palimbang in the province of Sultan Kudarat. Based on the modeling results, the value progressively decreases from said point till it reaches to only about 30 cm at Lagung Point in Maguindanao province NW of Palimbang and about 1 m in the town of Kiamba, east of Palimbang. Dr. Tanioka also modeled the tsunami waves at 1, 5 and 10 minutes after the earthquake. The 1minute map (Figure 7a) indicates a recession of sea water, the results of which are consistent with the observations of the local people. Likewise, the 5- (Figure 7b) and 10-minute (Figure 7b) maps show that waves had already came back to inundate the shore five and ten minutes after the main shock.

Fig 7a. Model of tsunami waves one minute after the main shock (after Y. Tanioka, 2002). Blue shaded areas show the location of areas where recession of water is expected. Eyewitness accounts in the town of Palimbang in Sultan Kudarat and in Maitum and Kiamba towns in Sarangani province conform to the results of the model.

Fig 7b. Model of tsunami waves five minutes after the main shock (after Y. Tanioka, 2002). The figure shows that five minutes after the main shock, the water has already rushed back to the shore. Again, this is consistent with the eyewitness accounts.

Fig 7c. Model of tsunami waves10 minutes after the main shock (after Y. Tanioka, 2002)

Meanwhile, the Pacific Tsunami Warning Center (PTWC) issued a tsunami bulletin at 5:41 AM that stated that "no destructive Pacific-wide tsunami exists" (Figure 8). This was received at PHIVOLCS at 6:42 AM by fax. Since the area affected was in the Celebes Sea and away from the Pacific Ocean, the PTWC bulletin was quite accurate.

Fig 8. Copy of the fax sent by the Pacific Tsunami Warning Center (PTWC) to PHIVOLCS by fax. Actual Observations:

1. Palimbang, Sultan Kudarat Palimbang is a coastal town of 40,000 people (NCS), 1995). In this place, a concrete chapel collapsed due to intense shaking (PHIVOLCS QRT Report). No one died as a result of the collapse because the church has previously been abandoned due to military operations in the area. However, one person was reported dead and seven wfrom Barangays Poblacion, Badiangan and Colubo were injured. Two people were injured and were hospitalized (OCD Region XII, March 9, 2002). General Magsino reported to PHIVOLCS Main Office that the sea was observed to have receded 150 m from the shoreline. It then went back 75 m inland damaging two boats (General Magsino and PHIVOLCS QRT Report). 2. Maitum, Sarangani Maitum (pop: 35,000) is the neighboring town of Palimbang. It belongs to the province of Sarangani province. The highway linking Maitum and Palimbang and places in Barangays Pinol and Lipo were affected by landslides. A repeater antenna fell. A Baptist Church and a water tank tilted. In Barangay Mabay and Sitio Talikod, three sandboils measuring 8-10 cm wide and 12 cm deep were observed. Cracks on the ground measuring 5-10 cm wide, 2 cm deep and 30 cm long were observed at Sitio Saub in Barangay Mabay and in Nolasco St. Water was observed to have receded 300 m. A pumpboat was totally damaged and dead fish were observed soon after (PHIVOLCS QRT Report). Two persons died at Barangay Mabay due to falling hollow blocks while five others were injured in Barangays Kawa, Old Poblacion and Kiambing (OCD Region XII, March 7 and 9, 2002). One church, eight houses, a school stage and a five-span concrete fence of Rude Elementary School were totally damaged (OCD Region XII, March 7, 2002). One convent, 18 houses, a water reservoir, the Edenson Mission College Building and two business establishments were partially damaged (OCD Region XII, March 7, 2002). A vehicle and two fishponds were damaged (OCD Region XII, March 7, 2002). About 10,000 people were evacuated (OCD Region XII, March 7, 2002) and stayed in schools and gymnasium (Ms. Fe Falgui, local resident (083) 509 4038/509 4028). 3. Kiamba, Sarangani

Kiamba (pop: 39,000) is the next shore town after Maitum. Two public markets made of wood located in Barangay Kiamba and Lagundi collapsed. A local resident described the quake as very frightening and "parang hinahalungkay galing sa ilalim" (Ms. Fe Falgui, local resident (083) 509 4038/509 4028). The Sarangani Cable Office was totally damaged. Walls of several houses collapsed leaving only posts and beams behind. Two walls in two classrooms of the Suli Elementary School fell. At the Southern Cotabato Academy and at the Bagtasan Elementary School, a fence and a wall collapsed, respectively. Three churches sustained slight damage. Ten houses were partially damaged while five houses built in 1976 were totally destroyed. Piles of softdrink bottes toppled down. Cracks were observed along downtown Kiamba. Tual Bridge sank by 6 cm. Water pipes were destroyed. Thirteen persons from Barangays Nalus, Suli, Poblacion, Laabuk-Tual, Kling, Tablao, Tual, Lagundi, Kapate and Tabak-Tual were injured (OCD Region XII, March 7 and 9, 2002; PHIVOLCS QRT Report). Water receded 5-8 m three times (Mr. Rommel Palge, local govt ofc (083) 509 4038). Afterwards, water was again observed to rise (Mr. Leonardo Esteban, local resident (083) 509 4069). As a result, people went up the mountain. As of 1 PM of March 7, the Mayor of Kiamba refuses to allow people to return back to their homes. NDCC requested for an advisory from PHIVOLCS that it was safe to go back to their homes. About 32,000 people or more than 80% of its local population were evacuated at the Tumadang Elementary School and Iglesia ni Cristo Church (OCD Region XII, March 7, 2002). 4. Maasim, Sarangani Maasim (pop: 32,000) is the next town after Kiamba following the highway. A four-span school fence of San Felipe Central Elementary School located in Barangay Colon collapsed (PDCC Alabel; (OCD Region XII, March 7, 2002). Three houses in Barangay Tinoto and Lomatil were partially damaged (OCD Region XII, March 7, 2002; PHIVOLCS QRT Report). 5. Alabel, Sarangani Alabel (pop: 46,000) is the capital of Sarangani province. A child died when hollow blocks fell on him while another one man died at the time of the ground shaking (Source: Ms. Gay Melani Palma of DSWD Alabel tel no. (083) 508 2177 as relayed by Mr. Milo Tabique of PHIVOLCS Kidapawan Station). 6. Glan, Sarangani Glan (pop: 74,000) is another coastal town along Sarangani Bay. In this place, a big rock fell disrupting traffic. Landslides were also reported in Barangays Kapatan and Alegado (OCD Region XII, March 7, 2002; Malaya, March 7, 2002). A bridge collapsed in Barangay Small Margus isolating the barangays of Batulaki, Kaltuad and Santo Nino (OCD Region XII, March 7, 2002). The quake caused a one-m wide depression on the concrete road at the Glan subport (Philippine Daily Inquirer, March 7, 2002). A mosque in Barangay Burias and a Barangay Multipurpose hall at Barangay Baliton collapsed (OCD Region XII, March 7, 2002). An old school building in Barangay Kapatan was totally damaged (OCD Region XII, March 7, 2002). 7. Bula, Sarangani An undetermined number of residents were evacuated to higher grounds (Malaya, March 7, 2002). 8. Malapatan, Sarangani Malapatan (pop: 48,000) is the next town NE of Glan. Landslides were reported by the Office of Vice Governor) 9. Lutayan, Sultan Kudarat In this town of 34,000 people, a market collapsed (Malaya, March 7, 2002; OCD Region XII, March 11, 2002).

10. Isulan, Sultan Kudarat Three houses in Kolambog, seven houses in Barangay Tayugo, Barangay Hall in Kolambog, a Catholic Church in Barangay Kalawag were partially damaged (OCD Region XII, March 11, 2002). The public market building in Baraangay Kalawag, municipal hall building in Barangay Kalaaawaag II and the department store building in Kalawag III sustained cracks (OCD Region XII, March 11, 2002). The fence of barangay plaza in Kolambog was damaged (OCD Region XII, March 11, 2002). 11. Tacurong City A 74-old man in Barangay Baras died of heart attack during the earthquake (OCD Region XII, March 7 and 9, 2002; Philippine Daily Inquirer, March 7, 2002). The walls of 14 houses in the LGU Resettlement Project in Barangay San Pablo cracked (OCD Region XII, March 7 and 11, 2002). One house in Barangay San Rafael and one church in Barangay New San Isabela were damaged (OCD, March 11, 2002). 12. Surallah, South Cotabato Surallah (pop: 62,000) is a town in Allah Valley located in the lowlands after the Sarangani mountain range. In this place, a house collapsed (Source: DSWD Alabel as relayed to Mr. Tabique of PHIVOLCS Kidapawan Seismic Station) and 158 others were damaged (OCD, March 11, 2002). A hanging bridge at Barangay Buenavista and a megadike at Barangay Centralla were partially damaged (OCD Region XII, March 7 and 11, 2002). Two were injured, one of them due to burns sustained while cooking (OCD Region XII, March 9, 2002). 274 families from the barangays of Lamsugod, Buenavista, Tubi-Allah, Veterans, Moloy, Duengas, Lolongbasing, Dalaay, Talahik and Central were affected (Provincial Disaster Coordinating Council, March 11, 2002). 13. Koronadal City, South Cotabato Koronadal (pop: 118,000) is the capital of South Cotabato and is found NE of the town of Surallah after the Roxas Mountain Range. Two classrooms of the Barangay Cacub Elementary School and the stage of the Siodina Elementary School Building collapsed. (OCD Region XII, March 7, 2002). The firewall of the Agreda Station and that of the Multipurpose Building of Barangay San Roque also collapsed (OCD Region XII, March 7, 2002). The NDMU Dormitory and 10 m of its concrete fence facing the DPWH Building collapsed (OCD Region XII, March 7, 2002). A 25-m portion of the wall of the LAPCO Warehouse collapsed (OCD Region XII, March 7, 2002). 15 other houses either collapsed or were damaged (OCD Region XII, March 7, 2002). The Masagan Bridge, concrete bridge and walls of Barangay Saravia Elementary School at Barangay Saravia, the approach of the Ferry Bridge, the San Roque Elementary School in Barangay San Roque, the MSST College of Technology Building, the KCC Mall, the overpass of the South Cotabato Provincial Hospital and the Elan Building suffered cracks (OCD Region XII, March 7, 2002; Malaya, Philippine Daily Inquirer and Manila Bulletin, March 7, 2002). The same were sustained by the walls of the Dungan Elementary School, Mangga Elementary School, Marbel 8 Elementary School, Mambucal Primary School, Mercury Drugstore and Chiu Kim Building (OCD Region XII, March 7, 2002). Two posts of the South Cotabato Electric Company were left leaning (OCD Region XII, March 7, 2002). The springs of the Dungan Lahek, Prisga and El Gawel were damaged (OCD Region XII, March 7, 2002). 14. T'boli, South Cotabato In T'boli (pop.: 54,000), a cooperative building and a health center in Barangay Dumangas collapsed (OCD Region XII, March 7 and 11, 2002; Malaya and Philippine Daily Inquirer, March 7, 2001). A solar drier, barangay stage, day care building, roads of Barangay Afus, barangay properties, a bridge approach at Barangay Dumangas and two houses were damaged (OCD Region XII, March 7, 2002). About 90% of fishpens along Allah River and four houses were washed out (OCD Region XII, March 7 and 11, 2002). Landslides were observed at Barangays Afus and Sabang zigzag road (OCD Region XII, March 7, 2002). 3,898 families from the barangays of New Dumangas,, Laconon, Datal Dlanag, Salacafe and Tudok were isolated when the Sapali/Gao Bridge was washed out by the floodwaters of Lake Maughan (OCD Region XII, March 6 and 9, 2002).

15. Lake Maughan, Parker Volcano Parker Volcano, one of the Philippine's 22 active volcanoes, has crater lake perched on its summit. The lake earlier breached in 1995 and repeat of this incident has been a serious concern for communities along the slopes of the volcano. After the earthquake, numerous landslides occurred at the crater wall near the Lake Maughan outlet. It covered two sabo dam areas. One of the landslides occurred at 3:20 AM coincident with a M5.0 aftershock (see list of aftershocks in this report; OCD Region XII, March 7, 2002). Water level at Gao reportedly increased after the earthquake. Brown smoke with accompanying sulfuric smell was observed at Mt. Melengbingay. Big cracks along trail Salacafe to Lake Maughan and from Salacafe to Barangay New Dumangas (OCD Region XII, March 7, 2002) were observed. The road from Sitio Montil to Barangay Salacafe proper upslope to Lake Maughan is unpassable. In the morning of March 7, 2002 at 5:20 AM, the walls of Lake Maughan collapsed and released waters that swept the town of New Dumangas, T'boli, South Cotabato and killed 3 people (Philippine Daily Inquirer, March 7, 2002 quoting a Major Julieto Ando of the Army's 6th Infantry Division in Mindanao). 16.Lake Sebu, South Cotabato Three persons from Barangays Lamhalak and Lamdalag died when hollow blocks fell on them (OCD Region XII, March 7 and 9, 2002; Malaya and Philippine Star, March 7, 2002). Two were also injured (OCD Region XII, March 7, 2002). Two houses collapsed (OCD Region XII, March 7, 2002) and 285 houses were partially damaged (OCD, March 11, 2002) The town hall cracked (Manila Bulletin, March 7, 2002). A health center, 15 houses in Barangays Poblacion, Kubli and Lake Lahit and 20 school buildings in Barangay Lake Lahit were partially damaged (OCD Region XII, March 7, 2002). A two-hectare area planted to gemelina and lamcade was covered by landslide (OCD Region XII, March 7 and 11, 2002). A total of 116 families from Barangay Lamcade, Sitio Bagong Silang, Sitio Kaunlaran and Barangays Maculan and Halilan were evacuated (OCD Region XII, March 7, 2002) while 1247 families from Baranagays Halilan, Lower Maculan, Upper Maculam, Denlag and Lamcade were isolated (PDCC, March 10, 2002). Damage to Maculan Bridge costs 2 million pesos (PDCC, March 10, 2002). 17.Santo Nino, South Cotabato The concrete fences of an elementary school in Baarangay Elementary School and the Barangay Teresita quadrangle collapsed (OCD Region XII, March 11, 2002). The old building of the publicc market also collapsed (OCD Region XII, March 11, 2002). A house in Barangay San Vicente was partially damaged (OCD Region XII, March 11, 2020). The terminal building and restaurants in Pob Public Market was damaged (OCD Region XII, March 11, 2002). A footbridge connecting Bagumbayan, Sultan Kudarat was damaged by floods amounting to six million pesos. Crops that were about to be harvested amounting to more than seven million pesos were destroyed by the flashfloods (PDCC, March 10, 2002). 18.Tupi, South Cotabato Eight houses at Barangays Tuben and Bunao were partially damaged (OCD Region XII, March 7 and 11, 2002). Two old churches collapsed (OCD Region XII, March 7, 2002; Malaya and Philippine Daily Inquirer, March 7, 2002) while four others (SDA, Southern Baptist, Calvarym Tuben and Bunao) were damaged (OCD Region XII, March 7 and 11, 2002). A 30-m wall of the Old Knights of Columbus Building collapsed while a street light was damaged (OCD Region XII, March 7 and 11, 2002). One street light was damaged (OCD, March 11, 2002). 19.Tantangan, South Cotabato Four houses were damaged while an old elementary school building at Barangay Libas collapsed (OCD, March 11, 2002). The walls and floors of the New Lambunao Elementary School and the multipurpose pavement and stage in Barangays New Lambunao and Cabuling cracked (OCD, March 11, 2002). The water system was also damaged (OCD, March 11, 2002). Two people were injured and were hospitalized (OCD Region XII, March 9, 2002).

20.Banga, South Cotabato Nine houses were partially damaged while three houses were totally damaged (OCD region XII, March 11, 2002). A span of the Rizal Elementary School collapsed while the altar of a Catholic Church in Barangay Kusan was partially damaged (OCD region XII, March 11, 2002). The Sapali Bridge cracked (OCD Region XII, March 11, 2002). A piggery farm at Barangay Liwanany was partially damaged (OCD Region XII, March 11, 2002). Eight barangays were affected including livestock (OCD Region XII, March 11, 2002). Two people from Barangay Cinco were injured (OCD Region XII, March 9, 2002). 21.Norala, South Cotabato Fourteen houses, two warehouses, a stage, a road, a motorcyle and the San Jose Elementary School Building were partially damaged (OCD, March 11, 2002). 22.General Santos City General Santos City (pop:327,000) is the prime city of South Cotabato. A house totally collapsed (OCD-Region XII, March 7 and 11, 2002). Walls of schools (MSU HS, J.P. Laurel and Divina Gracia Elem. School), hotel near the coast (Sydney Hotel), and that of the Philippine National Police Headquarters fell. The editing machine of ABS-CBN media station and a steel water tank servicing 1000 people of a subdivision (Dona Soledad Subd) also fell. Two police cars were crushed when the walls of the PNP headquarters fell on them. Cracks were sustained by the walls of the Notre Dame of Dadiangas College, Yap Mabuhay Building and stockyard, SAFI Building, Dolores Hospital, SMC Stockyard, Kimball Plaza and Department Store, City Christian Fellowship Center, 10 & 10 24-Hour Grocery Store, New Era University, City Hall Hotel Filipino Building, Gaisano Mall, Peninsula Rural Bank, Gillamac Building, WHC Building., District Building, Barrio 8 Hall Building, Lamco Building, firewall of the Cooperative Building, a building under construction and the approach of Palian Bridge (OCD-Region XII, March 7, 2002). The approach of a bridge in Barangay General Paulino Santos was damaged (Philippine Daily Inquirer, March 7, 2002). There was panic at a tuna factory causing injury to 100 people among the 1,000 workers (Malaya, March 7, 2002). Four electric posts in Barangay Bula fell causing power failure (OCD-Region XII, March 7, 2002). Water supply was also disrupted during the earthquake. Power resumed after lunchtime on the same day. One 54-year old died of heart attack during the earthquake while nine others were injured (OCD-Region XII, March 7 and 9, 2002). 23. Cotabato City Nine houses made of light materials at Barangay Kalanganan Mother collapsed (OCD Region XII, March 7 and 11, 2002). Two mosques were partially damaged Almonte Ext. and in Sitio Macaquiling, Barangay Sultan Kudarat. The Mega Market (OCD Region XII, March 7, 2002) suffered cracks . Power went off during the earthquake (Malaya, March 7, 2002). A man repairing a transmission line along Jose Lim Sr St. was electrocuted (OCD Region XII, March 7, 2002) Philippine Star, March 7, 2002). Two were injured by stray bullets which came from shots fired to rouse sleeping residents (OCD Region XII, March 7, 2002). Ten familiesin Barangay Kalanganan 2 were evacuated for fear of tsunami (OCD Region XII, March 7, 2002). 24. Zamboanga City Animals were frightened causing dogs to howl and chickens to crow. Plates and bottles rattled waking up sleeping people. People described the quake as strong and caused dizziness and fear. Fright caused them to hide under tables (Malaya, March 7, 2002)

25. Davao City A person died of heart attack during the earthquake. Guests in high-rise hotels panicked and were hesitant in going back to their rooms for fear of aftershocks (Philippine Star, March 7, 2002). Power went off during the earthquake (Malaya, March 7, 2002).

26. Iligan City An electric transformer in the commercial district fell (Philippine Star, March 7, 2002). 27. Other Places Kidapawan City Bislig, Surigao del Sur Pagadian City Mati, Davao Oriental Dipolog City Malaybalay, Bukidnon Butuan City Hibok Hibok Volcano Aftershocks:

: Intensity IV : Intensity III : Intensity III : Intensity III : Intensity III : Intensity III Power tripped off for some seconds after the quake : Intensity II : Intensity I

From March 6 to 9, parameters of 20 aftershocks were large enough to have been determined using the PHIVOLCS national seismic network and the USGS global network. Table 1 lists the parameters of these aftershocks while Figure 9 shows the plot of the locations of these events. The largest aftershock occurred at 2:27 AM on March 9. It was located south of the main shock, with depth of 5 km and Ms of 6.0. It was felt at Intensity III at General Santos City, Intensity II at Davao City and Intensity I at Cagayan de Oro City. Its mechanism as determined by USGS is normal faulting.

Fig 9. Aftershocks till March 9, 2002 as determined using PHIVOLCS and USGS data Date

March 6

Local Time 05:39 AM

Depth 33

Ms

5.0

mb

Mw

Epicenter

6.240 ; 124.100

Intensity

General Santos – Intensity III

March 6

11:25 AM

33

March 6

01:10 PM

33

March 6 March 6 March 6 March 7 March 7 March 7 March 7 March 7 March 7 March 8 March 8 March 8 March 8 March 8 March 9 March 9 March 9

12:08 PM 01:38 PM 10:36 PM

03:19 AM 11:30 AM 11:49 AM 06:37 PM 06:42 PM 09:56 PM 01:48 PM

03:56 AM 07:59 AM 08:07 AM 08:37 AM 02:27 AM 03:08 AM 03:35 AM

PHIVOLCS Response:

4.6

33

4.5

33

4.7

33

5.0

26 29 12 58 10 48 32 20 24 23 25 23 1

17

5.390 ; 126.940

4.6

6.160 ; 125.310 6.230 ; 124.320 6.230 ; 124.320

5.1

5.830 ; 124.79 0 5.170 ; 124.720

3.8 3.3 3.8 3.4

6.184 ; 124.657 4.6

3.5

2.9

3.2 3.5

5.459 ; 124.306 6.026 ; 123.871

General Santos City – Intensity III; Zambo

Caused another landslide at Lake Maugh

General Santos – Intensity I

5.475 ; 124.373

3.3 3.0

5.557 ; 124.392

Kiamba – Intensity IV

5.592 ; 124.018 4.9 4.9 4.7 6.0

5.329 ; 124.501 5.269 ; 124.674 5.700 ; 124.420 5.209 ; 123.184 5.288 ; 123.782

General Santos – Intensity I

General Santos – Intensity III; Davao – In General Santos – Intensity II General Santos – Intensity I

A few hours after the earthquake, PHIVOLCS sent a letter informing Her Excellency President Gloria Macapagal Arroyo about the details of the event. Copies were also furnished the National Disaster Coordinating Council (NDCC) and the Department of Science and Technology. PHIVOLCS also deployed technical personnel from its General Santos City seismic station to investigate the impacts of the earthquake especially to the coastal towns fronting the location of epicenter. The PHIVOLCS staff reached as far as Kiamba but was advised by local authorities to desist from going further to Palimbang due to peace and order problems. PHIVOLCS seismic stations in General Santos City, Zamboanga, Davao and Cotabato were swamped with calls and inquiries from the local public and media. At the same time, they had to attend to the picking earthquake phase readings for sending to Main Office for processing. PHIVOLCS Main Office personnel took

turns answering queries from the public especially 24 hours after the main shock. PHIVOLCS also issued several earthquake bulletins as more data came, the latest was bulletin no. 5. At the request of the NDCC, PHIVOLCS also issued an advisory to the coastal towns of Sarangani and Sultan Kudarat permitting them the local residents to go back to their homes. PHIVOLCS further advised them to seek the opinion of structural engineers regarding safety of their homes and places of work from future earthquakes.

Fig 10. Copy of the PHIVOLCS advisory issued in the afternoon of March 7, 2002 to the National Disaster Coordinating Council (NDCC) as requested. References:            



Bautista, Bartolome. 1996. Seismotectonic Implications of Recent Philippine Earthquakes. M.A. Thesis. State University of New York. Binghamton, New York. Gutenberg Fukushima and Tanaka (1990) Malaya. March 7, 2002 Manila Bulletin. March 7, 2002 Office of Civil Defense (OCD). March 9, 2002. Update on the Effects of the Earthquake in Mindanao as of 8:00 AM 09 March 2002. 2 pages Office of Civil Defense (OCD) Region XII. March 7, 2002. Consolidated Report Re Effects of the March 6, 2002 Tectonic Earthquake in Region XII. 8 pages. Office of Civil Defense (OCD) Region XII. March 11, 2002. Summary of Effects of March 6, 2002 Tectonic Earthquake in Region XII. 8 pages. Philippine Daily Inquirer. March 7, 2002 Philippine Star. March 7, 2002 PHIVOLCS Quick Response Team (QRT) report of the Investigation at Sarangani Province. March 7, 2002. Provincial Disaster Coordinating Council (PDCC). March 11, 2002. Status Report as of March 10, 2002, 3:00 PM pn the Extent of Damage Caused by Earthquake that Struck South Cotabato on March 6, 2002, 5:15 AM. 3 pages Tanioka, Yuchiro. 2002. Maps of tsunami models for the March 6, 2002 (Ms 6.8) Sarangani Earthquake. Unpubl. Data.

Bayugan Earthquake (1991 Ms 5.1)

1999 June 07 Ms5.1 Bayugan Earthquake

June 1999 Earthquakes in Agusan del Sur, Philippines Summary

On June 7, 1999, a Ms 5.1 earthquake occurred at 3:45 PM (local time) and damaged the town of Bayugan in Agusan del Sur. Using the PHIVOLCS seismic network, its epicenter was determined to be at 8.575 N lat, 125.754 E long or about 20 km south of the town of Bayugan. Its depth is estimated at 7 km. After two days in June 9, an aftershock occurred and damaged the town of Talacogon also in Agusan del Sur. The epicentral areas of both events are located in the Agusan Valley region, previously called by Maso (1911) as lying along

one of the “unstable portion of the line of fracture ...” due to the succession of damaging earthquakes that occurred in the area in the latter part of 19th century. Agusan River meanders through the valley floor at a general NW-trend. Mountain ranges bound the valley at its west and east sides. In the towns of Veruela and Talacogon, many swamps and lakes are found; the largest of which is Lake Lumao located west of Talacogon. The northern part of the valley belongs to Butuan City and the province of Agusan del Norte. Most part of the valley, however, belongs to the province of Agusan del Sur. The two Agusan provinces became distinct from each other from a presidential decree in 1969. Prior to this, there was only one Agusan province. The earthquake caused damages in the town of Bayugan in the province of Agusan del Sur. Bayugan has a population of 89,999 (National Statistics Office, 1996) distributed over 14 barangays. From among 9,752 dwelling units (National Census and Statistics Office, 1980), only 32 are classified as for commercial, industrial and/or agricultural purposes. About 98% of the total units were classified as “housing units”. Hence by 1995, the town of Bayugan may be described as small with few commercial buildings with most of its structures built for housing purposes. PHIVOLCS scientists who visited the town right after the earthquake noted that Bayugan has developed since then and by the time of the earthquake, had its own commercial buildings, elementary and high schools, a church, a funeral home and a municipal building (Roberto Tiglao, PHIVOLCS, personal communication, 1999). Most of these structures sustained damages from the earthquake (unpublished Quick Response Team reports, 1999). Talacogon, meanwhile, is slightly less well-developed than Bayugan. Tectonic Setting and Seismicity The affected areas are on the Agusan Valley region where the southern segment of the Philippine Fault passes through (Figure 1). The fault can be traced from the east side of the Malimono Ridge or west Lake Mainit and traverses the Agusan Valley floor in a NW direction. Another fault west of the ridge parallels the main trace and becomes indistinct in the town of Cabadbaran. Meanwhile, a branch of the main fault swerves into the southeast towards the shore town of Lianga. The town of Bayugan is at the junction where the SE-trending splay of the NW-trending Philippine Fault branches off towards Lianga. A plot of earthquakes with Ms > 4.0 from 1960 to 1997 shows that seismicity in the epicentral area that originates mainly from the activity of the Philippine Fault Zone (PFZ) is not so intense (Figure 2). The plot shows that most events are found along the west Lake Mainit fault segment. Seismicity is dispersed throughout the valley area; most of these have small magnitudes and shallow depths. Seismicity in the region is lower as compared to the offshore area between the PFZ and the Philippine Trench, which is also a source of significant seismic activity. Large earthquakes had affected the Agusan Valley area in the past and had also caused damage. Those that caused damages were the earthquakes in 1879, 1912 and 1990. Data show that previous earthquakes that originated from the Philippine Trench had not exacted that much damage to the Agusan Valley region.

Focal mechanism solutions of earthquakes from 1960 to present are shown in Figure 4. The data were derived from the Harvard University and the National Earthquake Information Center (NEIC) of the U. S. Geological Survey (USGS). In the offshore area east of Mindanao, most mechanisms show underthrusting possibly due to subduction-related activities. Offshore from the west Malimono segment, the mechanism of a shallow earthquake on May 4, 1993 showed thrust faulting (Figure 4). In the Lake Mainit area, a Ms 5.5 event that occurred on March 27, 1990 showed a left-lateral strike-slip faulting along a NW-trending fault line. This quake caused damage to Butuan City and the towns of Jabonga, Santiago and Cabadbaran, all in Agusan del Norte (Oanes and Salugsugan, 1990). There were also minor landslides at Diwata Range. Another event that showed strike-slip faulting occurred southeast of Lake Mainit at Diwata Range on May 1, 1979. After a few hours, this event was followed by an aftershock that showed normal faulting. Bautista (1996) related such type of mechanism to extension leading to the formation of a pull-apart basin. No damage was reported for these events. An earthquake with a normal mechanism occurred on August 12, 1989 near the Agusan del Sur town of Esperanza. Likewise, no damage was reported. Meanwhile, the fault swerves to the SE from the western Diwata Foothills area towards the seashore town of Lianga. Very near the town of Lianga, an earthquake in November 22, 1978 showed normal faulting. Three events (August 27, 1983, two on November 2, 1984) in the southeast part of the valley showed left-lateral strike-slip movement possibly related to movement of splays of the Philippine Fault. Sites, Sizes and Effects of Past Large Earthquakes in the Area

The review of historical earthquakes by Bautista (1996) and further refined by Bautista and Oike (2000) showed no earthquake event that may be related to the activity of the PFZ along the Agusan Valley segment prior to 1879. This could either be an artifact of lack of earthquake records, a true lack of strong earthquakes along this segment of the PFZ or miscalculation of earthquake size and epicenter due to sparse historical earthquake acocunts.

On July 1, 1879, a damaging earthquake affected the west Lake Mainit fault segment of the Philippine Fault Zone. The high intensity area covers the towns of Jabonga in Agusan del Norte, Mainit town in Surigao del Norte and a place simply called “Surigao” possibly referring to present-day Surigao City (Letter of Fr. Luengo, July 8, 1879). In the lakeshore towns of Jabonga and Mainit, hills and rocks slid down and the areas were described as highly disturbed. Numerous, large fissures opened to as much as 3.6 m wide along the lakeshore area. Some points along the lake disappeared while large landslides occurred. It caused a great fright among the few inhabitants in the area. At least 12 large aftershocks were counted within 24 hour of the main shock. Another place where significant damage to the environment was documented was in the town of Anao-oan (site of present day San Francisco) in the province of Surigao del Norte. In this place, there were 40 houses made of nipa. Fourteen tilted by as much as 25-30o while the rest collapsed. Two bridges were rendered useless. There were conspicuous landslides along the steep mountains near the shore. At Bilaa point, there was a report that a “fissure divided the promontory into two” (Letter of Fr. Luengo, July 8, 1879). While the reports are scarce, it is possible that the fissure described here is a ground rupture. This could be confirmed by detailed field investigations from the west Lake Mainit region northwards to the shoretown of San Francisco. Liquefaction was also observed in this town with the emission of NW-SE trending sandboils. The earthquake also created a potable water spring. Buildings sustained vertical cracks while floors tilted and windows and doors jammed. A mangrove area sank by 300 m at Bilan Port, near Bilaa Point. There were also numerous fissures, some measuring 4-6 cm wide. Some of these fissures later on closed. Movable objects were thrown down and two statues fell. A thick-walled church made of coralline limestone and with galvanized iron roof sustained both horizontal and vertical cracks (Letter of Fr. Luengo, July 8, 1879). Unlike Jabonga, there were 40 events felt within a 24-hour period. The direction of motion is SSE-NNW (Centeno y Garcia, November 1, 1879). Bautista (1999) inferred the epicenter to be at 9.350 N lat and 125.600 E. long and its Ms to be 6.9 using felt are-Ms relations. On July 11, 1912, a Ms 7.5 earthquake affected the NE Mindanao region. Damage and high intensity were experienced by towns (La Paz, Bunauan, Veruela and Talacogon) in the Agusan Valley area where intense ground shaking, liquefaction, widespread landslides and river/lake seiches occurred. Accuracy of location during the turn of the century is in the order of +/- 5o due to the few number of participating global stations during this time. While its epicenter was instrumentally located in the east Lake Mainit area, intensity data suggest that its epicenter was probably along the valley area where the fault passes through.

Epicenters, Magnitudes and Depths of the June 7 and 9 events The 3:45 PM event on June 7 was recorded by 20 PHIVOLCS seismic stations as far as Pasuquin Seismic Station in Ilocos Norte. Its epicenter was determined to be at 8.575 N lat, 125.754 E long using eight P-wave readings. Its depth is about seven km. The epicenter is about 15 kms from Bayugan, where the damages were reported, and which is inside the the range of error of PHIVOLCS plotting.

Based on the earthquake’s location, the source is believed to be from the movement of the segment of the Philippine Fault in Agusan Valley. While the magnitude of this quake may be considered as ‘moderate’, its expected intensity at Bayugan town could be as high as Intensity VII for medium soil using the attenuationrelation of Fukushima and Tanaka (1990). In Bayugan, poorly built structures collapsed, posts and foundations of buildings sank or tilted. There were also reports of injuries due to falling debris or objects. People in the epicentral area were frightened and most of them ran out when the earthquake occurred. After two days at 9:05 AM on June 9, a Ms 5.0 earthquake again occurred near the epicentral area. While this appears to be an aftershock of the June 7 event, its location is about 2 km north of the June 7 event. This event caused damages to bridges, roads, schools, a commercial complex, telephone station and municipal hall of the town of Talacogon also in Agusan del Sur. This was also felt in other towns inside Agusan Valley.

A large aftershock occurred in June 9 at 9:05 AM. By using 11 P-wave and 1 S-wave readings, the epicenter was determined to be at 8.604 N lat, 125.648 E long and its depth to be about 14 km. Unlike the June 7 event, this was most severely felt in the town of Talacogon. Focal Mechanism of the June 7 earthquake The NEIC which routinely publishes focal mechanism solutions of events with greater than Ms 5.5 did not issue solutions for the June 7 as its magnitude was below the Ms 5.5 cut off magnitude. Instead, we used the first motion data of 18 PHIVOLCS seismic stations to determine the solutions for the two events. Figure 4 shows the focal mechanism for this event. It shows a normal faulting with some strike slip component. This correlates with the August 12, 1989 event shown in Figure 3 and this could be interpreted as possibly due to movement along a pull-apart basin common in strike-slip environments. Intensity Distribution June 7 event

The most severely affected place is the town of Bayugan where structures collapsed. Based on the Philippine Earthquake Intensity Scale (PEIS) of I to X, the intensity is assessed as Intensity VII in this town. PHIVOLCS main office personnel and the QRT Bayugan team also conducted telephone and personal interviews to assess the intensities at different sites. The following are the results of both the intensity survey and reports of PHIVOLCS stations and as shown in Figure 5: Bayugan, Agusan del Sur - Intensity VII San Francisco, Agusan del Sur - Intensity V Butuan City - Intensity V Hinatuan, Surigao del Sur - Intensity IV Lianga, Surigao del Sur - Intensity II Cagayan de Oro City - Intensity II Bislig, Surigao del Sur - Intensity II Camiguin Island - Intensity I June 9 event

The June 9 event damaged the town of Talacogon. Talacogon, Agusan del Sur - Intensity VI Bayugan, Agusan del Sur - Intensity V Hinatuan, Surigao del Sur - Intensity IV Butuan City - Intensity IV Bislig, Surigao del Sur - Intensity IV San Francisco, Agusan del Sur - Intensity IV Lianga, Surigao del Sur - Intensity III Prosperidad, Agusan del Sur - Intensity III

Foreshocks and Aftershocks Figure 7 shows the background seismicity a month before the June 7 event. The events were small, ranging from Ms 3.3 to 4.8 with depths between 7 to 53 km. No earthquake, therefore, may be considered as a foreshock to this event based on the data. Table 1 lists the parameters of the background seismicity a month before the quake. There were no recorded events in the epicentral area.

Figure 8 shows the 51 earthquakes plotted from the June 7 main shock until June 30, 1999. Most of the events clustered around the June 7 main shock. Some, however, are located farther away (at most is 10 km) from the fault trace. The largest aftershock is the June 9 events at 9:05 AM. While some of the aftershocks cluster along the fault trace, some are spread out around the epicentral area. It is difficult to determine whether this was caused by the sparse seismic stations in the area resulting in a dispersed aftershock plot or possibly due to inhomogeneity of the rock units. Table 2 lists the aftershocks of the June 7 event, including their parameters, as detected by the PHIVOLCS seismic network.

PHIVOLCS’ response Since there was no PHIVOLCS station near the epicentral area and there was a need to determine actual intensity in the epicentral, PHIVOLCS conducted intensity survery right after the quake through telephone interviews of residents in Butuan City and the towns of Bayugan, Prosperidad and San Francisco.

During the same day , PHIVOLCS dispatched a Quick Response Team (QRT) composed of personnel from PHIVOLCS seismic stations in Surigao City and Bislig. They were tasked to conduct ocular inspection of reported damages, determine cause of damage, document impacts and to conduct information drive to allay the fears of the local people. Their reports relayed on June 8 and 9 are shown in Appendices ‘A’ and ‘B’. A second QRT team arrived on June 9 to assist the first team.

Two days after the main shock on June 9, a rumor that “an impending Intensity VIII earthquake in Agusan del Sur” caused panic and confusion in the affected area. Classes were suspended as a result. As a result, PHIVOLCS was swamped with calls regarding the veracity of this rumor. In the afternoon of June 9, PHIVOLCS released an “Earthquake Update” that “categorically denied issuing statements that there will be a strong earthquake in the area and that such rumors are ill-founded and do not have any scientific basis”. Conclusion and Recommendations The area where the June 7 and 9 earthquakes occurred is within a seismically active region. Data show that the two events were not the biggest events that had occurred in this area during historical times. The June 7 event in the Agusan Valley region may be considered as part of a ‘normal’ earthquake activity within a seismically active region. The event, despite its moderate magnitude, became damaging only because of possible site amplification due to thick sedimentary fill in the area and the poor construction prevalent in the locality of Bayugan. The area is also susceptible to liquefaction based on its local geology and as evidenced by past liquefaction in the area (e.g. 1879, 1912 earthquakes). We have also observed that earthquake intensities from previous earthquakes and also during the recent June 7 and 9 events were amplified due to the presence of thick, water-saturated sedimentary deposits along the valley floor.

Our concern right now is that should there be another moderate to large earthquake along this segment of the fault, some areas may experience strong ground shaking or may liquefy causing buildings and other structures to fail. We therefore advise that structures in the area be regularly inspected by structural engineers to evaluate their soundness. Buildings that are yet to be built should be properly anchored by pile driving through the liquefiable layers to counter the possible effects of liquefaction. The area which sustained the heaviest damage, Bayugan, is about 15 km from the epicenter determined using the current PHIVOLCS stations. Similarly, the June 9 event was also several kilometers away from the instrumental epicenter. We recommend that additional seismic stations in any of the valley towns (e. g. Talacogon, San Francisco, Veruela), in Bukidnon and another in either Cabadbaran, Bayugan or Butuan City be established to improve earthquake determination capability of PHIVOLCS. Site response evaluation studies are recommended to pinpoint the reason why the damages caused by the June 7 and 9 earthquakes were far from the instrumental epicenter. Mapping of the segment of the Philippine Fault should be done to finetune location of faults determined from seismicity and digital terrain data. Information campaign about the possible effects of future earthquakes in the Agusan Valley region should be started in order for local people to be prepared for future earthquakes. Acknowledgements The contents are quoted mostly from the work of Bautista et al (1999) entitled “Seismic Characteristics of the June 1999 Earthquakes in Agusan del Sur, Philippines: A Preliminary Report” References Allen, C. R. 1962. Geological Criteria for Evaluating Seismicity. Geological Society of America Bulletin. 86. 1041-1057.

Bautista, Bartolome. 1996. Seismotectonic Implications of Recent Philippine Earthquakes from 1980 to 1994. Unpubl. M. A. thesis. State University of New York at Binghamton, New York, USA. 218 pp. Bautista, Ma. Leonila P. 1996. Determination of the Epicenters and Magnitudes of Philippine Historical Earthquakes (1589 to 1896). Unpubl. M.S. thesis. Kyoto University.

Bautista, Maria Leonila P. and Kazuo Oike. 2000. Estimation of the Epicenters and Magnitudes of Philippine Historical Earthquakes. Tectonophysics. 317. 137-169. Bautista, Ma. Leonila P., B. C. Bautista, E. Q. Amin and J. C. Salcedo. 1999. Seismic Characteristics of the June 1999 Earthquakes in Agusan del Sur, Philippines: A Preliminary Report. PHIVOLCS Internal Report.

Centeno y Garcia, Jose. 1879. Report dated November 1, 1879 about the July 1, 1879 earthquake. Submitted to the Governor General of the Philippines. Fukushima, Y and T. Tanaka. 1990. A new attenuation relation for peak horizontal acceleration of strong earthquake ground motion in Japan. Bull. Seism. Soc. Am. 757-783.

Fr. Luengo, S.J. 1979. Letter of Fr. Luengo, S. J. dated July 8, 1879 about the July 1, 1879 earthquake in Surigao.

Maso, Miguel Saderra. 1912. The Earthquake of the Agusan Valley and the Eastern Coast of Mindanao, July 12, 1911. Bull.. Seism. Soc. Am. II. June 1912. 141-142. National Census and Statistics Office. 1980. Census of Population and Housing: Housing Characteristics of Occupied Dwelling Units by Region, Province, City and Municipality, Philippines. Special Report No. 5. Manila. 1-382. National Statistics Office. 1996. Census of Population. Report No. 1-P (Caraga): Population by Province, City/Municipality and Barangay. Manila. 1-44.

Oanes, Alejandro and Luisito M. Salugsugan. 1990. Investigation of Agusan del Norte Earthquake on 0647 H March 27, 1990. PHIVOLCS unpubl. Report of Investigation. 2 pp.

Quebral, Ramon D., Manuel Pubellier and Claude Rangin. 1996. The Onset of the Movement on the Philippine Fault in Eastern Mindanao: A Transition from a Collision to a Strike-Slip Environment. Tectonics. 15. 4.713726. Focal Mechanism

Strk1 197 Dip1 50 Rake1 -135 Strk2 75 Dip 2 57 Rake2 -50

P-axis Plunge 57 Azimuth 41

T-axis Plunge 4 Azimuth 137 Focal mechanism solution of the 07 June 1999 event using first motion data of 18 PHIVOLCS seismic stations.

Isoseismal Map of the 07 June 1999 (Ms=5.1) earthquake in Bayugan, Agusan del Sur. Intensity data based on PHIVOLCS Station reports, investigation and interviews Mindoro Earthquake (1994 Ms 7.1)

1994 November 15 Ms7.1 Mindoro Earthquake *INSERT PDF HERE*

Luzon Earthquake (1990 Ms 7.9)

1990 July 16 Ms7.9 Luzon Earthquake

Contents:

Foreword

We conceived of this technical monograph on the July 16, 1990 Earthquake in a symbolic way as a medium by which we can remember those who have died, were seriously injured and have lost their loved ones. It is also a most fitting way to honor those who have helped us and who continue to help us through rehabilitation efforts or the studies that will tame earthquakes through preparedness and risk management.

Beyond this symbolic value, this monograph has utmost practical importance for the Philippines, a country which has now realized itself not only to be in the "developing" stage but also in the "disaster-prone" category. I am sure that the prospective readers of this Compendium share both our experience and concern, being members of the same "Ring of Fire" club or simply being concerned scientists and experts who believe that the search for knowledge is best served by sharing knowledge. During the last two decades, the Philippines has suffered several devastating earthquakes. In 1968, 200 people died when the Ruby Tower collapsed in a 7.3 magnitude earthquake in Manila. More than 6,000 people died when a tsunami caused by a 7.9 magnitude earthquake in the Moro Gulf hit them in 1976. Then we had the July 16, 1990 earthquake whose devastation is so far unequaled in deaths, property damage and psychological shock. The Department of Environment and Natural Resources (DENR) immediately initiated rehabilitation efforts after the July 16 earthquake. Livelihood programs for the victims and the rehabilitation of damaged watersheds were implemented. These, as well as the efforts of other government agencies and nongovernmental organizations, however, are mostly of a curative nature and are not enough. There must be preventive approaches developed, if not against earthquakes (which we cannot prevent), at least in terms of early warning, land use planning, improved building codes, and the like. President Aquino thus signed on August 6, 1990 the Memorandum Order creating the "Inter-Agency Committee on Documenting and Establishing Database on the July 1990 Luzon Earthquake". This Inter-Agency Committee chaired by DENR and the Department of Science and Technology (DOST) was tasked to undertake a unified, systematic and scientific documentation of information on earthquakes, particularly the July 16 killer quake for future planning and research. This is one product of the Inter Agency Committee. The results of direct or related studies on the July 16 earthquake gain additional importance now that there is a need to again generate public interest and concern at the highest levels of government on how best to deal with earthquakes. Over the past months, the July 16 "Killer Earthquake" seemed to have taken the backseat in a series of prominent happenings including the eruption of Mt. Pinatubo and the national elections. For a short while, interest was revived by the release in the news media of the Marikina or "Punongbayan" Fault. But time has again eroded the attention focused accorded it. Those who have been to Baguio City, one of the most devastated areas during the July 16 earthquake, can hardly see any of the fear, sorrow, shock and ruins that characterized the city then. Almost everything has gone back to normal including the persistent and increasing applications for subdivision development in the steep slopes of the city. We certainly do not want to generate intense interest and concern on earthquakes with another earthquake. The scientific information and the practical recommendations coming from the contributors to this monograph would surely be more than enough. In a sense, this is much better as we can look at what had happened and what we can do in a more objective, organized and even optimistic outlook. To have learned nothing from the tragic events of July 16 would be callousness of the highest degree. To have done nothing to develop strategies to prevent injury and damage should another earthquake occur would be irresponsibility of the most serious proportions. It may well be for us to remember how those trying to save schoolchildren pinned by concrete slabs, for lack of proper equipment, had to commandeer hydraulic jacks from passing vehicles. Those who have relatives in Baguio City can still recall the fear and the anxiety of not knowing what happened to their loved ones because of broken communication lines. In the mosaic of collapsed buildings we should again take note of questions related to siting considerations, risk assessment, structural foundations, architectural and civil engineering designs, buffer Ones and safety corridors. Fulgencio S. Factoran, Jr. The July 16 1990 Luzon Earthquake Rupture

Raymundo S. Punongbayan*, Rolly E. Rimando*, Jessie A. Daligdig* Glenda M. Besana*, Arturo S. Daag*, Takashi Nakata**, and Hiroyuki Tsutsumi** *Philippine Institute of Volcanology and Seismology ** Hiroshima University, Japan

ABSTRACT The 16 July 1990 earthquake (Ms = 7.8) produced a 125 km-long ground rupture that stretches from Dingalan, Aurora to Kayapa, Nueva Vlzcaya as a result of strike-slip movements along the NW segment of the Philippine Fault Zone and its splay, the Digdig Fault. The earthquake epicenter was placed at 15º 42' N and 121º 7' E near the town of Rizal, Nueva Ecija. The surface rupture essentially followed the pre-earthquake active fault trace along previously identified fault-related geologic and geomorphic features such as mole tracks, sag ponds, offset streams and fluvial terraces, shutter and pressure ridges, scarplets, and similar features, with only slight deviations in certain places. Secondary shears are present as localized features along portions of the main rupture trace. Ground rupture, which had a general trend of N 40-60º W, was predominantly left-lateral with measured vertical and horizontal displacements varying from 0.1-2.5 m and 0.2-6.2 m, respectively. Movement is concentrated along the main rupture although right-stepping en echelon faults and trace discontinuities interrupt the narrow fault trace. The spatial distribution of these en echelon faults suggests that these are surface expressions of fault bends and are more pronounced to the SE of the epicenter. Asymmetry of fault length with respect to the epicenter, rupture arrest and displacement distribution can be explained in terms of these rupture propagation barriers. Variation of horizontal and vertical displacement values with distance shows a wavy pattern with some observed scissor-like reversals in the vertical displacement along some segments of the fault. Rupture length and measured maximum horizontal (6.2 m) and vertical (2.5 m) displacements are within the range of values observed worldwide, for earthquakes of this magnitude. Damage to buildings, infrastructures, and properties amounted to at least P 10B, a part of which was caused by ground rupturing. Structures directly straddling the ground rupture were totally damaged as a result of large lateral shifting and substantial vertical displacement. However, some houses within 1-2 m on either side of the ground rupture survived owing to their light-weight construction while those built of reinforced concrete within this zone suffered partial damage. Damages beyond 2m depended mainly on the structural integrity of the building and effects of local topography and ground conditions. These observations underscore the advantage of using lightweight materials for construction purposes as well as the need to observe sound construction and design of buildings particularly in areas close to the ground rupture and in places that may be affected by future movements along active faults.

INTRODUCTION The 16 July 1990 earthquake (Ms=7.8) produced a 125 km-long ground rupture that stretches from Dingalan, Aurora to Kayapa, Nueva Vizcaya along a general N 40-60º W trend ( Figure 1 ). The earthquake epicenter was placed at 15º 42' N and 121º 7' E near the town of Rizal, Nueva Ecija and was caused by strike-slip movements along the NW segment of the Philippine Fault Zone and its splay, the Digdig Fault. Prior to the earthquake, portions of the Digdig Fault along which the ground rupture later formed, were mapped and documented by the authors and Takashi Nakata of Hiroshima University in connection with PHIVOLCS's active faults mapping project. The 16 July event thus provided the corroboration to the recency of the fault's activity. While no short-term prediction for the active fault was issued, its possible future reactivation was anticipated and presented as among the constrains in planning in a seminar conducted in Baguio City a month before the earthquake struck. The seminar was attended by planners and local officials from Region I and the Cordillera Administrative Region. Mapping of the extent of the ground rupture and the documentation of fault-related features, aside from their value, will have significant implications for future land use and zonation planning, especially in areas close to or transected by active faults. Future movements of these faults are expected to follow pre-existing fault traces. Post-earthquake studies of the ground rupture may likewise provide additional insights into possible scenarios that might be expected when the Digdig Fault or other active faults in the Philippines next move. Furthermore, a study into the structural controls that affect rupture propagation arrest might provide us clues where strong ground motions that generally and enable us to identify sites along the fault trace where strong ground motions that generally account for a majority of earthquake-related damages might be expected HISTORICAL SEISMICITY ALONG THE DIGDIG FAULT Based on historical records, the 30 November 1645 earthquake may have originated from the same segment of the Philippine Fault which ruptured on 16 July 1990. This particular event is one of the most destructive earthquakes that affected Manila and large parts of Luzon, particularly in the uplands of Gapan, Nueva Ecija

(SEASEE, 1985). The earthquake was so strong that at least 25 % of the houses in Manila along were totally damaged, leaving some 3,000 casualties. Earthquake-related effects ranged from fissuring of rice fields, landsliding that dammed rivers, and river-flooding of settlement areas. The earthquake was said to have been accompanied by a terrifying noise with felt aftershocks lasting until the following year. In 1892, a similarly strong earthquake caused extensive damage over a wide area in Luzon but accounts are unclear as to its possible epicenter. Some writers ascribe the earthquake to the Digdig Fault but the Puncan Church, an eighteenth-century church located just 200 m from the 16 July ground rupture, was not affected by the 1892 earthquake, yet collapsed during the 16 July 1990 event.

THE GROUND RUPTURE Location, Style and Pattern of Surface Faulting Documentation and field mapping of the active fault prior to and after its movement on 16 July 1990 showed ground rupture that the 1990 ground rupture closely followed the pre-existing active fault trace, with come minor deviations in places. Earlier movements of the active fault before the July 16 event left many geologic and geomorphic evidence, (such as mole tracks, sag ponds, offset streams and fluvial terraces, shutter and pressure ridges and scarplets) suggesting its recent activity. Because diaplacements along active faults essentially occur along or near the same fault plane and nearly always with the same sense of movement, the location and pattern of future strike slip faulting can thus be predicted within reasonably close limits by mapping such fault-related features. A ground rupture is rarely confined to a simple narrow and distinct line. Instead, a complex fault pattern results where the main fault zone is complicated by branching and formation of secondary faults where lesser displacements occur. The main trace on the ground surface can be a single rupture or can consist of parallel, branching, or interlacing fractures, the width of which may vary from a few centimeters to hundreds of meters. In the case of the 16 July 1990 earthquake ground rupture, this zone is up to several meters wide depending on local ground conditions. The fault zone is generally narrower in areas underlying firm foundations (e.g., bedrock, adobe) than in places with soft substrate, such as sand and other fine sediments. Secondary faults in the form of synthetic, antithetic and subparallel fractures are as far as 300 meters from the main fault zone, as illustrated by the occurrence of secondary shears at Digdig, Carranglan, Nueva Ecija ( Figure 2 ) while branch faults may extend for a distance of 500 meters, as illustrated by the branch fault at Rizal, Nueva Ecija ( Figure 3 ). In places, the narrow distinct fault trace is often interrupted by en echelon traces and mole tracks ( Figure 4 ). In other locations, however, the compressional mounds between the ends of en echelon faults are not very prominent ( Figure 5 ).

Variations in Amount of Slip Along the Fault In general, displacement is not symmetrical along fault traces but may reach a maximum or a minimum near one end of the fault and may show several high points of offset along the fault trace. For the 16 July ground rupture, movement was predominantly left-lateral with measured horizontal and vertical displacement values ranging from 0.1 - 6.2 m and 1.0 - 2.5 m respectively. The horizontal (H) and vertical (V) displacements when plotted against distance from the NW terminus of the ground rupture ( Figure 6) show a wavy pattern having points of sharp increase and decrease within relatively short distances. From NW to SE, a general decrease in horizontal displacement is observed except near the termini where both horizontal and vertical displacements approach zero values. Likewise, a plot of the net slip with distance from NW to SE ( Figure 7 ) shows a similar pattern owing to the predominantly strike-slip nature of movement along the ground rupture. Moreover, inherent difficulties in estimating full displacement in some cases and possible imprecisions in displacement measurements may also have contributed, albeit to a lesser degree, to the observed wary pattern of displacement values along the fault trace. Figure 8 shows the variation in the sense and amount of vertical displacement with distance along the ground rupture. From the figure, the amount of vertical displacement generally increases from NW to SE, accounting for decreasing H/V values ( Figure 9 ) along this direction. This is attributed to the tendency of the rupture for releasing stress by extension at the termination point rather than propagating further to the SE. The sense of vertical displacement is likewise not uniform (i.e., the upthrown block shifts from one side of the fault to the other) along the whole stretch of the fault owing to local ground conditions. Thus as a whole, measured displacement values and sense of vertical displacement along the ground rupture are characterised by unevenness and irregularities. Table 1summarizes the displacement data along the ground rupture.

Length of Surface Rupture and Maximum Displacement Vs. Earthquake Magnitude Empirical relations between dimensions of faulting and earthquake magnitude have been derived from studies of historical large earthquakes worldwide. Correlations have been made between magnitude and fault length and between magnitude and maximum displacement q (Bonilla). Figure 10 for example, shows one correlation between earthquake magnitude and length of surface rupture. Figure 11a illustrates the empirical relation between magnitude and maximum surface displacement while Figure 11b shows the empirical relation of length of surface rupture with maximum surface displacement. Plotting the pertinent data from the 16 July 1990 event into these figures indicates that the rupture length, magnitude and maximum surface displacement values for the 16 July ground rupture are within the range of expected values based on worldwide empirical data. THEORY Structural and Geometric Barriers The presence of a barrier or a series of barriers along the fault rupture may constrain rupture propagation. Barriers could be in the form of irregularities along the fault plane as manifested by bends and jogs, a geometrical barrier near the rupture terminus, inhomogeneous barriers, or a combination of all of the above. Fault jogs and bends are usually classified as either dilational or antidilational. Dilational jogs are associated with small pull-apart basins while antidilational jogs are related to pop-up structures. Antidilational jogs clearly perturb ruptures and form obstacles to both short- and long-term slip transfer along a fault. However, dilational fault jogs seem to play an especially important role as preferred sites for rupture arrest (Sibson, 1987). Dilational fault jogs appear to behave as kinetic barriers which impede rapid slip transfer, but allow slip to occur slowly due to the difficulties of quickly opening a linking extensional fracture mesh in a fluid saturated crust (Sibson, 1987). Inhomogeneous barriers are those in which there are no obvious geometric obstacles and are related to differences in resistance to breaking along parts of a fault. Rupture Propagation Rupture propagation is a function of the magnitude of the applied stress, barrier strength and the areal extent of the barrier (Das and Aki, 1977). Crack tip propagation will be arrested if the areal extent of a barrier is large (Das and Aki, 1977). Hence, when the areal extent of the barrier is small, it is broken as the crack tip passes when the tectonic stress is relatively high or the crack tip proceeds beyond the barrier without breaking it when the tectonic stress is relatively low. However, when the tectonic stress is neither high nor low, then the barrier is initially unbroken as the crack tip passes but eventually breaks because of subsequent increase in dynamic stress (Das and Aki, 1977).

DISCUSSION The southeast extension of the fault (with respect to the epicenter) is more restricted compared to the northern segment indicating that this part of the fault must have been more resistant to movement. Southeast propagation of the ground rupture could have been constrained by the presence of a barrier or a series of barriers. Despite the considerable change in strike of as much as 30 degrees over a large area near Bato Ferry and Bateria and the presence of a series of more limited bends south east of the epicenter ( Figures 12 and 13 ), the rupture somehow still managed to propagate from the epicenter up to its south eastern terminus. The ability of the fault rupture to overcome such a bend can be attributed to the large stress applied during rupture propagation at this point on account of the proximity of the area to the epicenter. However, much of the available energy may have been conceivably spent to overcome this particular barrier thus slowing down rupture propagation in the process. In addition to this, irregularities along the fault plane might have contributed to the slowing down of the propagation causing it to proceed in a series of jerks. As a consequence of the rupture's uneven track, propagation was not smooth possibly resulting in the observed irregular values of slip along the whole stretch of the surface rupture. As the rupture encountered more barriers on its southeastward course, more energy was spent, until finally the remaining energy was no longer sufficient for the rupture process to progress beyond its termination point between Gabaldon and Dingalan. Furthermore, the presence of a formidable geometric barrier due to the abrupt bending of the Philippine Fault Zone from Dingalan to an almost southward direction ( Figure 14 ) may have greatly influenced the arrest and subsequent termination of the ground rupture.

In the vicinity of the epicenter near Rizal, the fault formed a branch with the main trace of the fault propagating farther northwestward while the splay did not go beyond Rizal town proper. The termination of the fault branch northwest of Rizal proper may likewise be attributed to the presence of a barrier. The nature of the barrier, however, is uncertain, there being no obvious change in rupture strike. However, intersecting and transverse faults along the trace of the rupture near the terraces at the branch terminus may have locally increased resistance of the fault to rupture propagation thus constituting an inhomogeneous barrier Additional evidence suggesting the presence of a barrier at the branch termination point is the high ground acceleration (g) value of at least 1 g in this area where many boulders where thrown-out as far as 20 cm from their former positions (Umeda et. al., 19?). Similarly thrown out boulders were also observed in Laur. Thus, high frequency waves induced by strong ground motion (Sibson, 1987) are enhanced by irregular rupture propagation with non-uniform slip due to the presence of a barrier (Aki, 1979). From Rizal going northwestward, rupture propagation was relatively smooth. However, localized bends are occasionally encountered accounting for the non-uniform slip along the trace ( Figures 15 & 16 ). Near Cabalatan, Kayapa, the trace started to encounter significant bends until its terminus at Bisong, Kayapa ( Figure 17 ). The nature of the barrier at this particular point is quite unclear but it was apparently strong enough to resist extension owing to the already weakened applied stress. Later, increased stress concentration at the northwestern terminus probably caused a sequence of aftershocks. In contrast, relatively few aftershocks occurred near the SE terminus of the rupture ( Figure 18 ). When a stress increase of great magnitude occurs, various things may happen (Aki, 1979). Stress could be concentrated near the stopping point and only partially released, thus accounting for the few aftershocks at the southeastern terminus. As the rupture could propagate no further, stress release in the form of greater vertical displacements thus occurred. In contrast, the spatial distribution of aftershocks at the northwestern terminus indicates a clustering near the end of the rupture zone and migration away from it, gradually extending the aftershock area. In this case, the stress concentration is relieved at least partially by brittle fracture. Alternatively the SE barrier may be ductile, relaxing stress by non elastic deformation, resulting in fewer aftershocks, while the NW barrier may be brittle and relieve stress by fracturing, producing numerous aftershocks in the process.

IMPACTS The earthquake left in its wake about 1,200 casualties and at least P 10 B of damages to buildings, infrastructures, and properties. Part of this damage was due to ground rupturing, although no casualty was reported as a result of this particular hazard. Areas heavily-impacted by the effects of ground rupturing, were Rizal town proper and nearby towns of Laur, Bongabon, and Gabaldon to the southeast, and Digdig, Puncan (Carranglan), in Nueva Ecija, and Imugan (Sta. Fe) in Nueva Vizcaya to the northwest. Danger within the epicentral area were confined to a narrow zone 1-2 m on both sides of the rupture. Buildings and houses regardless of make and design directly straddling the ground rupture were totally damaged ( Figure 19 ), mainly due to large lateral shifting and vertical displacement. In many instances, however, a number of lightlyconstructed buildings within this zone surprisingly survived ( Figure 20 ). In comparison, buildings made up of reinforced concrete within this 4-m zone suffered partial damage, underscoring the apparent flexibility of lightweight materials that tend to sway with and not resist, ground shaking. Beyond 2 m from the ground rupture but still within the epicentral area, damage to structures becomes minimal except for poorlyconstructed buildings and on account of the local ground conditions. In places, substantial vertical movement along the ground rupture resulted to disruption of normal agricultural activities in the Rizal-Bongabon-LaurCarranglan areas (Nueva Ecija) as formerly irrigated rice paddies can no longer be irrigated as these are on the upthrown side while those on the downthrown side cannot be planted to rice due to flooding ( Figure 21 ). Localised large-scale flooding occurred in areas where the streams and water systems were cut-off, dammed and at times, diverted by ground displacements ( Figure 22 ). Infrastructures such as roads ( Figures 23a and 23b ) and bridges ( Figures 24a and 24b ) along the ground rupture were also damaged as a result of both horizontal and vertical ground shifting. Other earthquake-related damages can be attributed to strong ground vibration and related landslides and liquefaction. Vibration was widely felt in Luzon affecting areas more than a hundred kilometers away from the epicenter with the intensity varying with local ground conditions. Landslides affected Nueva Ecija, Nueva Vizcaya and Benguet. The effects of the landslides were more intense near the areas traversed by the ground rupture, areas undercut by streams, road cuts, and in highly-steepened slopes specially where the rocks are

highly fractured and devoid of vegetation. Subsequent heavy rains carried down log-laden debris that sometimes resulted in the damming of river courses. Subsequent breaching of these natural dams resulted in widespread inundation of nearby settlements and agric ultural lands. Liquefaction and settling damaged mostly the swampy areas in and near Dagupan, Agoo and Aringay (La Union). In Gerona and Pura, Tarlac, liquefaction effects consist of subsidence, sand boiling and tilting of structures. Localised subsidence was also observed, in Metro Manila, particularly in the Cultural Center of the Philippines Complex along Manila Bay.

SUMMARY AND CONCLUSIONS The ground rupture of 16 July 1990 Earthquake is not confined to a simple narrow and distinct line. Instead, a complex fault pattern resulted where the main fault zone is complicated by branching and secondary faults of lesser displacements. Secondary faults in the form of synthetic, antithetic and subparallel fractures are as far as 300 meters from the main fault zone and branch faults may extend for a distance of 500 meters from the bifurcation point. The narrow distinct fault trace is also often interrupted by en echelon traces and mole tracks reflecting the type of local material (e.g., firm vs. soft foundation) and the localised effects of rupture propagation. The ground rupture closely followed the pre-existing active fault trace, although minor deviations in places were observed. The amount of surface displacement along the fault rupture varies greatly reaching a maximum or a minimum near one end of the fault and shows several high points of offset along the fault trace. Thus, displacement values and sense of vertical displacement along the fault is characterised by unevenness or irregularity, consistent with the location of barriers or asperities along the fault plane that prevent smooth rupture and displacement propagation. Asymmetry of the NW and SE rupture lengths, irregular rupture propagation, termination, slowing down and jerky rupture propagation and high ground acceleration are controlled by rupture propagation barriers interrupting the trace and fault plane. Damage to buildings, infrastructures, and properties amounted to at least P 10 B. a small part of which was caused directly by ground rupturing. Structures directly straddling the ground rupture were totally damaged as a result of large lateral shifting and substantial vertical displacement. However, some houses within 1-2 m on either side of the ground rupture survived owing to their light-weight construction while those built of reinforced concrete within this zone suffered partial damage. Damages beyond 2m but still within the epicentral zone was minimal depending on the structural integrity of the building and effects of local ground conditions. These observations underscore the advantage of using lightweight materials for construction purposes as well as the need to observe sound construction and design of buildings particularly in areas close to the ground rupture and in places that may be affected by future movements along active faults. RECOMMENDATIONS As shown by the results of this study, the 16 July earthquake essentially followed the previously-mapped active fault trace. The techniques and methodology used herein may thus be applied in the identification, delineation, and future studies of other active faults in the Philippines. More detailed mapping of the location and areal extent of the 16 July ground rupture should be undertaken, especially where the latter transects or passes close to settlement areas. As far as practicable, it is likewise proposed that efforts be exerted to mark the location of t the ground rupture at strategic points for easy reference and shall serve as among the constraints in future land utilization planning and development in these areas. Based on the observations of damages with respect to distance from the ground rupture, it is herein proposed that an easement of at least 5 m. on either side of the line of rupture be observed and strictly implemented for purposes of zoning and future land utilization and development scheme in areas transected by the ground rupture. Existing codes and regulations governing the structural design and construction of buildings close to the ground rupture be reviewed, rectified, and most importantly, strictly implemented. Specific sites along the ground rupture may be identified, developed, and preserved as national geologic monuments to instill awareness of earthquake-related hazards among the people and serve as concrete reminder of the 1990 earthquake and its damaging effects in view of the fact that disasters such as the 16 July 1990 earthquake are easily forgotten.

Multi-sectoral effort should be undertaken to embark on a nation-wide information campaign on the nature and hazards posed by earthquakes and their proper mitigation to educate the general public, policy planners, and decision-makers considering that the Philippines is an earthquake-prone country. Existing contingency measures designed to cope up with disasters should be reviewed and modified to incorporate the various lessons learned from the effects of the 16 July earthquake. ACKNOWLEDGMENT The authors would like to acknowledge the partial funding extended to this project by the Department of Environment and Natural Resources through Secretary Fulgencio S. Factoran, Jr. without which this endeavor would not have been possible. The help extended by the following PHIVOLCS personnel in the preparation of some figures used in this paper are likewise acknowledged: Messrs. Eleuterio Diao and Reynaldo Macaspac, and Ms. Shirley Inmenzo.

REFERENCES Aki, K. 1979. Characterization of barriers on an earthquake fault, Journal of Geophysical Research,84,61406148. Beanland, S., Berryman Kelvin R., and G. H. Blick.1989. Geological investigations of the 1987 Edgecumbe earthquake, New Zealand. New Zealand Journal of Geology and Geophysics,32, 73-90. Bonilla, M. G. and Buchanan, J. M.,1970. Interim report on worldwide historic surface faulting. U.S. Geol. Survey Open File Report.,32 p. Besana, G. M., R. S. Punongbayan, J. A. Daligdig, J. V. Umbal, and B. C. Bautista. 1990. Preliminary analysis of the 16 July 1990 earthquake aftershock distribution in relation to ground rupture. 1990. Proceedings of the 3rd Annual Geological Convention, December 5-7. Crone, A. J., M. N. Machete, M. G. Bonilla, J. J. Lienkaemper, K. L. Pierce, W. E. Scott, and Robert C. Bucknam. 1987. Surface Faulting Accompanying the Borah Peak Earthquake and Segmentation of the Lost River Fault, Central Idaho, Bulletin of the Seismological Society of America. 77: 739-770. Das, S. and K. Aki .1977. Fault planes with barriers: A versatile model, Journal of Geophysical Research. 82: 5648-5670. Nakata, T., H. Tsutsumi, R. S. Punongbayan, R. E. Rimando, J. A. Daligdig, G. M. Besana and A. S. Daag.1990. Surface faulting associated with the Philippine earthquake of 1990. J. & eogr., 99 (5): 95-112. Punongbayan, R. S., and J. V. Umbal. 1990. Overview and impacts of the July 16, 1990 earthquake. Proceedings of the 3rd Annual Geological Convention, December 5-7, 1990. Southeast Asia Association of Seismology and Earthquake Engineering, 1985. Edited by E.P. Arnold., Series on Seismology, Vol. IV: Philippines, Washington D.C.: Government Printing Office. Sibson, Richard S. 1986. Rupture interaction with fault jogs. In Earthquake source mechanics, edited by Das, S., J. Boatwright, and C. H. Scholz. American Geophysical Union Monograph 37, p.157-167. _________.1987. Effects of fault heterogeneity on rupture propagation. In The behaviour of seismogenic faults, Edited by A. J. Crone, and Omdahl, E. M. Directions in Paleoseismology Conference, XXXIXth Proceedings, p.362373. Ziony, J. I. and R. F. Yerkes. 1985. Evaluating Earthquake and Surface Faulting Potential Evaluating Earthquake Hazards in the Los Angeles Region - An Earth Science Perspective (U.S. Geological Survey Professional Paper 1360). Washington United States Government Printing Office. pp. 43-89. Inventory and Characterization of Landslides induced by the 16 July 1990 Luzon Earthquake Mapping of Areas Affected by Liquefaction during the 16 July 1990 Earthquake The 16 July 1990 Luzon Earthquake and its Aftershock Activity

Soil Study of Area Damage due to Liquefaction during the 16 July 1990 Philippine Earthquake Vital Engineering Lessons from the Earthquake of July 16, 1990

Quantifying Spatial and Temporal Dimensions of Premonitory Animal Behavior of the July 16, 1990 Luzon Earthquake

Households and Communities in a Post-Earthquake Situation: Lessons on Survival and Self-Reliance Organizational Response to the July 1990 Luzon Earthquake Disaster Psychosocial Issues in Disasters

Management Strategies for Earthquake-Related Psychosocial Problems Community-Based Interventions

Some Implications of the July 16, 1990 Earthquake on Urban and Regional Planning in the Philippines

Panay Earthquake (1990 Ms. 7.1)

1990 June 14 Ms7.1 Panay Earthquake

On 14 June 1990, an earthquake measuring 7.1 in the Richter Scale hit Panay Island at 3:41 P.M., killing 8 and injuring 41 people. The epicenter was located at 11.34°North latitude; 122.10° East longitude, in the vicinity of Culasi, Antique. The depth was computed to be 15 kilometers. It was generated by fault movement in the collisional zone off western Panay Island. The tremor was felt in the following places (in PEIScale): Intensity VII Culasi, Antique; Libacao, Aklan Intensity VI Balete, Kalibo, Madalag, Numancia, Altavas & Makato in Aklan; Sigma & Cuartero in Capiz, and; Calinog in Iloilo. Intensity V Northern part of Negros Island Intensity IV Romblon and the rest of Negros Island Intensity III Cebu City; Taal; Mayon; Palo, Leyte Intensity II Cagayan de Oro & Camiguin Island A quick response team dispatched to the area reported the following observations: Culasi, Antique  Seven persons perished and 31 others suffered mild to severe injuries.  About 15% of the residential houses collapsed, the rest were partially damaged.  Several commercial buildings, namely: San Miguel Beer and Coca-cola warehouse; half portion of the Rural Bank of Culasi building; the Esperanza Elementary School, and the Seventh-Day Adventist church collapsed.  Four bridges totally collapsed.  Fissures measuring 82.5 x 0.8 x 0.91meters, and 4 x 0.8 x 0.9meters were noted in two barangays.  Upliftment occurred in Barangay Bagacay of 0.6 meters with an approximate area of 3,000 square meters.  Landslides were noted along the slope of Mt. Madya-as. The volume of materials carried by the landslide was approximately 30,000 cubic meters in Bagacay.



Fifty-seven families (about 342 persons) were evacuated.

Libacao, Aklan  Five concrete residential buildings were totally damaged, while thirty structures were partially damaged.  Two churches and a river control project were heavily damaged.  Five highway bridges were partially damaged. Balete,       

Aklan The Baptist church and the public market were heavily damaged, while an icon was toppled down. The Rural Health Center and a rice mill collapsed. The Balete district hospital was badly damaged and was declared dangerous for future use. Partial damage to another public market and on the approach of some bridges. One residential house totally collapsed and ten others were partially damaged. Thirty-five people were evacuated to the Catholic Church. A fissure measuring 2 km long and 136 cm wide, trending N50W was noted along Jaro River.

Madalag, Aklan  The municipal and district hospital sustained some cracks. Kalibo,   

Aklan Aklan Science High School and Alan Cinema were partially damaged. The Catholic Church of Kalibo that is made of bricks suffered cracks on its walls. A house made of ceramics was partially damaged.

Numancia, Aklan  Sandboil was observed.

Altavas, Aklan  The wharf was partially damaged.  There were cracks on the walls of the Cathedral and the head of an icon was damaged. Makato, Aklan  The sports complex was partially damaged.  The posts and beams of the public market were damaged.

Kalinog, Iloilo  Various buildings of the Philippine Constabulary Regional Command were damaged.  The Catholic Church was partially damaged. Cuartero, Capiz  A church and several houses were partially damaged.

Sigma, Capiz  A bridge and a communication tower were partially damaged.

Estimated total amount of damage is about 30 million pesos. References: 1. PHIVOLCS Observer. . . . . .July 1990 ISSN 01-16-07-45 2. PHIVOLCS Observer. . . . . .January-March 1990

Acknowledgement: The Quick Response Team (QRT) Members: Jimmy Sincioco - Team Leader; Ariel Rasdas; Rod Medrano; Jerry Diolata; Dindo Javier; Nelson Mondia Pictures used were taken by Mr. Dindo Javier, and through the kindness of Ms. Louie de Guzman of the TID. Pictures

Focal Mechanism

Strk1 225

Dip1 75 Rake1 -172 Strk2 133 Dip2 83 Rake2 -15

P-axis Plunge 16 Azimuth 88 T-axis Plunge 5 Azimuth 179

Isoseismal map

Intensity VII Intensity VI and; Calinog in Intensity V Intensity IV Intensity III Intensity II

Culasi, Antique; Libacao, Aklan Balete, Kalibo, Madalag, Numancia, Altavas & Makato in Aklan; Sigma & Cuartero in Capiz, Iloilo. Northern part of Negros Island Romblon and the rest of Negros Island Cebu City; Taal; Mayon; Palo, Leyte Cagayan de Oro & Camiguin Island

Laoag Earthquake (1983 Ms 6.5)

1983 August 17 Ms6.5 Laoag Earthquake

At 8:18 P.M. of 17 August 1983, an earthquake with a magnitude of 5.3 (Ml) on the Richter Scale and an intensity of VII on the Rossi-Forel Scale hit the province of Ilocos Norte. The tremor was perceptible over a distance of 400 kilometers from the epicenter. This was the most sever earthquake in North-western Luzon in

52 years and probably the second largest earthquake event to hit Laoag city and it's immediate vicinity in historical times. This earthquake has caused death of 16 people and injuries of forty seven persons (PDE). Date of Event August 17, 1983 Origin Time 8:17 pm (12:17 GMT) Epicenter 18.231 N Latitude 120.860 E Longitude or approximately 30 aerial kilometers east-northeast of Laoag City. Magnitude 6.5 Ms ( 5.3 Ml on the Richter Scale) Depth approximately 42 km from the surface. Intensity Report: Intensity VII Laoag City, Pasuquin- Ilocos Norte Intensity VI Vigan-Ilocos Norte Intensity V Aparri-Cagayan, Santa-locos Sur Intensity IV Tuguegarao-Cagayan, Baguio City Intensity III Dagupan City, Callao-Cagayan, Manila Note: Intensity scale used in these observation was the Rossi-Forel Earthquake Intensity Scale.

Historical Background: Since 1862 upto 1981, (excepting the years 1941 to 1949) fifty-six earthquakes have affected Laoag City. Of these, the strongest was recorded on 19 March, 1931. This earthquake reportedly had an intensity of VII - IX. Prior to the 17 August earthquake, two tremors were recorded on the eleventh and the thirteenth of August 1983. These were believed to be foreshocks of the intensity VII earthquake (Macalincag, T. G., personal communication). The first had an intensity of V and the succeeding one an intensity of II in the Rossi-Forrel Scale. Summary of Damages: Damages on buildings:

A number of reinforced concrete buildings either totally crumbled or sustained major structural damage beyond rehabilitation. The failure in most of the damaged buildings can be attributed to shear and compressional waves, thereby producing horizontal and vertical stresses. The most heavily damaged structures in Laoag City are those situated near the Laoag River flood plain and along reclaimed stream channels. These buildings were condemned by the City Engineer's Office. Nearly all the damaged buildings in the area were of reinforced concrete frame. Most of the external walls and internal partitions were of concrete hollow blocks. There are however, some buildings with wood partitions. List of buildings that totally collapsed, suffered severe structural damage, sustained considerable damage or considerable non-structural damage: Laoag City

The Laoag Cathedral

Sinking Bell Tower Yabes Juan Department Store Building Denson Building Golden Hardware Building Laeno Building Avenue Lumber and Hardware Philippine Veterans Bank Building Teresita Building

Philippine National Bank Building Shirley Building Far East Building Sunrise Building Isabel Building SE Asia building Julian Building Pichay's Building Castro Building Mique Residence Gerardo Building E. Ang Residence San Nicolas Kaunlaran Building San Nicolas Church and Bell Tower Bacarra Bacarra Sarrat

Church and Bell Tower

Sta. Monica Church and Bell Tower

Sarrat Municipal Hall Marcos Guesthouse Vintar Vintar Church

San Nicolas Academy Vintar Municipal Hall Burgos Bojeador Lighthouse Minor Effects:

Landslides: Several earthquake induced landslides were observed in places where the slopes along road cuts were steep to very steep. This condition had been aggravated by prolonged rainy days, absence of vegetation to hold the soil, moderately weathered and indurated rocks. Areas affected by landslides were the Sarong Valley in Vintar and Patapat Mountains in Pagudpud, both in Ilocos Norte.

Sandboils or Sandblows: Several sandboils were reportedly observed in Barangay Zamboanga, Laoag City; Barangay Puyupuyan, Pasuquin; and Barangay Calayab, Paoay. The diameter of their craters vary from a few centimeters to 2.5 meters. Sandblows or sandboils are the spouting of hydrated sand caused by moderate to severe earthquakes. This connate water that has been entrapped in the interstices of sediments at the time of deposition may have come from either South China Sea or Laoag River. Differential Settlement: Majority of the bridges in Ilocos Norte had experienced differtial settlement of approach and or abutments. Some of the buildings were also observed to have differential settlement in addition to being out of plumb. Step fractures due to collapse of foundation were observed at Marcos Guesthouse in Sarrat. Magnitude of differential settlement measured range from a few centimeters to approximately 30 centimeters.

Shear Fractures: A tilted road pavement along J. P. Rizal Street, Laoag City was observed after the main tremor. Gaping tension fractures along Vintar-Bacarra Road and along asphalt pavement on the southern approach of Bacarra Bridge were also observed. Gaping Step tension fracture along Vintar Poblacion-Tamdagan road was found. Numerous irregular cracks and small fissures were discovered along seashores, river banks and alluvial fans. References: Santiago, N.G and Rillon, E.A ( December 1983): Assessment on the effects of the August 17, 1983 Earthquake in Laoag City: Bureau of Mines and Geo-sciences.

Valenzuela, R.G. and Garcia, L.C. (10 October 1983) Laoag Earthquake of 17 August 1983 SummaryReport: PAGASA. Focal Mechanism

Strk1 149 Dip1 35 Rake1 -146 Strk2 30 Dip 2 71 Rake2 -60

P-axis Plunge 54 Azimuth 337 T-axis Plunge 21 Azimuth 98 Isoseismal Map

Intensity VII Laoag City, Pasuquin- Ilocos Norte Intensity VI Vigan-Ilocos Norte Intensity V Aparri-Cagayan, Santa-locos Sur Intensity IV Tuguegarao-Cagayan, Baguio City Intensity III Dagupan City, Callao-Cagayan, Manila

Casiguran Earthquake (1968 Ms 6.3)

1968 August 02 Ms7.3 Casiguran Earthquake

At 4:19 AM (local time) on August 02, 1968 an earthquake with an intensity of VIII in the Rossi-Forel Intensity Scale rocked the town of Casiguran, Aurora. This was considered the most severe and destructive earthquake experienced in the Philippines during the last 20 years. Two hundred seventy (270) persons were killed and 261 were injured as a result of the earthquake. A six-storey building in Binondo, (Ruby Tower) Manila collapsed instantly during the quake while several major buildings near Binondo and Escolta area in Manila sustained varying levels of structural damages. The cost of property damage was several million dollars. Extensive landslides and large fissures were observed in the mountainous part of the epicentral area. Tsunami was also observed and recorded as far as observation in tide gauge station in Japan. Date of Event Origin Time Epicenter Magnitude Depth

August 02, 1968 4:19 am (20:19 GMT) 16.3 N Latitude 122.11 E Longitude or approximately Ms: 7.3 Mb: 5.9 (ISC) approximately 31 km from the surface.

Intensity Report: Intensity VIII Casiguran, Quezon Intensity VII Manila and Palanan Intensity VI Baler, Quezon City, Tuguegarao, Aparri, Baguio, Dagupan, Iba, Cabanatuan, Alabat, Intensity V Tarlac, Ambulong, Infanta, Jomalig Intensity IV Legaspi, Lucena, Calapan, Aurora, Laoag, Catarman, Virac Intensity III Romblon, Vigan

Note: Intensity scale used in these observations was the Adapted Rossi-Forel Earthquake Intensity Scale of IIX. Casualties: Place

No. of persons killed

Plunge (degrees)

Manila (Sta. Ana Tenement House)

-

1

Manila (Ruby Tower)

268

260

Aurora Sub Province

1

-

Guagua, Pampanga

1

-

Total

270

261

Summary of Damages: Damage to Particular Buildings in Manila

The severe damage area was concentrated in a relatively small part of Greater Manila. This part of Manila lies in the mouth of Pasig River (a major river system in Metro Manila) and includes the deepest and most recent alluvial deposits in the city. Ruby Tower The Ruby Tower was a large six-storey building containing 38 commercial units in its first two floors and 76 residential units in its upper four floors. Most of the building collapsed except for a part of the northern end of first and second floors (Photo 1 & 2), killing 268 persons and injuring 260 of the occupants. The upper floors fell southwards while the southern end of the roof moved about 30 feet or 9.15 meters south and 10 feet or3.05 meters east (Photo 3). The lower floors appeared to fall close to their plan position (Photo 4). Philippine Bar Association Building The Philippine bar Association (PBA) was a medium rise six-storey commercial building with offices and club rooms (Photo 5). Most of the first storey columns suffered total collapsed or very severe damage and shortening (Photo 6, 7, 8 & 9). The intensity of damage increased towards the southeast end of the building. Exterior columns suffered greater damage and shortening the nearby interior columns. This resulted in very a severe deformation of some interior beams and part of the floor at each storey level. Aloha Theater The Aloha Theater is a large eight-storey building that suffered severe damage near its southern end. The damage was initiated by the collapsed of a few very short columns towards the southern end of the fourth floor. It spread out and severely damaging the southern end of the building (Photo 10 & 11). Tuason Building Tuason Building is a medium sized six-storey building which suffered severe damage or very close to collapse (Photo 12 & 13). The columns along the southern side wrecked or severely damage (Photo 14). Other buildings that suffered considerable structural and nNon-structural damage (Manila) Trinity Building (Photo 15 & 16) Diamond Tower (Photo 17 & 18) Liwayway Hotel (Photo 19 & 20) National Library (Photo 21) Old Philippine National Bank Boie Building Araneta and Tuason Building (Photo 22) Development Bank of the Philippines Phoenix Building La Tondeña Building New Philippine National Bank (Photo 23) Overseas Terminal (Photo 24) Far Eastern University Arts and Sciences Building

Landslides Landslides occurred in several places on the steep slopes of surrounding mountains near the epicentral area. Landslides produced by the main shock were mostly on the slopes of mountains north of the town of Casiguran, while those that accompanied the big aftershocks were observed on mountains both to the north

and to the west. The largest landslide took place on the cliff at Dinajawan Point facing Casiguran Bay (Photo 25) while another landslide was observed in Manglad River, a tributary of Cagayan River (Photo 26). Manglad River traverses behind a cornfield and beside this, the transported unconsolidated sediments produced a small hill (Photo 27). Ground Ruptures In the epicentral area, around the town of Casiguran, cracks that were parallel to the nearest rivers were observed. Surface soil in this part is mostly loose deltaic sand. The length of the fissures varies from 10 to 20 meters but in some areas, it reached a length of 400 to 500 meters. The space between the cracks varies from 5 to 20 meters Fissures on the road from Casiguran to Barrio Tabas produced a 0.5 meters crack and the surface subsidence varied to as much as two meters (Photo 28). This road is approximately 8 meters from the Casiguran River at the top of a steep bank approximately 2.5 meters high. Photo 29, shows another fissure on a logging road, 30 meters away from and parallel to river bank in Casiguran area. References: Osome, S., Osawa, Y., Skinner, I., Yoshima, and Y., PHILIPPINES: Luzon Earthquake of August 02, 1968, UNESCO, Serial No. 977/BMS. RD/SCE. NR, Paris, January 1969. Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE). Series in Seismology, Volume IV (Philippines), 1985 Focal Mechanism:

Pole of 1st Nodal Plane

Pole of 2nd Nodal Plane

Axis of Compression (P) Axis of Tension (T) Null Axis (B)

Trend(degrees)

Plunge (degrees)

261.5

15.14

137.5 98.13

234.92 357.27

64.18 26.86 55.2 20.4

Intensity VIII Casiguran, Aurora Intensity VII Manila and Palanan-Isabela Intensity VI Baler, Quezon City, Tuguegarao and Cagayan, Aparri, Baguio City, Dagupan City, IbaZambales, Cabanatuan City, Alabat-Quezon Intensity V Tarlac, Ambulong, Infanta-Quezon, Jomalig Intensity IV Legaspi City, Lucena City, Calapan-Oriental Mindoro, Aurora, Laoag City, Catarman Samar, Virac Catanduanes Intensity III Romblon, Vigan Moro Gulf Earthquake (1976 Ms 7.9)

1976 August 17 Ms7.9 Moro Gulf Earthquake

A few minutes after the last stroke of midnight on August 17, 1976, a violent earthquake occurred in the island of Mindanao spawning a tsunami that devastated more than 700 kms of coastline bordering Moro Gulf in the North Celebes Sea. This offshore event generated by Cotabato trench, a less prominent trench system in the Philippines, was the largest tsunamigenic earthquake to have occurred in Mindanao in the last two decades. It was an earthquake that resulted in massive destruction of properties and great loss of lives. The tsunami generated contributed immensely to the devastation. The cities and provinces of Cotabato took the brunt of the earthquake while the tsunami generated cast its doom on the provinces bordering Moro Gulf especially on the shores of Pagadian City. According to surveys during the event, the tsunami was responsible for 85% of deaths, 65% of injuries and 95% of those missing. After the sea spent its fury and rolled back to its natural flow, thousands of people were left dead, others homeless or missing and millions of pesos lost with the damages of properties. Properties lost not only include establishments for residential and commercial use, but also bancas that, as a whole, represents the livelihood of hundreds of families.

Date of Event 17 August 1976 Time 12:11 A.M. (Local) Epicenter 06.3° N, 124.0° E Magnitude 7.9

FORESHOCKS Analysis of seismic records for August 1976 prior to August 17 of the same year showed that there were six events recorded that had epicenters in the same area as the main shock and could be considered as foreshocks of the Moro Gulf earthquake. Also, about a month before that, two quakes were reportedly felt in Zamboanga City that also had epicenters near the area of the main shock. These two events were not recorded at the PAGASA Observatory in Quezon City nor in any of its field stations. This brings to eight the total number of foreshocks, three of which are felt events with intensities ranging from I to IV. (Stratta et.al., 1977) AFTERSHOCKS There were approximately forty (40) aftershocks that were plotted using available data from the seismic network of PAGASA. But it was reported that more aftershocks were felt and recorded locally most of which were felt in the area with Rossi-Forel intensities of up to Intensity VI. Aftershocks in Cotabato City were monitored by the Commission on Volcanology (now Philippine Institute of Volcanology and Seismology) and during the span of time that the aftershocks were monitored, an average of about 140 aftershocks per day were recorded. Monitoring started on the 18th of August 1976 using a three component Hosaka seismograph and a single-component Kinemetrics seismometer. COTABATO CITY DAMAGE TO BUILDINGS SCHOOLS

COTABATO CHINESE SCHOOL GYMNASIUM A reinforced concrete and wood structure built in 1962. Its walls fell outward during the earthquake and the roof fell in.

ADMINISTRATION BUILDING This is a two-story building (designed for three stories) with reinforced concrete frame built in late 1973. The building suffered little damage on some of its walls. A pile foundation had been used.

HARVADIAN COLLEGE The campus includes several buildings but only a partially collapsed five-story structure was investigated. The building was built in 1962. It had reinforced concrete frame with reinforced concrete slabs at the second level and at the exterior walkways at the third, fourth and fifth levels. The fifth story was constructed completely of wood and the roof had GI sheeting. The building was reportedly designed for three stories with the fourth and fifth floor added later with no strengthening of the lower stories. NOTRE DAME UNIVERSITY This University is located on Notre Dame Avenue approximately 1.5 km southeast of the downtown area. The site has wet and soft marshy ground. Ground water appeared to be very near the surface as ponds were evident throughout the site.

THE ADMINISTRATION BUILDING A rectangular three-story structure with reinforced concrete columns and girders and concrete floors built in 1960s. Damage to the building was light. There was a 3 m concrete panel at the entrance that was heavily cracked and damage to the frame was minor.

THE AUDITORIUM AND SCIENCE BUILDING This is a 48 x 30 m auditorium crossed at its entrance by a three-story 51 x 12 m science wing. It had reinforced concrete frame with masonry infills built in 1969. The roofs of the auditorium and science wing had the same elevation. After the earthquake, a fire broke out in the science wing. Its first and second story columns sustained heavy damage. The long span beams also have heavy cracks. After the fire had burned for several hours, the science wing collapsed. The auditorium suffered heavy fire damage. The roof trusses in the stage area sagged heavily due to the intense heat. Large areas of the trusses dropped simultaneously. The infilled walls did not suffer structural damage but the entrance of the auditorium was destroyed when the science wing collapsed. THE NEW RESIDENCE HALL This is a rectangular, three-story structure nearing completion and unoccupied at the time of the earthquake. It consists of concrete exterior columns, thin concrete exterior walls, timber interior columns and floor systems, and plywood interior partitions. Damage to the building was light. There were cracks at the floor line, some columns were damaged at the sill line, interior partitions were torn apart, some ceiling panels fell and considerable cracks of the ground floor slab. THE TECHNICAL SCHOOL A two-story building built in 1965. It had a concrete frame with a concrete two-way slab floor. This building was linked to an adjacent building by a common wood canopy. Damage to the structure was moderate. The first story columns and fins were damaged at the head and sill levels and the canopy collapsed at its end bay. HOTELS and RESTAURANTS

DAWNS HOTEL A six-story reinforced concrete frame and wall building. There was no damage except for a little working on the floor joints of the south wall of the building.

D’MAX RESTAURANT A two-story building constructed in 1968. It was a combination of reinforced concrete and wood. The building collapsed completely. (Go to Amicus Building for additional information.) IMPERIAL HOTEL #1 Imperial Hotel #1, Imperial Hotel #2 and Rita Theatre are situated close together. Imperial Hotel #1 and Rita Theatre drifted to the west and pushed against Imperial Hotel #2. Imperial Hotel #1 is a four-story reinforced concrete building with masonry infills built in 1963. The building experienced a 38 cm permanent offset in the first story and the rear portion of the building collapsed.

IMPERIAL HOTEL #2 This is a six-story building with reinforced concrete frame built in 1967. There was superficial damage to the building that consisted of cracks in a column, its infill panels and part of the slab grade caused by the impact force from the fronts of Rita Theater and Imperial Hotel #1. The impact also caused shear failure of the second story column. MELBOURNE HOTEL A three-story building with reinforced concrete frame with masonry infilled panels built in 1970. The first story suffered a permanent offset to the south after the quake and its columns sustained heavy damage. On

the east side of the building, the panels were pushed out and window infills in its mezzanine floor buckled outward.

NEW SOCIETY HOTEL A four-story reinforced concrete frame building with shear walls and reinforced concrete slabs for its floors and roof. It was built in 1968. It is located within 30 m of Rio Grande and its elevation is 2 m below street level. The columns were founded on woodpiles with reinforced concrete pile caps. The pile cap and water table were nearly coincidental. Proximity of the river and the high water table would suggest a very strong ground shaking but there had been factors that ruled out this possibility and instead, flaws in the structure were considered as the principal causes of failure. The building collapsed as the building twisted in a counterclockwise motion; the northwest corner of the second floor dropped down to the street; and the opposite southeast corner suffered torsional failure of the corner pilaster and out-of-plane shearing of the adjacent walls. The frame and walls above the first story was practically undamaged.

SAGUITTARIUS HOTEL A four-story structure with reinforced concrete frame built in 1965. The building collapsed completely. (Go to Amicus Building for additional information.) SULTAN HOTEL A five-story building that suffered collapse of the first floor. The collapse of the building must have been slow because the portion above the second floor remained intact. THEATERS

COTABATO CINEMA This is a large structure to the rear of Sultan Hotel. When the hotel collapsed, it caused severe structural damage to the theater complex. It was hard to determine whether the collapse of the hotel caused failure to the theater or merely contributed to an already damaged structure. FRANCEL THEATER A reinforced concrete and wood building built in 1966. The reinforced concrete portion of the building collapsed causing failure of the wood trusses of the roof.

RITA THEATRE This was a two-story 12 m tall reinforced concrete frame building in front. Its rear portion was a combined reinforced concrete frame with masonry infills and wood and serves as the auditorium. The auditorium roof has two elevations. The front frame drifted to the west along with Imperial Hotel #1. The auditorium frame and its infilled east wall were knocked over by Imperial Hotel #1 and the roof on this part of the structure collapsed. Further to the rear, the roof did not collapsed because the roof elevation was lower. (Go to Imperial Hotel #1 for additional information.) CHURCHES

IMMACULATE CONCEPTION CHURCH This church was located across the street from Tison Building. Its grounds were very soft and the church was obviously not built on piles. The church tower settled by about 15 cm.

TAMONTAKA CATHOLIC CHURCH A structure made of unreinforced brick walls with interior timber columns and wooden roof said to have been built by the Spaniards around 1872. It was built on soft marshy soil. Before the earthquake, the building already had some structural cracks that could be either due to a previous earthquake or a differential settlement. The church suffered severe damage. OTHERS

AMICUS BUILDING The Amicus Building, Sagittarius Hotel and D'Max Restaurant formed a complex of three adjacent buildigs that collapsed. BOSTON BAKERY A two-story reinforced concrete building built in 1965. The first story of the build drifted about 60 cm to the west.

COTABATO AUTO SUPPLY A three-story building built in 1968 with reinforced concrete frame, concrete floor and masonry infilled exterior walls. Its partitions were made of timber and plywood. The first two floors were used for auto parts sales and storage and the third floor served as the owner's living quarters. The first story collapsed gradually, according to the proprietor, with the upper stories coming to rest approximately 3 m west of its original location. The rest of the building only sustained minor cracks. Storage shelves in the second floor were still standing after the quake. A one-story concrete lean-to behind this building also collapsed.

COTABATO FIRE and POLICE STATION The building settled out of plumb toward the river during the earthquake. Otherwise, there was no structural failure noted on the building itself. FIRST GIFT AND BOOKSTORE A four-story building built around 1968 to 69 with reinforced concrete frame resting on a timber pile foundation. This was also known as the Yap building after its owner. The first floor of this building collapsed during the initial earthquake tremor and fire broke out within the structure. Five to six hours later, the structure collapsed completely. It was noted that this structure leaned into an adjacent three-story building knocking it into a third building, the City Evangelical Church. Damage to the church was light. LCT HARDWARE AND AUTO SUPPLY A two-story reinforced concrete structure with wood trusses and GI sheet roof. The first story collapsed toward the west during the earthquake. MELINEEN BUILDING A two-story reinforced concrete building that pancaked.

SOUTH SEAS TRADING This was a three-story building built in 1967. It had a concrete frame and floor slab. This building completely collapsed. TAN BO BUILDING A four-story building constructed around 1971. Its frame was of reinforced concrete while the walls are infilled hollow blocks. The whole structure was built on timber piles. The only damage noted was the cracks on the walls near the stairs.

TISON BUILDING This was the only building in Cotabato City known to have been designed with seismic considerations. It was built on precast concrete friction piles on good soil. It survived the earthquake with only a slight crack in a concrete block partition.

WATERFRONT WAREHOUSES A large number of warehouses were located at the edge of Rio Grande west of Manday River. They look like they were made of masonry walls, timber trusses, and corrugated GI sheets. They were poorly built. All of the warehouses collapsed. DAMAGE TO BRIDGES

QUIRINO BRIDGE This is a four-span structural steel bridge over the Rio Grande. Each span is 40 m long. The second span from the south end collapsed into the river during the earthquake. The third span from the south end nearly collapsed and cracks appeared several centimeters below the base of the south abutment.

TAMONTAKA BRIDGE This bridge spans about 230 m across Tamontaka River approximately 6 kms south-southwest of Cotabato City. The bridge is made up of six spans resting on pile-supported piers. The girders, piers and piles are made of reinforced concrete. The bridge was constructed in three sections. After the earthquake, the center section moved east and west in excess of 38 cm each way evidenced by the broken concrete keepers on each end of the supporting piers. The northern section moved even greater distances. The southern section moved but with lesser distance. There was damage to the railings at the abutments and the expansion joints.

ZAMBOANGA CITY Fourteen buildings in this City of Flowers were partially damaged while twenty-six buildings sustained minor damage. The City Hall bore noticeable cracks along its façade. Ateneo de Zamboanga sustained failures at the sill level of its columns on the fourth floor due probably to poor concreting and column weakening because of water seepage from the GI downpipes embedded in the columns. Zamboanga Agricultural and Engineering College sustained damage to columns due to failure at end moments. Zamboanga City was spared from the onslaught of the tsunami on account of the strategic geographic location of Basilan and Santa Cruz Islands that served as buffers and deflected the waves that otherwise could have inflicted heavy damage along Zamboanga City's coastline. Damage in buildings consisted mostly of cracks on its masonry walls and insufficient lateral ties in some columns. PAGADIAN CITY The coastal districts of Santa Lucia, Santiago, San Pablo, San Roque and White Beach Barangay were hardest hit by the tsunami. Almost all of the houses along the coast within 500 meters inland were destroyed. Some

houses made of reinforced concrete hollow blocks were able to withstand the force of the waves and also served as protection to other house made of light materials. The approach to the Pagadian City wharf settled down, causing cracks in the slabs of the approach area and in the concrete deck. The five-story reinforced concrete building of Saint Columban College had noticeable cracks in the masonry infilled walls. Shear cracks in two columns were observed at the junction.

DAMAGE DUE TO TSUNAMI Just after the earthquake stopped, the sea, stirred by the powerful movement of the earthquake, swelled and moved away from the coastline for about three kilometers. About ten minutes later, it roared back to the shore and beyond in three succeeding waves soaring as high as the treetops according to some reports. The sea unloaded its fury on everything near the shore. Houses and properties along the coastal beaches of Lanao del Sur and Pagadian were practically washed out. Bits of houses littered the sea and bodies littered the shore. The casualties and victims of the earthquake and tsunami numbered thousands just in Regions 9 and 12. (Region 9 covers Pagadian City, Zamboanga del Sur, Zamboanga City, Basilan, and Sulu while Region 12 covers the areas of Sultan Kudarat, Maguindanao, Cotabato City, Lanao del Sur and Lanao del Norte.) A tabulation of the victims and casualties in these regions is as follows. Area

Dead

Missing

Injured

Homeless*

Region 12

3,351

1,379

2,227

43,534

Region 9

1,440

909

Source: Badillo, V.L. and Astilla, Z.C.: Moro Gulf Tsunami of August 17 1976 *Some of the data in this section was estimated at 6 members per family

7,701

49,848

The major cause of the great number of casualties during the event could be attributed to the fact that (1) the tremor happened just after midnight when most people were sleeping; (2) a great tsunami was spawned, struck the coasts from different directions and caught the people unaware. REFERENCES: Stratta, James L., et. al.; 1977, EERI Reconnaissance Report Mindanao, Philippines Earthquake August 17, 1976, 106 pp. Badillo, Victor L. and Astilla, Zinnia C.; 1978, Moro Gulf Tsunami of 17 August 1976, 41 pp.

Stewart, Gordon S. and Cohn, Stephen N.; 1978, The 1976 August 16, Mindanao, Philippine

Earthquake (Ms = 7.8) -- evidence for a subduction zone south of Mindanao, 14 pp. Acharya, H.K., 1978, Mindanao Earthquake of August 16, 1976: Preliminary Seismological Assessment : Bulletin of the Seismological Society of America Vol. 68, 1459-1468.

Southeast Asia Association of Seismology and Earthquake Engineering, 1985, Series on Seismology Volume IV Philippines, 489-515.

Intensity VII Intensity VI Intensity V Intensity IV Intensity II

Cotabato City; Jolo-Sulu; Zamboanga City Basilan City; Pagadian City; Dipolog City; Malaybalay-Bukidnon Cagayan de Oro City; Davao City; General Santos City Dumaguete City; Hinatuan Surigao del Sur; Tagbilaran-Bohol; Cebu City; SurigaoSurigao del Norte Roxas City; Iloilo City; Tacloban City; Legaspi City; Palo-Leyte; Catbalogan-Samar

Summary of PAGASA field reports on main shock ground motion Ragay Gulf Earthquake (1973 Ms 7.0)

1973 March 17 Ms7.0 Ragay Gulf Earthquake Date of Event: March 17, 1973 Epicenter: 13.41N ; 122.87E Ms = 7.0 Focal Mechanism: Strike Slip

Summary of Damages: Buildings and Other Civil Structures The town worst hit by the earthquake is Calauag, Quezon where 98 houses were totally destroyed and 270 others were partially destroyed. In barrio Sumulong of the same town, 70% of the school buildings were damaged. Most of the partially to completely destroyed houses and buildings were situated along the seashore in the northern section of the town proper. The damaged houses were largely wooden and some were poorly built concrete buildings. The town of Lopez ranks next to Calauag with respect to the extent of destruction. The place is relatively farther from the causative fault and the epicenter of the mainshock, but soft underlying sediments present in Calauag are similarly found in Lopez.

The concrete hollow block retaining walls of a 5-room PTA building of Lopez Provincial School collapsed on both sides of the building along the N-S direction. A residential 3-story concrete building was severely tilted to the north.

The facade of the Sto. Rosario Catholic Church of Lopez suffered cracks and some parts of the CHB walls on both sides toppled down.

The 1 km. long concrete seawall along the ESE coast of Calauag suffered minor cracks mostly along construction joints. About its mid-section in one of its stairways there was a 10 cms. crack. One section was displaced 5 cms to the north from the other section.

In Barrio Hondagua, Lopez 5 km east of Calauag, some buildings were totally or partially damaged. The CHB wall of one of the classrooms of Hondagua Elementary School toppled down.

The Hondagua Theater which had been converted into a restaurant completely collapsed and the Catholic chapel of the Barrio was partially destroyed. The concrete columns of the housings of the coveyor machines of the Philippine Flour Mills in Hondagua buckled down. There was differential settlement of the ground along fills in the pier such that floorings of some of the buildings became uneven and were cracked. Transportation / Communication Lines and Underground Pipes The earthquake wrought damages to roads, railroads and bridges. This hampered travel to and from Bicol Region. At least four highway bridges on the Manila South Road were reported to have suffered damages ranging from a partial to total collapse. The bridge which totally collapsed was the Sumulong highway bridge in Sumulong, Calauag. A PNR bridge crossing the Calauag River and situated about 600 meters north of the highway bridge was badly damaged although it did not collapse. The rails along the bridge were badly twisted.

A slight movement was detected at the PNR bridge in Morato Tagkawayan. Its ties were observed to have moved to 8 cms. to the east, and base plate of its western abutment was moved 5 cms. to south.

Damages to national and municipal roads were limited to cracking of the concrete slabs along the Manila South Road. Subsidence occurred along the Sumulong-Guinayangan road. Minor cracks were observed along the national highway from Km 217, up to Km 234 in Calauag. Betweeen the town of Lopez and Calauag the rails of the PNR were reported to have been badly twisted. The major twisting of the raiways however occurred some 300 meters from the southwestern approach of the PNR trestle bridge in Sumulong. This provided a remarkable manifestation of the lateral movement of the ground.

Electric systems, waterwoks systems and telegraph systems in the town of Lopez, Calauag and Guinayangan were severely disrupted.. In Calauag, water main pipes were either fracured or severed. Electric and telegraph lines snapped due to appreciable horizontal movements of the ground. Fires which broke out during the earthquake were immediatel controlled by alert local firefighters. In the town of Lopez, a concrete electric post broke on its base and toppled down, pinning 5 youngsters who embraced the post to prevent themselves from being thrown off their feet due to the strong shaking of the ground. Disruption to water system was minimal. In Barrio Hondagua, Lopez a 3’4’’ water pipe was subjected to tensional forces which resulted in the breaking and seperation of the pipes.

Agriculture The agricultural industry in the epicentral area is based mainly on coconut. The effect of earthquake on this industry was not felt immediately, but after a few months had elapsed. Coconut production registered low because of the shaking down of young nuts during the earthquake.

Furthermore, the roots of some trees situated within a distance of about 1 km. on both sides of the fault traces were cut off as a consequence of the horizontal ground movement, thus affecting their fruit bearing capability. Concrete fishpond dikes in the epicentral region were cracked although to a minor extent. Mud dikes which were loosely made toppled down while others were fissured.

Geologic Features and Effects Features and Effects Related to Faulting The most interesting feature in this earthquake was the remarkable extent of faulting. The farthest observable fault trace from the epicenter is 90 kms. away in the coastal barrio of Sumulong, Calauag.

Ground breakages were seen along the segment of the Philippine Fault, from the western coast of Ragay Gulf to Calauag Bay, a stretch of about 30 kms. The fault traces exhibited moletrack features with ground fissures arranged in enchelon to one another in an E-W trend. From Barrio Cibong towards barrio Sintones in the town of Guinayangan, some 6 kms. northwestward, the traces were observed to have followed a moderate deppression. During the second field survey to the epicentral area, a 3.4 meters offset of the shoreline in Barrio Cabong, Guinayangan was observed. Ground displacement was laso left lateral. Other Geologic Features: The strong shaking of the ground during the Ragay Gulf Earthquake caused two areas along the CalauagGuinayangan municipal road between kms 236-238 to subside. One of the resulting depressions was 225 meters long while the other was 95 meters long. The longer depression was 2 kms. NW from the first.

A fissure, 15 cms. Wide with 2 unknown lenth, lies along the foothills some 200 meter nw of the PNR terminal in Calauag. Its orientation is N80W. In Lopez, two fissures were observed along Lopez-Jaena St. These may be due to settlement of the bank of Talolong River. Close to the eastern bank of the Calauag River in Barrio Sumulong and Mabini, several sand boils were found. Mudboils are formed when water- laden sediments are subjected to compressional forces thereby causing the water and fine sands and muds to be injected into the air through fissures or to just upwellm towards the surface. Referemces: Southeat Asia Association of Seismology and Earthquake Engineering; Volume IV, Philippines, 1985 Journal of the Geological Society of the Philippines; Volume XXVIII, June, 1974 Summarized by: Erlinda Q. Amin

Intensity VIII Intensity VII Intensity VI Intensity V Intensity IV

Calauag-Quezon, Lopez-Quezon, Guinayangan-Quezon Alabat, Quezon San Francisco-Quezon, Manila Quezon City, Romblon, Romblon, Daet-Camarines Norte Tayabas-Quezon, MIA Pasay City, Ambulong Tanawan-Batangas, Legaspi-Albay, InfantaQuezon, Dagupan-Nueva Ecija Intensity III Virac-Catanduanes, Catbalogan-Samar  August 2, 1968 Ms 6.3 

March 17, 1973 Ms 7.0

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August 17, 1976 Ms 7.9

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August 17, 1983 Ms 6.5

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February 8, 1990 Ms 6.8

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June 14, 1990 Ms 7.1

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July 16, 1990 Ms 7.9

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November 15, 1994 Ms 7.1

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May 27, 1996 Ms 5.6

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June 7, 1999 Ms 5.1

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March 6, 2002 Ms 6.8

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February 15, 2003 Ms 6.2

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February 6, 2012 Ms 6.7

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October 15, 2013 Ms 7.2