1997- Mirzaei- Auniform Catalog Of Earthquake For Seismic Hazard Assesment In Iran.pdf

Vol. 10 No. 6 ( 7 1 3 ~ 7 2 6 )

ACTA SEISMOLOGICA SINICA

Nov. , 1997

A uniform catalog of earthquakes for seismic hazard assessment in Iran O

N o o r b a k h s h Mirzaei 1'2) MENG-TAN G A O 1 ) ( ~ ] ] ~ ) JIAN W A N G I ) ( q :-

YUN-TAI CHENX)(~:IF_~)

~)

1) Institute of Geophyo.cs, 8tare Se~nological Bureau, Beijmg 100081, China 2) ln~t~ute of Geophy~ca, Tehran Un~versily, Tehran 14394, lrau

Abstract The assessment of seismicity is strongly dependent on the recorded events as data base. A uniform catalog of earthquakes in Iran and neighbouring regions is provided to use for seismic hazard assessment of the country. Since the recurrence time of maximum credible earthquake can not be estimated directly from mb, empirical relationships for different seismotectonic provinces arc established to convert mb to Ms, which is a suitable magnitude scale for our purpose. It emerges from completeness study of the catalog that many small and moderate earthquakes are missed out, specially in the historical and early instrumental time-periods.

Key words : seismicity

uniform catalog

seismic hazard empirical relationships

completeness

Introduction Compilation of u n i f o r m catalog of earthquakes is of great importance which fulfills the condition of homogeneity of the basic data. T h e s e d a t a , t h e n , will be used for the s t u d y of seismicity, for the purpose of learning general laws of earthquake occurrence with the aim of protecting m a n k i n d from the disastrous effect of earthquakes. T h e area b o u n d e d in 220~-, 42"N and 42*"--66"E is selected for the s t u d y of seismicity in Iran. Selection of this area permit us to c o n t r i b u t e all the geological a n d seismological elements which can affect the c o u n t r y . A u n i f o r m catalog of earthquakes in I r a n and adjacent r e g i o n s , covering the period of 4th c e n t u r y B. C. t h r o u g h 1994 is provided. T h e catalog includes all events for which m a g n i t u d e (rob or M s ) have been determined b y i n t e r n a t i o n a l agencies and most reliable individual sources, which provides a basis for f u r t h e r statistical analysis of earthquakes in Iran. E a r t h q u a k e s with u n k n o w n m a g n i t u d e scale, and those for which no m a g n i t u d e have been assigned, assembled and retained in the data base as separate files to use for the different purposes (other t h a n statistical analysis). F i n a l l y , such information will be used in seismic hazard s t u d y of the country. Since the recurrence time of m a x i m u m credible e a r t h q u a k e can not be directly estimated from the rab(e, g. , W y s s and H a b e r m a n n , 1 9 8 2 ) , which is the o n l y m a g n i t u d e scale assigned for about 80 percent of earthquakes occurred in the area u n d e r s t u d y , following empirical formulae are established to convert mb to M s , which is suitable m a g n i t u d e scale for our purpose. Zagros

Ms = 1.79rob - - 4. 32

Received February 26, 1997; revised July l l , 1997; accepted July 1I, 1997. ContributionNo. 97A0101, Institute of Geophysics, SSB, China.

o'us = 0. 26

(1)

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ACTA SEISMOLOGICASINICA

Vol. 10

A z a r b a y e j a n - A l b o r z - K o p e h Dagh

Ms = 2. 01rob - - 5. 28

au s = 0 . 2 5

(2)

Central-East Iran

M s = 2. 0rob - - 5. 28

~u s = 0 . 2 3

(3)

Makran

Ms = 1.58rob - - 3. 11

a u s = 0. 30

(4)

6Us is the average s t a n d a r d deviation of dependent variables. Ms values, in each m a g n i t u d e class, range inside a unit of m a g n i t u d e .

Ms-rob relationships applied to those events for which no Ms are assigned, and the earthquakes with k n o w n Ms value excluded from the conversion and are added to the catalog directly.

1 Uniform catalog T h e assessment of seismicity is s t r o n g l y d e p e n d e n t o n the recorded events as data base. T h e data base provides u n i f o r m information on over 4770 e a r t h q u a k e s , including historical and i n s t r u m e n t a l l y recorded events from 4th c e n t u r y B . C . t h r o u g h 1994. I n a d d i t i o n , considerable n u m ber of events for which incomplete information is available, are assembled and retained in the data base as separate files to use in different purposes, other t h a n statistical analysis of data. Different sources which were useful in the compilation of the u n i f o r m catalog are listed in the following. 1.1

H i s t o r i c a l earthquakes A l t h o u g h the information concerning historical seismicity of ]_ran is collected in variety of ar-

ticles, such as Wilson ( 1930 ) , A m b r a s e y s ( 1968,

1974a ) , Berberian ( 1977 ) , S e y e d - N a b a v i

( 1 9 7 8 b ) , and Poirier and T a h e r ( 1 9 8 0 ) , more reliable information was found in A m b r a s e y s and Melville ( 1 9 8 2 ) , which is based on critical evaluation of d o c u m e n t a r y materials and also direct observation in the field. T h i s work has been continued and improved by A m b r a s e y s ( 1 9 8 8 , 1 9 8 9 ) in the s t u d y of seismicity in T u r k e y . T h e Ms values of these events are estimated based on the macroseismic i n f o r m a t i o n , i . e . m a x i m u m i n t e n s i t y and radius of perceptibility, calibrated against macroseismic i n f o r m a t i o n of sirnflar i n s t r u m e n t a l earthquakes. Epicenter coordinates of earthquakes are also adopted from the same source which are determined based on the macroseismic information and field investigations b y A m b r a s e y s and Melville ( 1 9 8 2 ) ,

and A m b r a s e y s ( 1 9 8 8 ,

1989). T o t a l n u m b e r of 277 historical earthquakes from 3rd m i l l e n n i u m B . C . to 1899 exist in the data base from which 163 events from 4th c e n t u r y B. C. to 1899 incorporated in the u n i f o r m catalog.

1.2

Instrumental earthquakes A M B Catalog of earthquakes provided b y A m b r a s e y s and M e l v i l l e ( 1 9 8 2 ) , and A m b r a s e y s

( 1 9 8 8 ) . T h e work of A m b r a s e y s and M e l v i l l e ( 1 9 8 2 ) , is the basic source of parameters for early i n s t r u m e n t a l time-period ( 1 9 0 0 ~ 1963). This work has been continued b y A m b r a s e y s ( 1 9 8 8 ) in the s t u d y of seismicity in T u r k e y . K A R Catalog of E u r o p e a n e a r t h q u a k e s ( 1 9 0 1 ~ 1 9 5 5 ) ,provided b y K a r n i k ( 1 9 6 9 ) . T h i s catalog covers n o r t h w e s t e r n part of the area u n d e r s t u d y and is considered as a reliable source of information to fill the gap of small size e v e n t s , which are not listed in A M B . M E A S T Catalog of earthquakes occurring in the Middle East ( 1 9 0 0 ~ 1 9 8 3 )

compiled at

National Oceanic and A t m o s p h e r i c A d m i n i s t r a t i o n ( N O A A ) , b y Riad and M e y e r s ( 1 9 8 5 ) . This catalog was useful for parameters of some earthquakes in early i n s t r u m e n t a l time-period. U S G S / N E I C Catalog of earthquakes located b y U S G S / N E I C and its predecessors in the U.S.

Coast and Geodetic S u r v e y , the National Oceanic S u r v e y and the E n v i r o n m e n t a l Research

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Laboratories of the Department of Commerce. Listings are in most cases the epicenters and magnitudes of the U S G S / N E I C which are listed in the Geological Survey Publication: "Preliminary Determination of Epicenters ( P D E ) - M o n t h l y Listing". Data extracted from Global Hypocenter Data Base CD-ROM ( U S G S / N E I C , 1994) for the period of 1963 to 1 9 9 2 and extended through 1994, using monthly listings of the PDE. ISC Catalog of earthquakes located by the International Seismological Center, extracted in part from a file prepared by ISC from 1964 to 1987 and completed and extended through 1993, using regional and monthly bulletins of earthquakes published by the same agency. MOF The data collection of earthquakes in Iran provided by Moinfar et al. (1994) based on various sources, covering historical and instrumental earthquakes up to 1992. This data collection was useful for few events in the early instrumental time-period. The other sources which were useful, in part, for our purpose, are listings of International Seismological Summary ( I S S ) , macroseismic epicenters determined by Berberian ( 1 9 7 7 ) , relocated epicenters by N o w r o o z i ( 1 9 7 6 ) , catalog of earthquakes in Pakistan, Afghanistan and southeastern Iran(Quittmeyer and Jacob, 1979). To get complete coverage of data, some other catalogs such as catalog of earthquakes occurring in Europe and Mediterranean region, compiled at the European-Mediterranean Seismological Center ( E M S C ) , catalog of earthquakes occurring in Turkey supplied by N O A A at 1985 ( U S G S / N E I C , 1994), catalog of significant earthquakes 2000 B.C. to 1 9 7 9 compiled at the World Data Center-A (WDC) by Ganse and Nelson(1981), catalog of large shallow earthquakes from 1 8 9 7 to 1980 provided by Abe ( 1 9 8 1 , 1 9 8 4 , 1 9 9 4 ) and Abe and Noguchi (1983) were searched. For early instrumental time-period, surface wave magnitudes determined by Ambraseys and Melville (1982), and Ambraseys(1988) which are calculated using more seismograms than others (Jackson and McKenzi, 1988), considered as more accurate than those reported in original catalogs or agency bulletins. These values are computed uniformly from amplitude recorded and period readings using the Vanek et al. ~s(1962) formula (Ambraseys, 1988), the same as the current procedure in U S G S / N E I C and ISC to estimate Ms, since 1968. Surface wave magnitudes in K A R which are calculated uniformly using the suggested formula for world-wide use (see Karnik, 1969) constitute the second source for magnitude determination. In original sources, in the cases that instrumental data have not been available , magnitudes have been converted from macroseismic parameters (see Karnik, 1969; Ambraseys and Melville, 1982). Finally, magnitudes for the remainder of earthquakes adopted from original sources, whenever distinguished as reliable. For modern instrumental time-period ( 1 9 6 4 ~ 1994), the main sources of magnitudes are those values reported by ISC and U S G S / N E I C , respectively. Magnitudes obtained from special studies ,were preferred, whenever available. For early instrumental time-period, it is generally accepted that, the macroseismic epicenters are more accurate than instrumental ones (e. g . , Karnik, 1969; Ambraseys and Melville, 1982). Careful field studies have shown that this is a fact regarding earthquakes in Iran. More discussions and details of field investigations can be found in Ambraseys and Moinfar (1973, 1974a, 1974b, 1976), A m b r a s e y s ( 1 9 7 4 b , 1976, 1978, 1988), Ambraseys et al. ( 1 9 7 3 ) , Tchalen-ko and B r a u d ( 1 9 7 4 ) , Moinfar (1976), Berberian(1979), and Ambraseys and Melville (1982), among others. A reasonable criterion applied for the selection of epicenters in instrumental time-period is as follows : For early instrumental time-period, macroseismie epicenters in AMB which are based on the field investigations, distinguished as more reliable than those published originally based on teleseismic determinations, and preferred whenever available; otherwise, macroseismic epicenters

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ACTA SEISMOLOGICASINICA

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proposed b y Berberian ( 1 9 7 7 ) , relocated epicenters b y N o w r o o z i ( 1 9 7 6 ) , epicenter coordinates in and M E A S T considered as m o r e reliable t h a n o t h e r sources respectively. F o r m o d e r n i n s t r u m e n t a l t i m e - p e r i o d , location d e t e r m i n a t i o n s b y ISC and U S G S / N E I C

KAR,

,

constitute t h e m a i n sources of locations respectively. In most cases, epicenters d e t e r m i n e d b y ISC which are based on the m o r e station r e a d i n g s , considered as m o r e reliable t h a n those assigned b y U S G S / N E I C ; and in a few eases that maeroseismic epicenters were available, w e r e p r e f e r r e d . Q u a l i t y of i n s t r u m e n t a l location is d e p e n d e n t on different factors such as velocity m o d e l , station d i s t r i b u t i o n , n u m b e r and quality of seismic p h a s e s , as well as the location p r o c e d u r e ( A h j o s and U s k i , 1 9 9 2 ) . A l t h o u g h w i t h t h e a d v e n t of the W o r l d - W i d e S t a n d a r d S e i s m o g r a p h i c N e t w o r k ( W W S S N ) in the early 1960~s and of digital n e t w o r k in t h e late 1970~s, quality of location d e t e r m i n a t i o n s have i n c r e a s e d , n e v e r t h e l e s s , blind acceptance of the locations d e t e r m i n e d b y international a g e n c i e s , and even national and local n e t w o r k s , can lead to erroneous inferences. T h e uncertainties of t h e m o s t reliable epicenters are p r o b a b l y at least 10 kin. D e p t h s of foci are cons i d e r a b l y m o r e u n c e r t a i n t h a n epicentral coordinates~ a c t u a l l y , reliable focal d e p t h s can be o b tained only for events inside a dense seismic n e t w o r k ~ the distance from the epicenter to the nearest station should be less t h a n the focal d e p t h and average station spacing not exceeding twice t h e d e p t h ( L e e and S t e w a r t , 1 9 8 1 ) .

C o n s e q u e n t l y , all teleseismically located d e p t h s , specially for

shallow d e p t h e a r t h q u a k e s should be viewed with suspicion, unless o t h e r information is used to s u p p o r t the d e p t h ( M o l n a r and C h e n , 1982). In exception of the N o v e m b e r 9 , 1 9 7 0 m b = 5. 4 event in Z a g r o s - M a k r a n convergence zone ( K a d i n s k y - C a d e and B a r a z a n g i , 1 9 8 2 ) , t h e r e is no reliable evidence of i n t e r m e d i a t e focal d e p t h e a r t h q u a k e in Iran. Results of w a v e f o r m modeling and m i c r o e a r t h q u a k e studies s h o w that the m a j o r i t y of e a r t h q u a k e s in Iran occur in d e p t h s of 8 ~ 1 5 k m in Z a g r o s , 7 ~ 3 5 k m in n o r t h e r n Iran and s o u t h e r n C a s p i a n , and 8 ~ 2 0 km in east and southeast of the c o u n t r y . F r o m M a k r a n coast of s o u t h e a s t e r n I r a n to M a k r a n coast of s o u t h w e s t e r n P a k i s t a n , focal d e p t h s increase up to about 60 k m . It is w o r t h to note that several collection of i n s t r u m e n t a l l y recorded e a r t h q u a k e s covering different time and spaces have been published previously concerning I r a n i a n e a r t h quakes~ e. g . ,

Banisadr ( 1 9 6 9 ) ,

Moazami-Goudarzi(1972),

Seyed-Nabavi(1978a,

1979),

I G T U ( 1 9 9 3 ) , M o i n f a r et al. ( 1 9 9 4 ) . P r e p a r a t i o n of the most complete catalog of e a r t h q u a k e s in Iran and n e i g h b o r i n g regions m a i n l y based on the original source m a t e r i a l s , disclosed unacceptable inefficiencies in r e c e n t l y published d a t a collection b y I G T U ( t 9 9 3 ) , which m a k e it i n a p p r o p r i a t e and even misleading for seismicity s t u d y . Lack of at least 1 / 4 of e a r t h q u a k e s (in all r a n g e of m a g n i t u d e s ) , m a n y errors in e x t r a c t i o n and t y p i n g of d a t a from original sources and u n k n o w n sources of some p a r a m e t e r s (e. g. , m a g n i t u d e s of e a r t h q u a k e s located b y ISS ) , and uncritical using of available i n f o r m a t i o n which is resulted in the existence of gross location errors in this catalog [for e x a m p l e , the location of S i l a k h o r e a r t h q u a k e of 23 J a n u a r y 1909, M s = 7 . 4

, in w e s t -

ern I_ran which is reliably located in 33. 4 t ° N , 4 9 . 1 3 ° E b y A m b a r s e y s and M e l v i U e ( 1 9 8 2 ) based on the detail field w o r k s ( A m b r a s e y s and M o i n f a r , 1973~ A m b r a s e y s , 1974b~ T c h a l e n k o and B r a u d , 1 9 7 4 ) , is still considered to be at 3 3 . 0 * N , 50°E about 100 k m far from the g e n e r a l l y accepted and well d o c u m e n t e d location of this event-] are e x a m p l e s of pointed inefficiencies. T h e more recent published d a t a collection of e a r t h q u a k e s in I.ran ( M o i n f a r et a l . , 1 9 9 4 ) , covering historical and i n s t r u m e n t a l t i m e - p e r i o d , up to 1992, is more complete than all previously p u b lished c a t a l o g s , and even t h o u g h needs completion and revision, is an a p p r o p r i a t e source of d a t a for events in the e a r l y i n s t r u m e n t a l t i m e - p e r i o d .

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2 M s - m b relationships 2. I

Ms-rob relationships for different earthquake regions in Iran Body wave m a g n i t u d e , mh, is a world-wide scale d e t e r m i n e d b y the m a x i m u m amplitude of

the first few s e c o n d s ( u s u a l l y about one second) of short-period P waves on the vertical compon e n t seismogram. Surface wave m a g n i t u d e , M s , is a world-wide scale d e t e r m i n e d from the a m plitude of surface waves ( u s u a l l y Rayleigh w a v e s ) with a period of about 20 seconds ( R e i t e r ,

1990). For large e a r t h q u a k e s , fftb saturates beyond 6 . 2 or so (Singh et a l . , 1983) and can not fully describe the s t r e n g t h of the large earthquakes that release energy along several kilometers of fault r u p t u r e ( R e i t e r , 1990). Ms scale tends to saturate and lose its validity for earthquakes beyond m a g n i t u d e 8. T h i s s a t u r a t i o n of Ms scale m a y be a t t r i b u t e d to e x t r e m e l y large source d i m e n sions, generating e x t r e m e l y long period seismic waves ( K a s a h a r a , 1985). C l e a r l y , the most appropriate m a g n i t u d e scale to use in most aspects of seismic studies is m o m e n t m a g n i t u d e , Mw, ( K a n a m o r i , 1 9 7 7 ) which does not suffer from s a t u r a t i o n , and has a s o u n d e r physical basis ( K a s a h a r a , 1985). H o w e v e r , Mw is available only for a very few portion of earthquakes. Since, Mw agrees very well with M s for m a n y earthquakes below the s a t u r a t i o n level ( K a n a m o r i , 1977 ; H a n k s and K a n a m o r i , 1 9 7 9 ) , and only a few of the earthquakes in the interest r e g i o n , have m a g n i t u d e s near to the limit of saturation of M s , it was concluded t h a t , surface wave m a g n i t u d e , which is the only cited m a g n i t u d e for older e a r t h q u a k e s , is an appropriate scale for our purpose. D e t e r m i n a t i o n of Ms in U S G S / N E I C b e g a n in mid 1968, and it was o n l y since the data-year 1978 that effort has been made in the ISC ( M c G r e g o r e t a l . , 1988) for d e t e r m i n a t i o n of surface wave m a g n i t u d e . F o r the period from 1963 t h r o u g h 1994, a total n u m b e r of over 3900 mb values have been reported by ISC or U S G S / N E I C for earthquakes in I.ran and n e i g h b o r i n g regions (an area covering 2 2 " ~ 4 2 " N and 4 2 ° ~ 6 6 ° E ) , for about 800 of t h e s e , also, M s values are assigned. C o n s e q u e n t l y , we need to convert mb to Ms ; even t h o u g h such relations are approximate ( B a t h , 1981) and error-prone process (Singh e t a l . , 1983). Relation b e t w e e n mb and Ms depends on several p a r a m e t e r s , such as focal d e p t h , focal m e c h a n i s m , earth s t r u c t u r e ( W y s s and H a b e r m a n n , 1 9 8 2 ) , and significant difference from one earthquake region to a n o t h e r ( B a t h , 1981). C o n t i n e n t a l - c o n t i n e n t a l collision zone of the active Zagros f o l d - t h r u s t b e l t , on the north m a r g i n of the A r a b i a n c o n t i n e n t a l c r u s t , Kopeh Dagh active fold belt on the s o u t h e r n m a r g i n of the T u r a n c o n t i n e n t a l crust ( s o u t h e r n E u r a s i a ) , active oceanic-continental s u b d u c t i o n zone of M a k r a n , rigid intraplate zone of C e n t r a l - E a s t 1ran and A l b o r z - A z a r b a y e j a n region in s o u t h e r n border of the Caspian and n o r t h w e s t I r a n , are k n o w n as areas with different tectonic settings (for more i n f o r m a t i o n about active tectonics of I r a n , see for e x a m p l e , B e r b e r i a n , 1 9 8 1 ) ; for which four empirical formulae based on all available pairs of mb and Ms values from ISC and U S G S / N E I C data fries were established ( T a b l e 1 ) • Because of the lack of sufficient data to Table 1 Ms-rob relationships for different earthquake regions in Iran Earthquake regions

Ms-rob relationships

Zagros

M s = l . 79rae--4.32

4, O~rab~<6.2

484

ATarbayejan-Alborz-KopehDagh Central-East h'an Makran

M s = 2 . 0imp--5. 28

4. 0~mb~6.2 4. l~mb~8.2 4. 0~mb~5.9

263 201 78

Ms = 2, Omb--5. 28 Ms=l. 58mb-3.11

Amplitute range

Data(ISCand USGS)

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establish a robust relationship for K o p e h Dagh , e x i s t e n c e of some geological similarities and results of p r e l i m i n a r y s t u d i e s , p e r m i t us to establish a unique relationship for A z a r b a y e j a n - A l b o r z - K o p e h D a g h ; w h i c h is v e r y close to t h e relationship obtained for C e n t r a l - E a s t Iran ( s e e , T a b l e 1). 2.2

Procedure Following B a t h ( 1 9 8 1 ) ,

and Wyss and H a b e r m a n n ( 1 9 8 2 ) , w e

assume that relationship be-

t w e e n mb and M s can be w r i t t e n in the following f o r m : M s = d m b-~- c

(5)

W h e r e d and c are constants. W y s s and H a b e r m a n n ( 1 9 8 2 ) found t h a t the slope d , is estim a t e d based on t h e cases w h e r e standard deviations of the errors of i n d e p e n d e n t variable mb and d e p e n d e n t variable M s , or error ratio ( r , = a , / q ~ )

w e r e k n o w n , yield the m o s t satisfactory solu-

tion for estimation o f d :

d

+=1

=

,=1

~=1

~=1

~=,

(6)

2 2AyiAzi where

ax+

=

~=t

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--

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=

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--

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• i event = 2 event

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Ms

versus mb based on the ISC and

data files for Zagros (Circles and crosses refer to the data from ISC and U S G S / N E I C respectively)

Figure 2

M s versus mu based on the ISC and U S G S / NEIC

data files for Azarbaycjan-Alborz-

K o p e h Dagh (Cizclcs and crosses refer to the data from ISC and U S G S / N E I C respectively)

No. 6

MIRZAEI, N. et at.

1 event 7.0

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~ 2 event

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=

A UNIFORM CATALOG OF EARTHQUAKES IN IRAN

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~,/Ib

Figure 3

Ms versus mb based on the ISC and USGS/NEIC data files for CentralEast Iran (Circles and crosses refer to the data from ISC and USGS/NEIC respectively)

Figure 4

Ms versus mb based on the ISC and USGS/NEIC data files for Makran (Circles and crosses refer to the data from ISC and USGS/NEIC respectively)

Because of discrepancies in the values of mb and M s d e t e r m i n e d by ISC and U S G S / N E I C for the same e v e n t s , it was concluded that for obtaining the best r e s u l t s , it is m o r e reasonable to combine available data from t w o data files t o g e t h e r . Results from this c o m b i n a t i o n , was much better t h a n those obtained from each source separately. F i t t e d lines to d a t a for different regions are s h o w n in F i g u r e s 1 ~

4 . It can be inferred from the figures that the best estimates of Ms

m a y obtain from the conversion of mb ranging from 4. 6 to 5. 7 in Z a g r o s ,

4. 5 to 5. 5 in

A z a r b a y e j a n - A l b o r z - K o p e h D a g h , 4. 6 to 6. 0 in C e n t r a l - E a s t I r a n , and 4 . 4 to 5 . 5 in M a k r a n . F o r smaller m a g n i t u d e s , M s values from the conversion probably will be u n d e r e s t i m a t e d ~ while for larger m a g n i t u d e s even t h o u g h in s o m e cases appear to be o v e r e s t i m a t e d , overall results seems to be satisfactory. It is w o r t h to note that for most of larger events (almost all) in t h e interest area from 1968 to 1994, Ms estimates by ISC or U S G S / N E I C are available. F o r i n s t r u m e n t a l e a r t h q u a k e s prior to 1963, for 668 out of 707 e a r t h q u a k e s , also, Ms estimates are available. T h e obtained relationships were applied only to those events for w h i c h no Ms values are e s t i m a t e d , and those with k n o w n M s are excluded from the conversion and added to the data base directly. Plots of Ms versus mb exhibit a large scatter. Such scatters are due both to errors of the m e a s u r e m e n t s and to the fact that the formulae for calculation of m a g n i t u d e s are simplifications of t h e complex physical processes at t h e seismic source ( B a t h , 1981~ K a s a h a r a , 1985).

720

ACTA SEISMOLOOICA SINICA

VoL 10

3 Completeness of the catalog T h e question of completeness is p a r a m o u n t ; nothing of value can be e s t i m a t e d from a statistical analysis with significant a m o u n t s of the d a t a missing ( V e r e - J o n e s , 1992). T h i s can be the chief reason for t h e expression o f H u a n g et al. ( 1 9 9 4 ) , w h o pointed out that " t h e completeness and reliability of d a t a are t h e base of e a r t h q u a k e r e s e a r c h " . Since t h e catalog covers a long period of t i m e , e x p e c t e d l y spatial and t e m p o r a l inconsistencies exists. F o r historical e a r t h q u a k e s in I r a n , a l t h o u g h scattered indications of e a r t h q u a k e effects go back as far as the third millennium B . C .

( A m b r a s e y s and M e l v i l l e , 1 9 8 2 ) , adequate indications

for individual events becomes available o n l y from 7th c e n t u r y A . D . ( S e y e d - N a b a v i , 1 9 7 8 b ; A m b r a s e y s and Melville, 1982). R a p i d progress in d a t a survival takes off very late in 19th cent u r y , so t h a t , in the one t h o u s a n d years period from 800 to 1800, n u m b e r of r e p o r t e d shocks are only 160(one e v e n t , a p p r o x i m a t e l y e v e r y six y e a r s ) , which almost all events have been m o r e or less d a m a g i n g ( M e l v i l l e , 1984). It emerges from the s t u d y of d a t a that m a n y small and m o d e r a t e m a g n i t u d e e a r t h q u a k e s have been missed o u t , particularly in desert regions. It r e m a i n s v e r y p r o b a b l e , that a n y m a j o r or great e a r t h q u a k e has been n o t e d , although not necessarily fully identified. It is reasonable to s u p p o s e , t h e r e f o r e , that t h e available historical d a t a for the whole region is incomplete for all r a n g e of m a g n i t u d e s ( A m b r a s e y s and Melville, 1982; Melville, 1984; A m b r a s e y s , 1989). T h e t e m p o r a l completeness of the catalog evaluated using two alternative m e t h o d s applied b y A h j o s and U s k i ( 1 9 9 2 ) to F e n n o s c a n d i a n catalog. In the first alternative which is the so called S t e p p l s m e t h o d , it is assumed that the e a r t h q u a k e processes are s t a t i o n a r y with the m e a n occurrence rate of e v e n t s in a given m a g n i t u d e class r e m a i n i n g constant during an observation period (i. e. , n = N / A N , w h e r e n is mean occurrence r a t e and N is cumulative n u m b e r of e a r t h q u a k e s w i t h i n a given m a g n i t u d e class A M , during an observation period At y e a r s ) . In this m e t h o d , for s t a t i o n a r y , it is expected that t h e variance ~ of n is equal to n/At and t h e s t a n d a r d deviation ~r. is parallel to 1/-.,/"~ ( A h j u s and U s k i , 1992). F i g u r e s 5 show t h e s t a n d a r d deviation of the m e a n n u m b e r of events in a m a g n i t u d e class AM versus s a m p l e observation period t in five major seismotectonic provinces of Iran. In the figu r e s , reference lines s h o w g r a d i e n t s c o r r e s p o n d i n g to t h e plot versus t , which m u s t be equal to t h e g r a d i e n t of the plot versus t. D e p a r t u r e from the reference line ( p a r a l l e l i s m ) implies that either At includes i n c o m p l e t e l y r e p o r t e d m a g n i t u d e s or t h a t is not long enough to yield a good estimates of the m e a n occurrence rate ( A m b r a s e y s , 1985; A h j o s and U s k i , 1992). F r o m our a n a l y ses it is concluded t h a t the S t e p p l s m e t h o d could be applied sueeessfuUy to those classes of m a g n i tudes for which the n u m b e r of events is large enough (short m e a n occurrence r a t e ) , which is t h e case for 4 . 0 ~ M s <

6 . 0 in our catalog. It is probable that for larger m a g n i t u d e s , At is not long

enough to provide a good e s t i m a t i o n of t h e m e a n occurrence rate of t h e events. T h e second alternative requires to plot the n u m b e r of e a r t h q u a k e s in each m a g n i t u d e interval versus time of occurrence of the events. T h i s is a simple and effective m e t h o d to present available d a t a clearly to judge and declare the beginning year of t h e complete detection of data. W e applied this m e t h o d to d e t e r m i n e the beginning year of complete reporting of larger events ( M s ~ 6 ) . F i g u r e s 6 are e x a m p l e s of this application to the catalog of e a r t h q u a k e s in the m a j o r S e i s m o t e c t o n ic provinces of I.ran.

No. 6

M I R Z A E I , N. et al. : A U N I F O R M C A T A L O G OF E A R T H Q U A K E S IN IRAN

-r~

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Figure 5

C o m p l e t e n e s s test of t h e catalog of e a r t h q u a k e s in Z a g r o s , A l b o r z - A z a b a y e j a n , Kopeh Dagh, Central-East Iran and Makran major seismotectonic provinces : stand a r d deviation o f t h e m e a n n u m b e r of e v e n t s ¢. w i t h i n a m a g n i t u d e class A M vers u s t h e s a m p l e l e n g t h A t ( o b s e r v a t i o n period in y e a r s b e f o r e 1 9 9 4 ) [The reference line connects the reference points(see t e x t ) ]

10~

722

ACTA SEISMOLOGICASINICA

7_agtos

..... 1400

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.

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!,i[.

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Vol. 10

1800

2000

6. O~Ms<6.5

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t/a

Figure 6

Frequency-time distribution of earthquakes within the given magnitude interval for completeness test in Zagros, Alborz-Azarbayejan, Kopth Dagh, Central-East Iran and Makran

M a g n i t u d e - T i m e ( M - t ) diagrams ( F i g u r e 7) for different seismoteetonic provinces are d r a w n to represent m a g n i t u d e distribution of events in respect to t h e time of occurrence of the events. F r o m such d i a g r a m s active periods for larger m a g n i t u d e s and a shade of detectability of the n e t w o r k s and c o m p l e t e n e s s of the catalog will be obtained. F r o m the f i g u r e , s i m p l y , it is concluded that in t h e historical t i m e p e r i o d , m a n y m o d e r a t e and almost all of small e v e n t s are missed out ; and at least t w o s h a r p increases in detectability of t h e events and data survival in 20th c e n t u r y following the installation of seismographic i n s t r u m e n t s in around the w o r l d , and at t h e early 1960~s in c o n s e q u e n c e of installation of W W S S N exist.

M o r e o v e r , d a t a survival v a r y g r e a t l y

from one province to a n o t h e r . In the early i n s t r u m e n t a l t i m e - p e r i o d , m o r e complete data is available for A l b o r z - A z a r b a y e j a n , because of t h e w o r k of K a r n i k ( 1 9 6 9 ) w h i c h covers this area. F o r the same t i m e - p e r i o d , i n f o r m a t i o n for M a k r a n is v e r y incomplete , so that t h e r e is no complete and reliable i n f o r m a t i o n available for a n y event up to 1919.

Results of completeness s t u d y are

s u m m a r i z e d in T a b l e 2. Table 2

Beginning year of complete reporting of earthquakes in major seismotectonic provinces of Iran Regions

4 . 0 ~ Ms~4. 5

4 . 5 ~ Ms<~5. 0

5. 0~ Ms(5. 5

5 . 5 ~ Ms~6.0

6. 0~Ms

Albor~Azarbaye~n KopehDagh Zagros Centre-East ~an Makran

]975 1975 1975 1975 1975

1945 1963 1965 1955 1965

1900 1925 1944 1955 1950

1900 1904 1925 1900 1919

1860 1850 1860 1900 1919

No.

MIRZAEI, N. et at. : A UNIFORM CATALOGOF EARTHQUAKES IN ]RAN

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Magnitude-time distribution of earthquakes in major seismoteetonic provinces of Iran

In order to u n d e r s t a n d the completeness of earthquake catalog it is useful to recaptulate the history of the I r a n i a n seismic n e t w o r k . T h e following description is m a i n l y adopted from A m braseys and M e l v i l l e ( 1 9 8 2 ) , and A k a s h e h et a l . ( 1 9 7 6 ) . T h e seismographic station in T e h r a n , the first to be installed in I_ran, came into operation in 1958 and was followed a year latter b y the station in Shiraz. In 1962 a secondary station was set up at Sefid-Rud and soon after in 1 9 6 4 ~ 1965, the stations in T a b r i z , M a s h h a d , K e r m a n s h a h came into o p e r a t i o n ; Shiraz, T a b r i z and M a s h h a d being equipped with i n s t r u m e n t s of the W W S S N . I n 1971, M a s h h a d began to expand its regional n e t w o r k with s u b o r d i n a t e s t a t i o n s , covering the northeast and east-central part of the country. A little later the A t o m i c E n e r g y Organization of Iran began to operate local n e t w o r k s in the regions of B u s h e h r ( 1 9 7 5 ) , E s f a h a n ( 1 9 7 6 ) and T e h r a n ( 1 9 7 7 ) .

I n 1976, I r a n i a n Long Peri-

od A r r a y ( I L P A ) ( A k a s h e h et a l . , 1 9 7 6 ) , was established in the region of S a v e h , southwest of T e h r a n , consisting of seven three c o m p o n e n t wideband borehole seismometers in a circular shape with six i n s t r u m e n t s forming a circle with the diameter of about 60 k m and the seventh located in the center. C o m p l e m e n t a r y i n f o r m a t i o n concerning seismic stations in Iran are summarized in T a b l e 3.

ACTA SEISMOLOGICA SINICA

724

Vol. 10

Table 3 Main (signed by asterisk) and secondary seismic stations in Iran by the year 1994, based on the data extracted from Regional Bulletins of ISC(1994) Station Name Station name

Code name

Tehran *

THE

35"44' 16"

51"23' 08"

1 360

Shiraz*

SHI

29"38' 40"

52"31'34"

1 595

WWSSN

1963-05-10

Tabriz*

TAB

38*04'03"

46°19'36"

1 430

WWSSN

1965-08-10

M ashhad *

MHI

36"18'30" t

59"28'18"

1 150

WWSSN

1975-09-15

Mashhad

MSH

36"18'4~Y' 59*35'16"

987

WWSSN

1965-09-17

MAIO

36"18'30'

Mashhad (SRO) *

9~s/(0)

)~/(0)

59"28'18"

Elevation/m

1 150

Network

Start time

Close time

1958

1975-09-06

1975-10

Mashhad Univ.

MUI

36"18'42"

59"36'18"

1 000

1972-01

Kermanshah*

KER

34"21' 08"

47*06' 21"

1 310

1965

Sefid Rud

SRI

36*45' 30"

49' 23' 00"

243

1965

]ran LP Array*

IR1

35*24'58. 2"

50°41'19. 5"

1 347

1976-01

11t2

35°39'46. 1" 50053'51.5"

i 172

1976-01

IR3

35*28'34. 0" 51"01'25. 5"

1 106

1976-01

IR4

35"14~19. 3" 50054'04. 2"

1 373

1976-01

IR5

35°12~46. 2" 50"34'52. 0"

1 350

1976-01

IR6

35028'25. 2" 50025'32. 2"

1 540

1976-01 1976-01

IR7

35042' 10. 1" 50036'32. 0'0

1 305

Kakhk

KHI

34*08' 36"

58*38'30'

1 600

1971

Shahrud

SHD

36*26' 00"

54*56'30"

1 500

1975

TaghiGharnbar

TGI

32*57' 48"

59*11' 36"

1 800

1975

* are main station

4 Conclusion T h e u n i f o r m catalog of e a r t h q u a k e s in Ixan a n d n e i g h b o r i n g r e g i o n s has provided a reliable a n d m o s t c o m p l e t e collection of available i n f o r m a t i o n for s e i s m i c s t u d y p r o g r a m s in t h e c o u n t r y . It e m e r g e s f r o m t e m p o r a l a n d s p a t i a l c o m p l e t e n e s s s t u d y o f t h e c a t a l o g t h a t s i g n i f i c a n t v a r i a t i o n in d a t a s u r v i v a l , s p e c i a l l y in h i s t o r i c a l a n d e a r l y i n s t r u m e n t a l t i m e - p e r i o d s , e a r t h q u a k e r e g i o n to a n o t h e r ,

W e w o u l d like to t h a n k Pakzad,

Mr.

exist from one

w h i c h s h o u l d b e c o n s i d e r e d in all r e s e a r c h p r o j e c t s .

Prof.

DE-CUN ZHANG,

PEI-SHAN C H E N ,

CDSN

Dr.

ZHONG-LIANG W U ,

Dr.

Mehrdad

staffs a n d d a t a c e n t e r of I G S S B for their help to access

earthquake data from original sources. References Abe K, 1981. Magnitudes of large shallow earthquakes from 1904 to 1980. Physgarth Planet lute'r, 2 7 : 7 2 ~ 9 2 Abe K, 1984. Complements to "Magnitudes of large shallow earthquakes from 1904 to 1980". Physgarth Plaaet later, 3 4 : t 7 423 Abe K, 1994. Instrumental magnitudes of historical earthquakes. BuU ,Se/wa •o¢ Amer, 8 4 : 4 1 5 ~ 4 2 5 Abe K, Nogucni S, 1983. Revision of magnitudes of large shallow earthquakes, 1897~1912. Phy~ Earth Plauet l~ter, 33 : l 11 Ahjes T , U s k i M , 1992. Earthquakes in northern Europe in 1375~1989. In: Freeman R, Muller S (eds), The European Geotraverse, Part 8. Tectonophy~ics, 2 0 7 : 1 ~ 2 3 Akasheh B, Eshghi I, Soltanian R, 1976. Iranian long period array (1LPA). J Geophys, 4 2 : 1 5 9 ~ 1 6 2 Ambraseys N N, 1968. Early earthquakes in north central Iran. Bull Eewn ,.~oc Amer, 5 8 : 8 5 ~ 4 9 6 Ambraseys N N, 1974a. The historical seismicity of North-Central Irma. Geo~ 8u~v lras, Rep 2 8 : 4 7 ~ 9 6 Ambraseys N N, 1974b. The seismicity of Iran: The SRakhor, Lorestan earthquake of 23rd January 1909, Part I . Attnal di Geofis, 2 7 : 3 9 9 ~ 4 2 7 Ambraseys N N, 1976. Earthquake epicenters in ]ran. Proc CENTO Semin Recent AdvanCes in garthq Hazard MiAimizatioa.

No. 6

MIRZAEI, N. el al. : A UNIFORM CATALOG OF EARTHQUAKES IN ]:RAN

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