Gps Earthquake Slip Nicoya Peninsula Costarica

GPS & Seismic Studies of Episodic Tremor & Slip on the Nicoya Peninsula, Costa Rica Timothy H Dixon MARGINS Lecturer February/March 2009

Observing Subduction Zone Deformation Seismology has been the traditional tool; limited to events with periods shorter than a few hours (dynamic offset due to earthquake waves) Ground-based geodesy (eg leveling) showed interseismic strain accumulation, static offset due to earthquakes, & post-seismic deformation (tool of choice until ~ 1990) Space geodesy (InSAR, GPS) began in late 1980’s - early 1990’s InSAR has excellent spatial resolution; good for co-seismic offset; time resolution limited to repeat time of satellite orbit (several weeks - several months) Campaign style GPS also misses rapid time variation

Observing slow slip events Observation gap: deformation phenomena with periods longer than a few hours (seismology) and shorter than a few weeks or months (InSAR, campaign GPS) could be missed Maturing GPS technology allows installation of Continuous GPS (C-GPS) networks; common in last decade Slow slip and other transient deformation phenomena may be common in subduction zones, perhaps plate boundaries Well-documented in Cascadia, Japan, Mexico

High Precision Geodesy with GPS Range to four or more satellites specifies 3D position + clock error Use dual frequency to make first order ionosphere correction Use precise phase and pseudo- range data to estimate range between satellite and ground point Use geophysical models to estimate and correct major error sources (orbits, troposphere, tides) Use global network to

Background Standard Seismic Cycle a Subduction Zone Interseismic strain accumulation (slow)

locke d

What GPS records during interseismic phase: Downwarp, inland motion Uplift No motion

slippe

(Displacements reverse during co-seismic phase) d

Earthquake: Coseismic offset (fast)

Simple seismic cycle as observed by GPS Position

Position

Strain Accumulation (“Interseismic”)

Strain Accumulation (“Interseismic”) “Co-seismic”

Seismic Cycle: more nuanced view First Interseismic Co-seismic Afterslip Post-seismic Next Interseismic (different velocity?) (Aseismic Creep)

Strain accumulation vs Creep

Creep can be stopped, it just takes more & stronger curbs*

*In the 2009 Stimulus Package

Aseismic Creep

What is Episodic Tremor and Slip? •A slow slip event (“ very slow earthquake”)

accompanied by seismic tremor •May repeat at regular intervals (every few months - every few years) •A new class of Earth deformation phenomenon, with characteristics intermediate between standard earthquake and aseismic creep

Characteristics of Subduction Episodic Tremor & Slip Fully Locked?

Temporally Variable Locking?

Observed in Cascadia, Mexico, Japan

Recurrence Interval – Identified in some regions

Depth Range - deeper than standard EQ - may reflect frictional Fully Locked: Earthquakes Temporally Variable Locking: ETS Partially slipping all the time? Fully slipping part of the time?

properties, fluid flow or thermal conditions

ETS: Global Perspective Cascadia

Bungo Channel, Japan

Guerrero, Mexico

2-4 mm

3 cm

Up to 6cm

Inferred fault 2-3 cm plane slip

7-9 cm

Several faults

Depth of max 30-50 km slip

30-50 km

> 30 km

Duration Equivalent magnitude

6-14 days 6.4-6.8

6 mo-1 yr 6.7-7.0

5-6 mo 7.1-7.5

Source

Dragert et al Ozawa et al Larson et al 2001 2001, 2004 2004, 2006

Max slip @ surface

Costa Rica Project Goals Set up network of continuous GPS in “typical” subduction zone (fast subduction, young crust, frequent earthquakes) Inspect data for phenomena that have so far escaped attention, eg ETS events Are ETS events common in Middle America subduction zone? How do their characeristics differ from other subduction zones? What can they tell us about subduction earthquakes? –Implications for Seismic Hazard –Implications for Earthquake Process

Costa Rica Geography Nicaragua

Caribbean Plate st Co a R a ic

Panama

~8-9 cm

Cocos Plate

Embedded Animation

Previous Work: Episodic GPS Norabuena et al, 2004

Campaign data 1994-2000

Spatial locking patterns

Good Spatial Resolution; Poor Temporal Resolution Are patches fully locked all the time?

2 Patches of Locking Shallow Patch: centered at 14 km  Seismogenic Deep Patch: centered at 39 km  Temporal Variation?

Why Build the Network Here? ETS Event observed Sept 2003 Duration 1 month 1.5 cm total slip Only 3 stations available; no seismic tremor recorded

Deep Brace ~10 m depth

Short Brace ~2 m depth - requires hard rock

Cement pillar ~ 2 m depth – base for 5700 spike mounts

Embedded Animation

Monumentation

Communication

Internet Communication

Data Analysis GIPSY precise point positioning – Each station’s position is independently determined

Ambiguity resolution – East/West component

Baseline to MANA (Managua, Nicaragua) to reduce common mode errors – ~250 km North

MANA: Records regional signal

Baselines

Network Stations: Record local plus regional Signal Subtract regional signal from network stations to get “pure” local signal

Transient Observed in May 2007 12 GPS, 10 seismic stations operating Most record event GPS data are noisy

Embedded Animation

GPS Station Coverage

Seismic Tremor

Seismic Network Configuration Borehole seismometers (100 m) Surface vault seismometers (2-8 m) Borehole Signal Surface Vault Signal Note: PNCB is the only site w/o signal Embedded Animation

Temporal Coincidence of Tremor and Slip

Minutes of tremor per day measured by Nicoya Seismic Network

Characterizing the Event Atmospheric noise is high in the tropics (humid, variable troposphere) Challenge: extract meaningful signal in presence of noise V(t) = V0 + V*t + (U/2)*(tanh((t-T)/tau)-1) (Larson et al. 2004)

t = Day of Year V0 = intercept V = adjusted background velocity U = surface offset Tau = duration of event T = mid point of event Embedded Animation

North Offset

1.0 cm

Start Time

May 7, 2007

Duration

32 days

Interseismic velocity plus slow slip displacement

Slip Inversion Results Maximum Slip 16 cm Depth ~25 km Equivalent Magnitude M = 5.8 RMS 3 mm

Smoothing Tests

Observations vs. Predictions

Inversion Results, Implications

Inversion: 16 cm of slip 30 km depth MEquiv = 5.9 Transient slip patch “fills in” earthquake rupture geometry. Will next EQ be smaller?

Implications for Earthquake cycle, slip budget – Stress: More updip stress after ETS event (makes next major earthquake more likely) – ETS event fills in “slip gap” from the 1950 (Mw = 7.7) event, meaning that the next earthquake could be smaller than the 1950 event (would require that ETS events recur every few years) – Need to keep looking for next 3-4 years to understand recurrence interval

Cascadia Bungo Channel, Japan

Guerrero, Mexico

Max slip @ 1.2 cm surface

2-4 mm

3 cm

Up to 6cm

Inferred 10 cm fault plane slip

2-3 cm

7-9 cm

Several faults

Depth of max slip

30 km

30-50 km 30-50 km > 30 km

Duration

30 days ~6.0

6-14 days 6.4-6.8

6 mo-1 yr 6.7-7.0

5-6 mo 7.1-7.5

This study

Dragert et al 2001

Ozawa et al 2001, 2004

Larson et al 2004, 2006

Equivalent magnitude

Source

Costa Rica

Future 3 new stations (gives~ 25 km average spacing Upgrade rcvrs, comms –Faster data recovery 2 new reference stations (back arc) for better baseline quality Model time dependant strain

Conclusions There was an ETS event beginning May 13, 2007 captured by at least 8 stations of the new Nicoya CGPS network. The ETS event occurred deeper than a previously observed locked patch offshore Costa Rica May have implications for the size of the next earthquake (more info in recurrence innterval needed)

Acknowledgements NSF-MARGINS, I&F UNAVCO OVSICORI Thank you!

Back Up

Seismic Equipment

Ambiguity Resolution: Ambizap (Blewitt, 2006)

WRMS Decreases .1

.9

.7

Monument Type Concrete Pillar

WRMS Range (North) 2.8-3.8

Short Brace Deep Brace

2.3-3.9 3.1-3.5

Monument Type Concrete Pillar

WRMS Range (East) 4.6-5.7

Short Brace

4.0-5.2

Deep Brace

4.7-4.9

Monument Type Concrete Pillar Short Brace Deep Brace

WRMS Range (Vertical) 8.4-10.7 8.4-9.6 9.2-10.4

92 19

Costa Rican Subduction zone earthquake history M

2 7. s=

= w ,M 1 19

= .2 M =7 1 50 M 6 7. 19 78 7.0 = 19 = M

7 7.

th

cr u

19 9 M 0 =6 .2

7.0-7.5 > 7.5 1 second = 10 yrs

Embedded Animation

19 39

st

Magnitude: 6.0-6.9

7 7.

(very slow rupture causing a discrepancy in magnitudes)

sm oo

M00 19

Nicaraguan “Tsunami Earthquake”

9

se am l Q ade o u Pl uep n nt at o ea s u

.

6

M =

199 6 M= 6. 2 1974 M= 6 .2

199

194 1

9

M= 7.3

198 3 M= 6.5 M

= 190 6 4M . =7. 9 1

Co Ri co dg s e