GPS & Seismic Studies of GPS & Seismic Studies of Episodic Tremor & Slip on the Episodic Tremor & Slip on the Nicoya Peninsula, Costa RicaNicoya Peninsula, Costa Rica

Timothy H DixonTimothy H DixonMARGINS LecturerMARGINS Lecturer

February/March 2009February/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’sInSAR 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 eventsObservation gap: deformation phenomena with periods longer than a few hours (seismology) and shorter than a few weeks or months (InSAR, campaign GPS) could be missedMaturing GPS technology allows installation of Continuous GPS (C-GPS) networks; common in last decadeSlow slip and other transient deformation phenomena may be common in subduction zones, perhaps plate boundariesWell-documented in Cascadia, Japan, Mexico

Range to four or more satellites specifies 3D position + clock errorUse 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 define global reference frame

High Precision Geodesy with GPS

BackgroundBackground

locked

slipped

What GPS records during interseismic phase:

Downwarp, inland motion Uplift No motion

Interseismic strain accumulation (slow)

Earthquake: Coseismic offset (fast)

Standard Seismic Cycle a Subduction Zone

(Displacements reverse during co-seismic phase)

Simple seismic cycle as observed by GPSSimple seismic cycle as observed by GPS

Strain Accumulation

Strain Accumulation(“Interseismic”)

(“Interseismic”)

“Co-seismic”

Position P

ositi

on

Seismic Cycle: more nuanced view

First InterseismicCo-seismicAfterslipPost-seismicNext Interseismic (different velocity?)(Aseismic Creep)

Strain accumulation vs CreepStrain accumulation vs Creep

Aseismic Aseismic CreepCreep

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

*In the 2009Stimulus Package

What is Episodic Tremor and Slip?What is Episodic Tremor and Slip?

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

Characteristics of Subduction Characteristics of Subduction Episodic Tremor & SlipEpisodic Tremor & Slip

Observed in Observed in Cascadia, Cascadia, Mexico, JapanMexico, Japan

Recurrence IntervalRecurrence Interval– Identified in some regionsIdentified in some regions

Depth RangeDepth Range - deeper than standard - deeper than standard

EQEQ- - may reflect fmay reflect frictional rictional

properties, fluid flow or properties, fluid flow or thermal conditionsthermal conditions

FullyLocked?

TemporallyVariable Locking?

Fully Locked: EarthquakesTemporally Variable Locking: ETS Partially slipping all the time? Fully slipping part of the time?

ETS: Global PerspectiveETS: Global PerspectiveCascadiaCascadia Bungo Bungo

Channel, Channel, JapanJapan

Guerrero, Guerrero, MexicoMexico

Max slip @ Max slip @ surfacesurface

2-4 mm2-4 mm 3 cm3 cm Up to 6cmUp to 6cm

Inferred fault Inferred fault plane slipplane slip

2-3 cm2-3 cm 7-9 cm7-9 cm Several Several faultsfaults

Depth of Depth of max slipmax slip

30-50 km30-50 km 30-50 km30-50 km > 30 km> 30 km

DurationDuration 6-14 days6-14 days 6 mo-1 yr6 mo-1 yr 5-6 mo5-6 moEquivalent Equivalent magnitudemagnitude

6.4-6.86.4-6.8 6.7-7.06.7-7.0 7.1-7.57.1-7.5

SourceSource Dragert et Dragert et al 2001al 2001

Ozawa et al Ozawa et al 2001, 20042001, 2004

Larson et al Larson et al 2004, 20062004, 2006

Costa Rica Project GoalsCosta Rica Project GoalsSet up network of continuous GPS in “typical” Set up network of continuous GPS in “typical”

subduction zone (fast subduction, young crust, subduction zone (fast subduction, young crust, frequent earthquakes)frequent earthquakes)

Inspect data for phenomena that have so far Inspect data for phenomena that have so far escaped attention, eg ETS eventsescaped attention, eg ETS events

Are ETS events common in Middle America Are ETS events common in Middle America subduction zone?subduction zone?

How do their characeristics differ from other How do their characeristics differ from other subduction zones?subduction zones?

What can they tell us about subduction What can they tell us about subduction earthquakes?earthquakes?

–Implications for Seismic HazardImplications for Seismic Hazard–Implications for Earthquake ProcessImplications for Earthquake Process

Costa Rica GeographyCosta Rica Geography

~8-9 cm

Cocos Plate

Caribbean Plate

Nicaragua

Panama

Costa Rica

Embedded Animation

Previous Work: Episodic GPSPrevious Work: Episodic GPSNorabuena et al, 2004Norabuena et al, 2004

2 Patches of LockingShallow Patch: centered at 14 km SeismogenicDeep Patch: centered at 39 km Temporal Variation?

Campaign data1994-2000

Spatial locking patterns

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

Why Build the Network Here?Why Build the Network Here?

ETS Event ETS Event observed Sept observed Sept 20032003Duration 1 Duration 1 monthmonth1.5 cm total slip1.5 cm total slipOnly 3 stations Only 3 stations available; no available; no seismic tremor seismic tremor recordedrecorded

MonumentationMonumentationDeep BraceDeep Brace~10 m depth~10 m depth

Short BraceShort Brace~2 m depth ~2 m depth - requires hard rock- requires hard rock

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

mountsmounts

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CommunicationCommunication

Internet CommunicationInternet Communication

Data AnalysisData Analysis

GIPSY precise point positioningGIPSY precise point positioning– Each station’s position is independently Each station’s position is independently

determineddeterminedAmbiguity resolutionAmbiguity resolution– East/West component East/West component Baseline to MANA (Managua, Nicaragua) Baseline to MANA (Managua, Nicaragua) to reduce common mode errorsto reduce common mode errors– ~250 km North~250 km North

BaselinesBaselinesMANA: Records regional signal

Network Stations: Record local plus regional Signal

Subtract regional signal from network stations to get “pure” local signal

Transient Observed in May 2007Transient Observed in May 2007

12 GPS, 10 seismic 12 GPS, 10 seismic stations operatingstations operatingMost record eventMost record eventGPS data are noisyGPS data are noisy

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GPS Station CoverageGPS Station Coverage

Seismic TremorSeismic Tremor

Borehole Signal Surface Vault Signal

Note: PNCB is the only site w/o signal

Seismic Network Configuration Borehole seismometers

(100 m) Surface vault seismometers

(2-8 m)

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Temporal Coincidence of Tremor and SlipTemporal Coincidence of Tremor and Slip

Minutes of tremor per day measured by Nicoya Seismic Network

Characterizing the EventCharacterizing the EventAtmospheric noise is high in the tropics Atmospheric noise is high in the tropics (humid, variable troposphere)(humid, variable troposphere)Challenge: extract meaningful signal in Challenge: extract meaningful signal in presence of noisepresence of noiseV(t) = VV(t) = V00 + V*t + (U/2)*(tanh((t-T)/tau)-1) + V*t + (U/2)*(tanh((t-T)/tau)-1)(Larson et al. 2004)

V0 = interceptV0 = interceptV = adjusted background velocityV = adjusted background velocityU = surface offsetU = surface offset

T = mid point of eventT = mid point of eventTau = duration of eventTau = duration of event

t = Day of Yeart = Day of Year

Embedded Animation

North North OffsetOffset

1.0 cm1.0 cm

Start Start TimeTime

May 7, May 7, 20072007

DurationDuration 32 days32 days

Interseismic velocity plus slow slip Interseismic velocity plus slow slip displacementdisplacement

Slip Inversion ResultsSlip Inversion ResultsMaximum Slip16 cm

Depth~25 km

Equivalent MagnitudeM = 5.8

RMS 3 mm

Smoothing TestsSmoothing Tests

Observations vs. PredictionsObservations vs. Predictions

Inversion Results, ImplicationsInversion 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 budgetImplications for Earthquake cycle, slip budget

– Stress: More updip stress after ETS event (makes next Stress: More updip stress after ETS event (makes next major earthquake more likely)major earthquake more likely)

– ETS event fills in “slip gap” from the 1950 (Mw = 7.7) ETS event fills in “slip gap” from the 1950 (Mw = 7.7) event, meaning that the next earthquake could be smaller event, meaning that the next earthquake could be smaller than the 1950 event (would require that ETS events recur than the 1950 event (would require that ETS events recur every few years)every few years)

– Need to keep looking for next 3-4 years to understand Need to keep looking for next 3-4 years to understand recurrence intervalrecurrence interval

Costa Costa RicaRica

CascadiaCascadia Bungo Bungo Channel, Channel, JapanJapan

Guerrero, Guerrero, MexicoMexico

Max slip @ Max slip @ surfacesurface

1.2 cm1.2 cm 2-4 mm2-4 mm 3 cm3 cm Up to 6cmUp to 6cm

Inferred Inferred fault plane fault plane slipslip

10 cm10 cm 2-3 cm2-3 cm 7-9 cm7-9 cm Several Several faultsfaults

Depth of Depth of max slipmax slip

30 km30 km 30-50 km30-50 km 30-50 km30-50 km > 30 km> 30 km

DurationDuration 30 days30 days 6-14 days6-14 days 6 mo-1 yr6 mo-1 yr 5-6 mo5-6 moEquivalent Equivalent magnitudemagnitude

~6.0~6.0 6.4-6.86.4-6.8 6.7-7.06.7-7.0 7.1-7.57.1-7.5

SourceSource This studyThis study Dragert et al Dragert et al 20012001

Ozawa et al Ozawa et al 2001, 20042001, 2004

Larson et al Larson et al 2004, 20062004, 2006

3 new stations (gives~ 3 new stations (gives~ 25 km average spacing25 km average spacing

Upgrade rcvrs, commsUpgrade rcvrs, comms–Faster data recoveryFaster data recovery

2 new reference stations 2 new reference stations (back arc) for better (back arc) for better baseline qualitybaseline quality

Model time dependant Model time dependant strainstrain

Future

ConclusionsConclusions

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

AcknowledgementsAcknowledgements

NSF-MARGINS, I&FNSF-MARGINS, I&FUNAVCOUNAVCO

OVSICORIOVSICORI

Thank you!Thank you!

Back UpBack Up

Seismic EquipmentSeismic Equipment

Ambiguity Resolution: Ambizap Ambiguity Resolution: Ambizap (Blewitt, 2006)(Blewitt, 2006)

WRMS Decreases

.1

.9

.7

Monument TypeMonument Type WRMS Range (North)WRMS Range (North)Concrete PillarConcrete Pillar 2.8-3.82.8-3.8Short BraceShort Brace 2.3-3.92.3-3.9Deep BraceDeep Brace 3.1-3.53.1-3.5

Monument TypeMonument Type WRMS Range (East)WRMS Range (East)Concrete PillarConcrete Pillar 4.6-5.74.6-5.7

Short BraceShort Brace 4.0-5.24.0-5.2

Deep BraceDeep Brace 4.7-4.94.7-4.9

Monument TypeMonument Type WRMS Range (Vertical)WRMS Range (Vertical)Concrete PillarConcrete Pillar 8.4-10.78.4-10.7Short BraceShort Brace 8.4-9.68.4-9.6Deep BraceDeep Brace 9.2-10.49.2-10.4

1904 M=7.1

Costa Rican Subduction zone Costa Rican Subduction zone earthquake historyearthquake history

1941 M=7.31916 M=7.2

1939

M=

6.

9

1999

M=

6.

9

1996 M=6.2 1974 M=6.2

1983 M=6.5

smooth crust

seamountladen

Quepos Plateau

Cocos Ridge1 second = 10 yrs

1992 Ms=7.2, Mw=7.7

Nicaraguan “Tsunami Earthquake”(very slow rupture causing a discrepancy in magnitudes)

Magnitude:6.0-6.97.0-7.5 > 7.5

1900 M=7.11950 M=7.71978 M=7.0

1990 M=6.2

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