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

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 define global reference frame High Precision Geodesy with GPS

Background 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 GPS Strain Accumulation (“Interseismic”) “Co-seismic” Position

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

Strain accumulation vs Creep

Aseismic Creep Creep Creep can be stopped, it just takes more & stronger curbs* *In the 2009 Stimulus Package

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

Characteristics of Subduction Episodic Tremor & Slip Observed in Cascadia, Mexico, Japan Recurrence Interval –Identified in some regions Depth Range - deeper than standard EQ - deeper than standard EQ - may reflect frictional properties, fluid flow or thermal conditions Fully Locked? Temporally Variable Locking? Fully Locked: Earthquakes Temporally Variable Locking: ETS Partially slipping all the time? Fully slipping part of the time?

ETS: Global Perspective Cascadia Bungo Channel, Japan Guerrero, Mexico Max surface 2-4 mm 3 cm Up to 6cm Inferred fault plane slip 2-3 cm 7-9 cm Several faults Depth of max slip km > 30 km Duration 6-14 days 6 mo-1 yr 5-6 mo Equivalent magnitude Source Dragert et al 2001 Ozawa et al 2001, 2004 Larson et al 2004, 2006

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 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 ~8-9 cm Cocos Plate Caribbean Plate Nicaragua Panama Costa Rica Embedded Animation

Previous Work: Episodic GPS Norabuena et al, Patches of Locking Shallow Patch: centered at 14 km  Seismogenic Deep Patch: centered at 39 km  Temporal Variation? Campaign data Spatial locking patterns Good Spatial Resolution; Poor Temporal Resolution Are patches fully locked all the time?

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

Monumentation 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

Communication

Internet Communication

Data Analysis 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

Baselines MANA: 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 GPS, 10 seismic stations operating Most record event GPS data are noisy Embedded Animation

GPS Station Coverage

Seismic 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) 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) = V 0 + V*t + (U/2)*(tanh((t-T)/tau)-1) (Larson et al. 2004) V0 = intercept V = adjusted background velocity U = surface offset T = mid point of event Tau = duration of event t = Day of Year Embedded Animation

North Offset 1.0 cm StartTime 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 M Equiv = 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

Costa Rica Cascadia Bungo Channel, Japan Guerrero, Mexico Max surface 1.2 cm 2-4 mm 3 cm Up to 6cm Inferred fault plane slip 10 cm 2-3 cm 7-9 cm Several faults Depth of max slip 30 km km > 30 km Duration 30 days 6-14 days 6 mo-1 yr 5-6 mo Equivalent magnitude ~ Source This study Dragert et al 2001 Ozawa et al 2001, 2004 Larson et al 2004, 2006

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 Future

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 UNAVCOOVSICORI Thank you!

Back Up

Seismic Equipment

Ambiguity Resolution: Ambizap (Blewitt, 2006) WRMS Decreases.1.9.7

Monument Type WRMS Range (North) Concrete Pillar Short Brace Deep Brace Monument Type WRMS Range (East) Concrete Pillar Short Brace Deep Brace Monument Type WRMS Range (Vertical) Concrete Pillar Short Brace Deep Brace

1904 M =7.1 Costa Rican Subduction zone earthquake history 1941 M = M = M = M = M = M = M = M = M = M = M = M =6.5 smooth crust seamount laden Quepos Plateau Cocos Ridge 1 second = 10 yrs 1992 Ms=7.2, Mw=7.7 Nicaraguan “Tsunami Earthquake” (very slow rupture causing a discrepancy in magnitudes) Magnitude: > M= M = M = M = M =6.2 Embedded Animation