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 define global reference frame
Background Standard Seismic Cycle a Subduction Zone Interseismic strain accumulation (slow) • What GPS records during interseismic phase: • Downwarp, inland motion • Uplift • No motion locked (Displacements reverse during co-seismic phase) slipped Earthquake: Coseismic offset (fast)
Simple seismic cycle as observed by GPS Position Strain Accumulation (“Interseismic”) Position Strain Accumulation (“Interseismic”) “Co-seismic”
Seismic Cycle: more nuanced view First Interseismic Co-seismic Afterslip Post-seismic Next Interseismic (different velocity?) (Aseismic Creep)
Aseismic 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 • 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 • Observed in Cascadia, Mexico, Japan • Recurrence Interval • Identified in some regions • Depth Range - deeper than standard EQ - may reflect frictional properties, fluid flow or thermal conditions Temporally Variable Locking? Fully Locked? Fully Locked: Earthquakes Temporally Variable Locking: ETS Partially slipping all the time? Fully slipping part of the time?
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 Costa Rica Panama ~8-9 cm Cocos Plate Embedded Animation
Previous Work: Episodic GPSNorabuena 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
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
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 Network Stations: Record local plus regional Signal Subtract regional signal from network stations to get “pure” local signal Baselines
Transient Observed in May 2007 • 12 GPS, 10 seismic stations operating • Most record event • GPS data are noisy Embedded Animation
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
Slip Inversion Results Maximum Slip 16 cm Depth ~25 km Equivalent Magnitude M = 5.8 RMS 3 mm
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
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!
Ambiguity Resolution: Ambizap (Blewitt, 2006) WRMS Decreases .1 .9 .7
Costa Rican Subduction zone earthquake history 1992 Ms=7.2, Mw=7.7 Nicaraguan “Tsunami Earthquake” (very slow rupture causing a discrepancy in magnitudes) 1950 M=7.7 1916 M=7.2 1978 M=7.0 1900 M=7.1 1996 M=6.2 1990 M=6.2 1974 M=6.2 1939 M=6.9 1941 M=7.3 1999 M=6.9 1983 M=6.5 1904 M=7.1 smooth crust Magnitude: 6.0-6.9 7.0-7.5 > 7.5 seamount laden Quepos Plateau Cocos Ridge 1 second = 10 yrs Embedded Animation