Andrew V. Newman, Grant. T. Farmer, Abhijit Ghosh, Amanda Thomas, Jaime Convers Georgia Institute of Technology, Atlanta, GA, USA Seismic and Geodetic Characteristics of the Middle America Trench: Focus on Northern Costa Rica and Nicaragua Susan Schwartz University of California Santa Cruz, CA, USA. Heather DeShon University of Memphis, TN, USA. J. Marino Protti, Victor Gonzales Universidad Nacional Costa Rica (OVSICORI) Timothy H. Dixon, Kim Psencik University of Miami, Miami, FL, USA. Edmundo Norabuena Instituto Geofisico del Peru, Lima, Peru. 09/23/2008

Nicoya CRSEIZE overview Identifying the updip limit of seismicity –Thermally effected –Transient shifts Microseismicity as an indicator of coupling Long-term Seismic Coupling Efficiency MAT Interface Model (Subduction Zone Geometry) Direction for MARGINS SEIZE …we must get to where the action is

12/99 –06/ land (18 mo) - 14 OBS (1st 6 mo) Mw=6.4 Outerrise EQ occurred just after OBS recovered. Main-shock and aftershocks were poorly located by our network Nicoya-CRSEIZE Analysts: (UCSC) - self (postdoc) - Heather DeShon (Grad) - Matt Densmore (UG) - Martin Valle (Grad) - Megan Avants (UG) - Dev Gobalkrishnan (UG) - Christina Bernot (UG) (OVSICORI) - Victor González - Marino Protti (GT) - Abhijit Ghosh (Grad) - Amanda Thomas (UG) - Alice Koerner (UG) - Jaime Convers (Grad) - Grant Farmer (UG/Grad) >10,000 regional events located About 50% at GT in past 3 years

After initial location in 1D velocity model, events are relocated using 3D Vp, Vp/Vs from Deshon et al., 2006.

Along-strike seismicity and Geodetic Locking After Norabuena et al., 2004

Along-strike seismicity No vertical exaggeration North (left) seismicity is below 20 km South (right) updip seismicity begins about depth. Red earthquakes are only upper portion of the seismogenic zone (avoiding crustal events). Newman et al., 2002

Cocos Plate origins from magnetic anomalies EPR = East Pacific Rise Crust CNS-1 = Cocos-Nazca Spreading Center Crust before rotation CNS-2 = Cocos-Nazca Spreading Center Crust after rotation Cocos plate origin - East Pacific Rise (EPR) in NW Nicoya - Cocos-Nazca spreading center (CNS) Origin in SE Subducted crusts are similar age but perpendicular production Direction.

Cocos plate seismicity CNS heat flow is appropriate for 20 Ma crust (~140 mW/m 2 ), EPR heat flow is very low (~10 mW/m 2 ) - Heat flow drops rapidly at transition from CNS to EPR - Extreme gradient in NW EPR from ~10 up to 650 mW/m 2 with heat flow

Getting updip seismicity in Southern Nicaragua

Nicoya and Nicaragua seismicity and networks Overlapping seismicity in space and time Nicaragua data: ; 1993-recent Nicoya-CRSEIZE data: Late mid-2001

Representative velocity profile from CRSEIZE After DeShon et al., GJI, 2006

Cross-network relocations Relocations of earthquakes identified in both Nicoya and Nicaragua networks Green: Nicoya locations Magenta: Nicaragua locations Red: Joint locations Farmer et al., in prep

A Transient updip limit to seismicity? Nicaraguan updip limit is very shallow Updip limit: Fundamentally different? or Transient postseismic from 1992 shallow rupturing Tsunami Earthquake? Farmer et al., in prep

Seismically Defined MAT interface

-Remove crustal events -Define normal to a priori surface -Select minimum cylinder containing n (35) events -bin results normal to functional form -Determine new 3D position from only maximum seismicity bin Interface Modeling (maximum seismicity method) : By minimizing error associated with poor locations, it should well approximate interface <~40 km depth (intra-slab events deeper)

Interface Modeling (maximum seismicity method) :

Using seismicity rates to infer locking

S. Stein and M. Wysession, 2003 Earthquake frequency-magnitude distribution Log 10 N = a – b M M = Magnitude N = # earthquakes > M a = activity b = slope b-value: ratio of number of smaller to larger earthquakes global average ~1 –10x more events with unit M decrease high b: more small events low b: more larger events

b-value in Nicoya Ghosh et al., 2008

Norabuena et al., 2004 Relation to geodetically derived locking Ghosh et al., 2008

Relation to geodetically derived locking Ghosh et al., 2008

Interface mapping of b-value EPR CNS

Moment Content with Magnitude

Coupling efficiency 108 year of seismicity - Pacheco and Sykes, globalcmt.org Convergence between 79 and 90 mm/yr Assumed -100 km wide fault - 30 GPa rigidity Highly variable, but mostly less than 20%. … or is it??

Coupling efficiency M 7+ M 5.5+ Seismicity from : Pacheco and Sykes, present: global CMT

Nicaragua and Costa Rica are seismically very active: –Coupling efficiency is hugely variable. Updip limit seems to be controlled by subducted slab –Possible temperature or topographic control –May be time-dependent: suggested by offset in Nicaragua b-value mapping vs. geodetic locking –Corresponds with GPS locking and ETS event location –Useful because it can maintain better resolution away from land (either at depth, or offshore with OBS measurements) High b-values indicates large component of moment release from small earthquakes. Interface modeling: Subduction zone geometry –Still preliminary, but shows offset in EPR-CNS transition Summary

Constraining shallow locking:

Intensive Ocean-bottom seismic and geodetic deployments are necessary to characterize seismicity and strain accumulation. Gagnon et al., 2005

Non-volcanic tremor and episodic slip in Nicoya in Nicoya Psencik, unpublished Protti et al., 2004 Schwartz, unpublished June 2007 event

Duration [s] Global Seismic Hazards Assessment Program Regional Seismic Hazards

Costa Rican Subduction zone “rough-smooth” boundary Cocos Ridge

Nicoya dataset