Andrew V. Newman, Grant. T. Farmer, Abhijit Ghosh, Amanda Thomas, Jaime Convers

1. Seismic and Geodetic Characteristics of the Middle America Trench: Focus on Northern Costa Rica and Nicaragua Andrew V. Newman, Grant. T. Farmer, Abhijit Ghosh, Amanda Thomas, Jaime Convers Georgia Institute of Technology, Atlanta, GA, USA 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

2. 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

3. Nicoya-CRSEIZE 12/99 –06/2001 - 20 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 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

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

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

6. Along-strike seismicity Red earthquakes are only upper portion of the seismogenic zone (avoiding crustal events). • North (left) seismicity is below 20 km • South (right) updip seismicity begins about 10-14 depth. Newman et al., 2002 No vertical exaggeration

7. 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 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

8. Cocos plate seismicity with heat flow CNS heat flow is appropriate for 20 Ma crust (~140 mW/m2), EPR heat flow is very low (~10 mW/m2) - Heat flow drops rapidly at transition from CNS to EPR - Extreme gradient in NW EPR from ~10 up to 650 mW/m2

9. Getting updip seismicity in Southern Nicaragua

10. Nicoya and Nicaragua seismicity and networks Nicaragua data: 1975-1982; 1993-recent Nicoya-CRSEIZE data: Late-1999 - mid-2001 Overlapping seismicity in space and time

11. Representative velocity profilefrom CRSEIZE After DeShon et al., GJI, 2006

12. 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

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

14. Seismically Defined MAT interface

15. Interface Modeling (maximum seismicity method): • 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 By minimizing error associated with poor locations, it should well approximate interface <~40 km depth (intra-slab events deeper)

16. Interface Modeling (maximum seismicity method):

17. Using seismicity rates to infer locking

18. Earthquake frequency-magnitude distribution Log10 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 S. Stein and M. Wysession, 2003

19. b-value in Nicoya Ghosh et al., 2008

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

21. Relation to geodetically derived locking Ghosh et al., 2008

22. Interface mapping of b-value EPR CNS

23. Moment Content with Magnitude

24. Coupling efficiency • 108 year of seismicity • - Pacheco and Sykes, 1992 • 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??

25. Coupling efficiency Seismicity from 1900-1976: Pacheco and Sykes, 1992 1976-present: global CMT M 7+ M 5.5+

26. Summary • 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

27. Constraining shallow locking:

28. Constraining shallow locking: Intensive Ocean-bottom seismicand geodeticdeployments are necessary to characterize seismicity and strain accumulation. Gagnon et al., 2005

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

31. Regional Seismic Hazards Global Seismic Hazards Assessment Program www.seismo.ethz.ch/gshap Duration [s]

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

33. Nicoya dataset