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

Georgia Institute of Technology, Atlanta, GA, USAGeorgia Institute of Technology, Atlanta, GA, USA

Seismic and Seismic and GeodeticGeodetic Characteristics Characteristics of the Middle America Trench: Focus of the Middle America Trench: Focus on Northern Costa Rica and on Northern Costa Rica and NicaraguaNicaragua

Susan SchwartzSusan SchwartzUniversity of California Santa Cruz, CA, USA.University of California Santa Cruz, CA, USA.

Heather DeShonHeather DeShonUniversity of Memphis, TN, USA.University of Memphis, TN, USA.

J. Marino Protti, Victor GonzalesJ. Marino Protti, Victor GonzalesUniversidad Nacional Costa Rica (OVSICORI)Universidad Nacional Costa Rica (OVSICORI)

Timothy H. Dixon, Kim PsencikTimothy H. Dixon, Kim PsencikUniversity of Miami, Miami, FL, USA.University of Miami, Miami, FL, USA.

Edmundo NorabuenaEdmundo NorabuenaInstituto Geofisico del Instituto Geofisico del

Peru, Lima, Peru.Peru, Lima, Peru.

09/23/200809/23/2008

• 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 –06/2001 - 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

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 10-14 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 CrustCNS-1 = Cocos-Nazca Spreading Center Crust before rotationCNS-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 butperpendicular production Direction.

Cocos plate seismicity

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

with heat flow

Getting updip seismicity in Southern Nicaragua

Nicoya and Nicaragua seismicity and networks

Overlapping seismicity in space and time

Nicaragua data: 1975-1982; 1993-recent

Nicoya-CRSEIZE data: Late-1999 - mid-2001

Representative velocity profilefrom CRSEIZE

After DeShon et al., GJI, 2006

Cross-network relocations

Relocations of earthquakes identified in both Nicoya and Nicaragua networks

Green: Nicoya locationsMagenta: 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 interfaceSeismically 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 Using seismicity rates to infer lockinglocking

S. Stein and M. Wysession, 2003

Earthquake frequency-magnitude Earthquake frequency-magnitude distributiondistribution

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

b-value in Nicoya

Ghosh et al., 2008

Norabuena et al., 2004

Relation to geodetically derived Relation to geodetically derived lockinglocking

Ghosh et al., 2008

Relation to geodetically derived Relation to geodetically derived lockinglocking

Ghosh et al., 2008

Interface mapping of Interface mapping of bb-value-value

EPR CNS

EPR CNS

Moment Content with Magnitude

Coupling efficiencyCoupling 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??

Coupling efficiency Coupling efficiency

M 7+

M 5.5+

Seismicity from 1900-1976: Pacheco and Sykes, 1992

1976-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:

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 Non-volcanic tremor and episodic slipand episodic slip

in Nicoyain Nicoya

Psencik, unpublished

Protti et al., 2004

Schwartz, unpublished

June 2007 event

Duration [s]

Global Seismic Hazards Assessment Program

Regional Seismic Hazards

www.seismo.ethz.ch/gshap

Costa Rican Subduction zoneCosta Rican Subduction zone

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Nicoya dataset