Transcript Seismic and Geodetic Characteristics of the Middle America Trench: Focus on Northern Costa Rica and Nicaragua Andrew V.

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
• 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
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
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
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
Cocos Plate origins from magnetic anomalies
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.
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 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
Getting updip seismicity in
Southern Nicaragua
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
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?
Farmer et al., in prep
Nicaraguan updip limit is
very shallow
Updip limit:
Fundamentally different?
or
Transient postseismic from
1992 shallow rupturing
Tsunami Earthquake?
Seismically Defined MAT interface
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)
Interface Modeling (maximum seismicity method):
Using seismicity rates to infer locking
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
b-value in Nicoya
Ghosh et al., 2008
Relation to geodetically derived locking
Ghosh et al., 2008
Norabuena et al., 2004
Relation to geodetically derived locking
Ghosh et al., 2008
Interface mapping of b-value
Moment Content with Magnitude
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??
Coupling efficiency
Seismicity from
1900-1976:
Pacheco and
Sykes, 1992
1976-present:
global CMT
M 7+
M 5.5+
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
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
and episodic slip
in Nicoya
June 2007 event
Protti et al., 2004
Psencik, unpublished
Schwartz, unpublished
Regional Seismic Hazards
Global Seismic Hazards Assessment Program
www.seismo.ethz.ch/gshap
Duration [s]
Costa Rican Subduction zone
Nicoya dataset