Imaging the seismogenic zone along the Middle America Trench, Costa Rica

a joint seismology/geodesy/fluid flux project between UCSC, UCSD, UMiami, OVSICORI-UNA, & GEOMAR
funded in part through the NSF MARGINS Seismogenic Zone Experiment program


CLICK HERE FOR
CRSEIZE DATA ARCHIVE


Introduction to the CRSEIZE Experiment
Most of the world's large and damaging (Mw>7.0) earthquakes occur along the shallow portion of the subducting plate interface.  This region of a subduction zone, termed the seismogenic zone, is responsible for ~80% of total world-wide energy release. Basic seismological characteristics (e.g.. seismicity patterns, P & S-wave velocity variability, crustal thicknesses, attenuation structure) and the degree of plate coupling across these regions are not well understood.  Microseismicity within seismogenic zones is difficult to study because these small earthquakes, while numerous, often occur offshore, and seismic stations are generally limited to onshore environments.   Such a geometry creates mathematical instabilities in constraining the exact locations and associated errors.  In Costa Rica both the Osa and Nicoya Peninsulas lie directly over the active seismogenic zone of the Middle America Trench, making the location perfect for combined onshore/offshore seismic arrays, collocated fluid flux meters, and high resolution geodetic studies.

 
*click on illustration for a larger view

This figure shows the configuration of the Costa Rica Seismogenic Zone Experiment (CRSEIZE). Approximately 4,000 preliminary earthquake locations are shown (red circles).  Black triangles represent the location of the Osa and Nicoya seismic arrays, and circles represent collocated fluid flux meters.  The Osa Experiment recorded ~1000 local and regional earthquakes; the Nicoya Experiment is currently being analyzed (50% picked) and has recorded ~3000 earthquakes.  Harvard CMT solutions for  the August 20, 1999 Mw=6.9 Quepos underthrusting earthquake and the July 21, 2000 Mw=6.4 outer-rise normal earthquake provided. Green diamonds are the location of new and re-occupied GPS sites that constitute the geodetic portion of the CRSEIZE project. Black squares are the locations of the permanent UNA-OVSICORI single component short period seismic network.

 Osa Seismic
Experiment




 
Land & OBS Deployment: September  - December 1999
station locations, uptimes, etc. in data archive
  
*click on illustration for a larger view


1 Broadband Garulp 40-T sensor
5 Short period L22 sensors
14 Broadband OBS (plus hydrophone) using PMD (7) and L4c sensors (7)





We have completed an aftershock relocation study of the 20 August, 1999 Quepos earthquake, reported in DeShon et al. [2003].  We used an a priori 3D model based on available P-wave velocity data (Stavenhagen et al., 1998; Ye et al., 1999; Walther et al., 2000) to compute absolute locations.  We also computed a minimum 1D P-wave model using VELEST and used these results to compute relative relocations utilizing hypoDD (Waldhauser and Ellsworth, 2000).  The 1999 aftershocks illuminated the seismogenic zone inboard of the Quepos Plateau; aftershocks extend from 10 km depth, 35 km from the MAT to 35 km depth, 95 km from the trench.  Results also support the idea that oceanic bathymetric highs along central Costa Rica  act as a local focus for rupture during large earthquakes and exhibit some control on aftershock location, previously suggested for the 1999 earthquake based on analysis of the moment release history of the mainshock (Bilek et al., 2003).







Nicoya Seismic
Experiment

Land Deployment: December 1999 - June 2001
OBS Deployment: December 1999 - June 2000
station locations, uptimes, etc. in data archive
 



10 Broadband Garulp 40-T or STS-2 sensors
10 Short period L22 sensors
14 Broadband OBS (plus hydrophone) using PMD (7) and L4c sensors (7)



During the Nicoya seismic deployment, we recorded several thousand small earthquakes occurring beneath the peninsula.  We initially relocated many of these events using a country-wide, high resolution three dimensional velocity model (Protti et al., 1996), and relative relocation techniques (using hypoDD, Waldhauser and Ellsworth 20000) to better determine seismogenic structure. Results of this study are reported in Newman et al. [2002].  Although interplate seismicity is present from 10 to 40 km depth, we find that most events concentrate along a narrow band between 14 to 22 km starting about 60 km land-ward of the trench. This activity, which best defines the upper limit of the seismogenic zone, shallows by 4 degrees to the southeast and may be partially controlled by varying thermal structure.

Post OBS deployment, a large normal faulting outer-rise earthquake occurred just south of our network. This event has raised several ideas about the development of large normal faults offshore the peninsula, the state of stress of the downgoing slab, and the role of outer-rise earthquakes and related processes on seismogenic zone coupling. There are still many questions to be answered about near surface subducting plate interactions here that we plan to further explore using data from this experiment (i.e., role of sedimentary diagenesis, effects from subducted seamounts, metamorphic reactions and aseismic creep in earthquake distributions).


We have calculated more precise earthquake locations for the Nicoya Peninsula region using local earthquake tomography (generalized cross-section shown left).  We use SIMULPS14, an iterative simultaneous inversion program, to calculate the best 3D P- and  S-wave velocity models and corresponding earthquake locations for the Nicoya Peninsula  region.  We have ~10,000 P and S-wave arrival times recorded by the land and OBS  seismic arrays.  Calculations of the minimum 1D P- and S-wave velocity models for the Nicoya region using the program VELEST have been completed [DeShon and Schwartz, 2004], and these results serve as a starting point for the 3D local  tomography study.    Hypocenter locations from the 1D and 3D inversions for the Nicoya region support the initial results presented by Newman et al. (2002) that microseismicity spatially mirrors thermal changes across the incoming oceanic plate (shown at left).  Additionally we see evidence of two mechanical transitions occurring along the plate interface [Schwartz and DeShon, in press].  The first transition as noted in geodetic data of Norabuena et al. [2004] marks the transition from aseismic to potentially seismic behavior, or the updip limit of rupture during large magnitude earthquakes.  The second transition occurs further downdip where small magnitude interseismic seismicity generates.  Finally, we see seismic velocity evidence for serpentinization of the forearc mantle wedge region, which may effect the contional stability along the downdip limit of the seismogenic zone [DeShon and Schwartz, 2004].   Publications on the 3D inversions results is under development.


Geodesy
Experiment

For details regarding geodetic results, please visit http://www.geodesy.miami.edu/
Fluid Flux
Experiments

For details regarding the fluid flux experiments, visit http://daneel.ucsd.edu/



CRSEIZE Principal Investigators:
Susan Schwartz - Univ. California Santa Cruz & IGPP (Seismology)
Tim Dixon - Univ. Miami, RSMAS (Geodesy)
LeRoy Dorman - Univ. California San Diego, Scripps Institute of Oceanography (Marine Seismology)
Ernst Flueh - GEOMAR (Seismology)
Paul Lungren - Jet Propulsion Laboratory (Geodesy)
Marino Protti - Universidad Nacional Costa Rica, OVSICORI (Seismology/Geodesy)


CRSEIZE Collaborators:

Kevin Brown - Univ. California San Diego, Scripps Institute of Oceanography (Geochemistry)
Susan Bilek - New Mexico Tech (Seismology)
Andrew Newman - Los Alamos National Laboratory (Seismology/Geodesy)
Heather DeShon -
Univ. California Santa Cruz (Seismology)
Samantha Hansen - Univ. California Santa Cruz (Seismology)
Claudia Flores - Univ. California Santa Cruz (Seismology)

Mike Tryon - Univ. California San Diego, Scripps Institute of Oceanography (Geochemistry)
Victor Gonzalez - Universidad Nacional Costa Rica, OVSICORI (Seismology)
Edmundo Norabuena - Univ. Miami, RSMAS (Geodesy)
Dan Sampson - Univ. California Santa Cruz (Instrument Specialist)
Alan Sauter - Scripps Institute of Oceanography (Instrument Specialist)


CRSEIZE Publications:
 
Bilek, S.L., S.Y. Schwartz, and H.R. DeShon, Control of seafloor roughness on earthquake rupture behavior, Geology, 31, 455-458, 2003 Download PDF version

Brown, K.M., M. Tryon, H.R. DeShon, L.M. Dorman, and S.Y. Schwartz, Transient fluid pulsing and seismic tremor: Evidence of episodic creep at the updip edge of the seismogenic zone, Costa Rica? Earth Planetary Science Letters, (in review).

DeShon H.R., Seismogenic zone structure along the Middle America subduction zone, Costa Rica, Ph.D. Thesis, pp. 359, University of California-Santa Cruz, Santa Cruz, CA, June 2004.  PDF version available upon request from H.R. DeShon

DeShon, H.R., and S.Y. Schwartz, Evidence for serpentinization of the forearc mantle wedge along the Nicoya Peninsula, Costa Rica, Geophysicsl Research Letters, 31, L21611, doi:10.1029/2004GL021179, 2004. Download PDF version


DeShon, H. R., S. Y. Schwartz, S. L. Bilek, L. M. Dorman, V. Gonzalez, J. M. Protti, E. R. Flueh, & T. H. Dixon, Seismogenic zone structure of the southern Middle America Trench, Costa Rica, J. Geophys. Res., 108(B10), 2491, doi:10.1029/2002JB002294, 2003 Download PDF version

DeShon, H.R., S.Y. Schwartz, A.V. Newman, V. Gonzalez,  J.M. Protti, L.M. Dorman, T. Dixon, E. Norabuena and E. Flueh, Seismogenic zone structure beneath the Nicoya Peninsula, Costa Rica, from 3D local earthquake P- and S-wave tomography, Geophysical J. International, (in review).

LeFemina, P., T.H. Dixon, W. Strauch, Bookshelf faulting in Nicaragua, Geology, 30, 751-754, 2002. Download PDF version

Newman, A. V., S. Y. Schwartz, V. Gonzalez, H. R. DeShon, J. M. Protti & L. Dorman, Along-Strike Variability in the Updip Limit of the Seismogenic Zone Below Nicoya Peninsula, Costa Rica Geoph. Res. Lett., 29 20, 38:1-4, doi:10.1029/2002GL015409, 2002 Download PDF version

Norabuena, E., T.H. Dixon, S.Y. Schwartz, H.R. DeShon, M. Protti, L. Dorman, E.R. Flueh, P. Lundgren, A. Newman, F. Pollitz, D. Sampson, Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica, J. Geophys. Res., 109, B11403,doi:10.1029/2003JB002931, 2004. Download PDF version

Schwartz, S.Y. and H.R. DeShon, Evidence for multiple mechanical transitions along the updip limit, Nicoya Peninsula, Costa Rica, in Interplate subduction zone seismogenesis, eds. T. Dixon et al., Columbia University Press, New York, (in press).  Download PDF preprint


Updated Jan. 4  10:29 CST 2005 | Webmaster: Heather DeShon hdeshon@geology.wisc.edu