Imaging the seismogenic zone along the Nicoya portion of the Middle America Trench in Costa Rica Andrew V. Newman Most of the world's great earthquakes occur along the active portion of the thrust interface at subduction zones. Basic seismological properties about plate coupling across these regions, termed seismogenic zones, are not well understood. These regions are difficult to study because they often occur offshore. However, The Nicoya Peninsula in Costa Rica, a ophiolite complex, lies directly over an active seismogenic zone, making it perfect for such a study. Currently, with Susan Schwartz (UCSC) and Tim Dixon (University of Miami), I have been working to better image the seismogenic zone in Costa Rica using both on/off-shore passive seismic arrays and GPS geodesy. Over the span of the 18 month seismic deployment, we recorded several thousand small earthquakes occurring beneath the peninsula. I located many of these events using a high resolution three dimensional localized velocity model, and relative relocation techniques to better determine seismogenic structure. Although interplate seismicity is present from 10 to 40 km depth, I have found 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 a varying thermal structure. During the seismic campaign, a large normal faulting outer-rise earthquake occurred just south of our network. This event has raised several ideas about the state of stress of the down-going slab and how it effects 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). *click on illustration for a larger view (364 kb) Multi-beam hydrosweep bathymetry and tracks of seismic data off of Costa Rica (including Nicaragua in inset) from Ranero and von Huene [Nature 2000]. Note the uniquely close approach of the Middle America Trench to the Nicoya and Osa Peninsulas allowing for land-based instruments to be very close to the uppermost portion of the seismogenic zone. This along with the seismic activity and wealth of existing seismic reflection/refraction data from GEOMAR and others have made this a very attractive region to study. Seismicity shown here were recorded by the OVSICORI local network prior to 1995. T1-T7 represent transects discussed in Ranero and von Huene [2000]. *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); ~1000 and ~3000 represent the complete Osa portion and ~30% of the Nicoya portions of the data. Also shown is the location of the seismic profile (blue line) and region of projected seismicity (dashed box) shown in figures below. Beach balls represent the August 20, 1999 Mw=6.9 under-thrust and July 21, 2000 Mw=6.4 outer-rise normal earthquakes, focal solutions. Green diamonds are the location of new and re-occupied GPS sites that constitute the geodetic portion of the CRSEIZE project (handled by University of Miami and JPL). Black squares are the locations of the permanent UNA-OVSICORI single-component short-period seismic network and black triangles represent the location of our 18 month land and 6 month off-shore seismic deployments consisting of both broad band and short-period instruments. *click on illustration for a larger view Shown here are ~2500 earthquakes (black cubes) relocated using the SIMULPS program (Thurber, 1992) and a 3D Nicoya specific P wave velocity structure. The data are rotated 57o CW from North to best illustrate the seismicly active plate interface. Events shown in black are those that are located nearest the land network thus having smaller errors in location. In the cross section, the seismogenic zone appears to begin at approximately 10 km depth and is active down to ~40 km. Below about ~50 km depth seismicity continues more diffusely and with less resolution along the subducting slab. *click on illustration for a larger view This is similar to the above figure except the data are rotated CCW 90o to illustrate along-strike variability in seismogenic zone seismicity. Data within the dashed lines on the cross-section projection are those that have higher resolution. The up-dip of the seismogenic zone is about 10 km depth in the south, however, at a cluster of seismicity in central Nicoya the up-dip limit begins to deepen quickly down to 20 km depth. This change corresponds to a change in the incoming oceanic plate from Cocos-Nazca spreading origin in the south to East Pacific Rise spreading in the North. *click on illustration for a larger view This shows first-motion focal solutions for some of the larger earthquakes that have been recorded by the Nicoya network. All of these events are shallow (around 20 km depth) and occur along the lineation that defines the up-dip limit of the seismogenic zone. All events have either entirely or obliquely thrusting focal mechanism which is consistent with compressional stick-slip behavior along the subduction interface. The yellow event (Ml=3) and 4 aftershocks are obliquely thrusting and occur at the bend in the seismogenic zone. References: DeShon, H. R., S. Y. Schwartz, L. M. Dorman, A. V. Newman, V. Gonzalaz, M. Protti, T. Dixon, E. Norabuena & E. Flueh, Seismogenic Zone Structure along the Middle America Trench, Nicoya Peninsula, Costa Rica, from 3D local earthquake tomography using P- and S-wave data , Geoph. Journ. Int., [In Review]. Norabuena, E., T. H. Dixon, S. Y. Schwartz, H. R. DeShon, A. V. Newman, M. Protti, V. Gonzalez, L. M. Dorman, E. Flueh, P. Lundgren, 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 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 Avants, M., S. Schwartz, A. V. Newman, H. DeShon, M. Protti & F. Guendel, Large Underthrusting Earthquakes Beneath the Nicoya Peninsula, Costa Rica, Trans. Amer. Geophys. Un. (EOS), 82, Fall 2001 Newman, A. V., S. Schwartz, H. Deshon, D. Sampson, M. Protti, V. Gonzales, F. Guendel, L. Dorman & M. Avants, Characteristics of the Nicoya Peninsula, Costa Rica Seismogenic Zone from Microseismicity, Trans. Amer. Geophys. Un. (EOS), 82, Fall 2001 Ranero, C.R., & R. von Huene, Subduction erosion along the Middle America convergent margin, Nature, 404, 748-752, 2000. Schwartz, S. Y., A. V. Newman, M. Protti, & M. Vallee, A Large Tensional Outer-Rise Earthquake in the Nicoya Seismic Gap, Costa Rica, Trans. Amer. Geophys. Un. (EOS), 82, Fall 2001 Thurber, C. H., Hypocenter-velocity structure coupling in local earthquake tomography, Phys. Earth. Planet Interiors, 75, 55-62, 1992. Please check out Heather DeShon's CRSEIZE website for more information. Current Research | Curriculum Vitae | Sites of interest | Home anewmangatech.edu | Updated: Sun Nov 5 13:41:15 EST 2006