Characterization of the Central Costa Rican Pacific Seismogenic Zone from an ``Amphibious'' Local Seismological Network

The convergent margin along the Middle America Trench off Costa Rica has been the focus of the first stage of the project SFB574 ``Volatiles and Fluids in Subduction Zones'', which aims to better understand the processes involved in the exchange of fluids on erosive margins. The central Pacific region of Costa Rica has the highest seismicity rate along this sector of the trench, coinciding with the subduction of rough-relief ocean floor, which includes seamounts and ridges. The area has generated earthquakes with magnitude up to seven in the past, most recently a Mw-6.9 event in 1999 and a Mw-6.4 in 2002. Precise local earthquake locations and detailed knowledge of the three-dimensional velocity structure provide valuable insights into the dynamics and structure of the shallow part of the subduction zones, including the limits and geometry of the seismogenic zone. As part of the project, 23 ocean bottom sensors and 15 land stations were deployed from April to October 2002 in an area where a subducted seamount has been detected. Additional readings from 10 of the permanent stations of the Red Sismológica Nacional (RSN) were included in the database improving the coverage to the expected downdip end of the seismogenic zone. More than 2000 earthquakes were recorded in the target area over the six-month period. Most of the earthquakes originated offshore, beneath the continental slope and along the plate interface. Fewer were generated close to trench and within the subducting slab. To achieve precise earthquake locations and to serve as an initial model for local earthquake tomography, we derived a minimum-1D model using a very high-quality subset of 262 events with 5004 P-wave arrivals with the lowest reading uncertainty (±0.05 s). This model was applied to relocate all recorded earthquakes, and a new subset of 540 events was selected for the 3D inversion of seismic velocities. We present the characteristics of the seismogenic zone based on the tomographic results and precise hypocenter locations, and review the results in the light of heat flow and controlled source seismology data for this area.