HR: 1330h
AN: S52F-0174    [PDF]
TI: Local Seismicity at the Cocos Ridge - Osa Peninsula Subduction Zone, Costa Rica
AU: Arroyo, I G
EM: iarroyo@geomar.de
AF: Marine Geodynamics - GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Arroyo, I G
EM: iarroyo@geomar.de
AF: Red Sismologica Nacional, University of Costa Rica, San Jose, 214 - 2060 Costa Rica
AU: Arroyo, I G
EM: iarroyo@geomar.de
AF: SFB 574, University of Kiel, Kiel, 24148 Germany
AU: Alvarado, G E
AF: Red Sismologica Nacional, University of Costa Rica, San Jose, 214 - 2060 Costa Rica
AU: Alvarado, G E
AF: SFB 574, University of Kiel, Kiel, 24148 Germany
AU: * Flueh, E R
EM: eflueh@geomar.de
AF: Marine Geodynamics - GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: * Flueh, E R
EM: eflueh@geomar.de
AF: SFB 574, University of Kiel, Kiel, 24148 Germany
AB: A local seismological network was operated from May, 1998 to November, 2001 to monitor the seismic activity in southern Costa Rica, where the Cocos Ridge is currently being subducted under the Panama block at the terminus of the Middle America Trench (MAT), leading to a rapid uplift of the Osa Peninsula. Although previous works assumed a shallow subduction angle (20-30$\deg$) or no evidences for subduction, the new seismological data show for the first time a well-defined, thick steepslab (50-55$\deg$ at depths $>$ 40 km), reaching depths up to 67 km, 60-75 km landwards from the trench. With this configuration, the subducting ridge could not be projected underneath the Caribbean coast, thus it is difficult to explain the uplift of the Cordillera de Talamanca with current models. Besides, the Talamanca adakites, located betweeen 80 to 110 km over the down-dip aseismic projection of the subducted slab, could not be produced by flat subduction, as previously suggested. A better constraining of the Cocos Ridge arrival time (models varying from 0.5 to 8 m.y.) is crucial to associate the observed geological features with the slab-window, crustal thickening by shortening due to the collision of the Panama block, or other models. The present work discuss these models in the framework of new seismological and geological data. Anomalously deep intracrustal seismicity (up to 40 km) in the Panama block was detected below the thrust belt of Fila Coste¤a and the piedmont of the Cordillera de Talamanca. It could indicate an anomalously cold crust with no astenospheric wedge underneath, and perhaps continuous crustal shortening. N-S dextral strike-slip faults and NW-SE thrust faults and related seismicity result from the complex stress pattern generated by the indentation effect of the Cocos Ridge in an oblique subduction, together with the Panama block movement. At least 4 strong destructive subduction-related earthquakes (M$>$7) in the last two centuries, and two more in the last century related to the Panama Fracture Zone, together with several seismic sequences, represent the potential seismic hazard of the region.
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting