Investigaciones Sismología

Abstract: The earthquake of 20 November 2004 was located north of Damas Island in the Pacific coast of Costa Rica, within the Costa Rica Deformed Belt. The earthquake was located at 24 km depth and reported with a magnitude (Mw) of 6.4 and a strike-slip mechanism with a large normal dip-slip motion. This mechanism agrees with mapped faults in the area that suggest transtensional deformation on the forearc and the entire western boundary of the Panama microplate. Aftershock locations do not delineate a preferable plane to distinguish between the two nodal planes and are distributed between 15 and 25 km depth. The slip distribution during the mainshock, modeled after teleseismic and local data, pictured a circular rupture 8 km in radius and 0.25 m of average displacement. The fault plane cannot be distinguished from the two nodal planes from the slip distribution because of the lack of directivity and resolution for this magnitude earthquake. Weak evidence from empirical Green’s function analysis suggests that the dextral northwest-oriented fault
could be the causative fault. Depth to the top of the slab, hypocenter location of the mainshock, its slip distribution, depth distribution of the aftershocks, and Quaternary fault activity at the surface suggest that deformation takes place throughout the whole thickness of the crust. This extended deformation might be caused by seamount subduction and strong basal friction on the upper plate, due to subduction of a thick, young, and buoyant oceanic plate, rough seafloor, and underplating of large seamounts.

Abstract:Tomographic images constrain the existence of a subducted seamount beneath the Gulf of Nicoya, Costa Rica. The subducted seamount is found at a depth of 30 km within the rupture area of
the March 25, 1990, Mw = 7.0 Gulf of Nicoya earthquake. The Gulf of Nicoya earthquake was a typical thrust-type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting Cocos plate and the overriding plate. Precise relocation of the mainshock and its aftershocks in a 3-D P-wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount. Most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate. We interpret the subducted seamount as an asperity whose rupture caused the 1990 Gulf of Nicoya earthquake. INDEX TERMS: 1734 History of Geophysics: Seismology; 6982 Radio Science:Tomography and imaging; 7209 Seismology: Earthquake dynamics and mechanics; 7230 Seismology: Seismicity and

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