Three-dimensional velocity structure of the upper mantle beneath Costa Rica from a teleseismic tomography study

Abstract

A tomographic study has been carried out in Costa Rica utilizing teleseismic events recorded by the national seismic network OVSICORI-UNA. The data consist of 130 earthquakes which occurred at epicentral distances of 23°–100° (P phases) and 110°–180° (PKP phases). 1378 IASP91 traveltime residuals were computed and inverted using an iterative linearized inversion procedure which incorporates 3-D minimum-traveltime ray tracing. The 3-D velocity structure of the upper mantle, as derived from teleseismic tomography, shows a northeastward-dipping high-velocity body beneath northern Costa Rica, interpreted as the subducting Cocos plate. Although less evident, a similar pattern of lateral heterogeneities characterizes the deep structure of central Costa Rica. There is no evidence for deep high-velocity bodies beneath southern Costa Rica. The 3-D reconstruction of the subducting Cocos Plate shows a steeply dipping slab (subvertical) beneath northern Costa Rica. Here the oceanic lithosphere is older and denser than in central Costa Rica, where the slab dips at about 60°. Southern Costa Rica does not show a dipping slab; this is in accordance with the subduction/collision of the thick and buoyant Cocos Ridge. The geometry of the slab, the maximum velocity anomalies and the sudden change in the local seismicity depth pattern suggest the existence of a tear in the subducted plate, located between the northern and central Costa Rican subduction zones. The strain field in the overriding plate indicates a strong structural control induced by the along-trench variations in subduction. Northern Costa Rica shows slight tensile deformation, while southern Costa Rica displays moderate to strong compressive deformation. The maximum horizontal gradient of the strain field is located in central Costa Rica where a sinistral transcurrent fault system crosses the country from the Pacific to the Atlantic coast. This system is likely to be generated by the along-trench variations in subduction.