Influence of the intra-plate geometry on earthquake nucleation: insights from the seismic and tectonic structure of Southern Costa Rica

Abstract

We present a wide-angle seismic (WAS) transect coincident with a multi-channel seismic (MCS) profile acquired along the Southern Costa Rica convergent margin, offshore Osa Peninsula. The WAS transect is 104 km-long and includes 19 OBS/OBH with an average separation of ~5 km. We have performed a joint refraction and reflection travel-time inversion of the WAS data following a layer-stripping procedure. Beneath a sedimentary cover of variable thickness, the seismic structure of the overriding plate can be separated into three different domains: (I) the frontal domain; corresponding to ~33 km-wide zone starting at the trench characterized by low velocities ranging between 3.8 and 4.5 km/s, an average vertical velocity gradient of 0.17 s-1, and a horizontal gradient of 0.02 s-1; (II) a 15 km-wide transition zone with velocities between 4.5 to 5.5 km/s ; and (III) a 9 km-thick and 40 km-wide crustal section in the NE with velocities between 4.5 and 6.5 km/s. The subducting plate shows a rough topography and a laterally-varying oceanic-like velocity structure, with a crustal thickness decreasing from 12-13 km just below the trench to 6-7 km-thick below the mid continental slope. The MCS profile shows characteristic features in the previously described overriding plate domains: (I) a 8 km-wide accretionary wedge in the frontal domain, and layered sequences that have been shortened by thrust faulting and folding and imbricately stacked showing internal stratification; (II) a highly fractured transitional zone showing landward-dipping reflectors and active deformation in the sedimentary cover; and (III) a northern zone showing rigid deformation, small extensional basins and tilted blocks overlain by ~2 km of sediments. Our data in combination with numerical modelling show that the 2002 Mw 6.4 Osa Earthquake and the subsequent aftershocks have possibly been triggered in front of two subducting seamounts due to the main tectonic stress concentration