Inter-plate coupling in the Nicoya Peninsula, Costa Rica, as deduced from a trans-peninsula GPS experiment

摘要

We investigated the state of plate coupling at the subduction zone beneath the Nicoya Peninsula, northwestern Costa Rica, from 1.5 years of trans-peninsula GPS campaign experiments. This area is recognized as a seismic gap located between the rupture areas of two M ≥ 7 earthquakes that occurred in the early 1990s, and it has not ruptured since 1950. We carried out campaign GPS observations beginning in autumn 2001 to obtain the profile of site velocities across the peninsula for the interseismic period. The obtained velocity field indicates a strong coupling at the plate interface. However, velocity directions deflect counterclockwise, which suggests a trench-parallel forearc sliver motion of Nicoya Peninsula at ～8.5 mm/year. An inversion analysis to infer inter-plate coupling at Nicoya seismic gap shows that the strongly coupled zone agrees well with seismogenic zone inferred from seismicity. Cumulative slip deficits since 1950 at the Nicoya seismic gap would produce a thrust earthquake as large as M_W ≥ 7.5.