Seismic cycle and plate margin deformation in Costa Rica : GPS observations 1994 - 1997

GPS observations in Costa Rica from 1994-1997 reveal a complex pattern of motion consistent with the superposition of seismic cycle and secular plate margin deformation. In the south, velocity vectors are consistent with motion of the Panama Block plus postseismic deformation following the 1991 Limon earthquake, and interseismic strain due to partial locking of the Middle America Trench (MAT) thrust. In the northwest, sites west of the volcanic arc are moving to the NW as a forearc sliver. Superimposed on this sliver motion are vertical and horizontal interseismic deformations from the adjacent Nicoya segment of he MAT. We apply two different inverse methods to understand the source of the seismic strain in NW Costa Rica. We compare fault locking models derived using a singular value decomposition inversion with that of a simulated annealing global optimization approach. Both methods yield similar models for partial locking of the thrust interface beneath the Nicoya Peninsula. Our results define an area of nearly fully locked fault beneath the outer coast of the southern portion of the peninsula, with somewhat lower coupling beneath the northern half, and with low coupling elsewhere. These initial results show the promise for detailed imaging of the locked portion of a thrust interface responsible for future large subduction zone earthquakes. Introduction Costa Rica lies in a complicated tectonic setting featuring fast subduction, deformation of the Caribbean Plate margin, recent large earthquakes (Fig. l), and the unusual location of the Nicoya Peninsula, which lies near the Middle America Trench (MAT) directly above the seismogenic zone of the subduction underthrust [Munn et al., 19901. Off the southwest coast of Costa Rica the