Seismic cycle and plate margin deformation in Costa Rica: GPS observationsfrom 1994 to 1997

Global Positioning System (GPS) observations in Costa Pica from 1994 to 199 7 reveal a complex pattern of motion consistent with the superposition of s eismic cycle and secular plate margin deformation. In the south, velocity v ectors are consistent with motion of the Panama Block plus postseismic defo rmation 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 are 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 the MAT. We apply two dif ferent inverse methods to understand the source of the seismic strain in NW Costa Pica. We compare fault-locking models derived using a singular value decomposition inversion with that of a simulated annealing global optimiza tion 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 portio n 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 f or future large subduction zone earthquakes.