GPS Measurements Athe Northern Caribbean Plate Boundary Zone: Impact of Postseismic Relaxation Following Historic Earthquakes

GPS measurements in the northern Caribbean suggest that the rate of Caribbean plate motion relative to North America is about 10 mm/yr faster than predicted by global plate motion model NUVEL-1A. Several of the key sites used in the GPS study are located in the Dominican Republic, near the rupture zones of large earthquakes in 1946 and in the previous two centuries. Postseismic relaxation of the crust and upper mantle is a possible explanation for the plate velocity discrepancy. We explore a range of fault mechanisms and crustal and mantle rheology to place an upper bound on postseismic relaxation effects. The upper bound velocity contribution in the southern Dominican Republic is 5–6 mm/yr, and the most plausible contribution is 1–2 mm/yr, suggesting that postseismic effects cannot account for the discrepancy. This implies that the NUVEL-1A model underestimates the rate of motion of the Caribbean plate