DeMets, Charles; Wiggins-Grandison, Margaret
Author Affiliation, Ana.
Deformation of Jamaica and motion of the Gonāve microplate from GPS and seismic data
Geophysical Journal International
Date of Publication
We use velocities from 20 GPS sites on the island of Jamaica and seismic data from the Jamaican Seismic Network to quantify motion of the proposed Gonāve microplate and study deformation and earthquake hazard in Jamaica. All 20 Jamaican GPS sites move significantly relative to both the Caribbean and North American plates, thereby confirming the existence of the Gonāve microplate. In a Caribbean Plate reference frame, the fastest site velocities in Jamaica are 8 ± 1 mm yr-1, representing a minimum estimate for the rate of Gonāve-Caribbean Plate motion. We estimate a firm upper limit of 13 ± 1 mm yr-1 for Gonāve-Caribbean Plate motion assuming that additional elastic or permanent deformation occurs north of Jamaica beyond the reach of our GPS network. The range of estimates for Gonāve-Caribbean Plate motion exceeds the 3-7 mm yr-1 range of geologically derived rates for post-10 Ma fault slip in Jamaica, suggesting that motion has either increased relative to the long-term deformation rate or that estimates of the long-term deformation rate are too slow. Minimum and maximum rates for Gonāve-North America Plate motion are 6 ± 1 mm yr-1 and 11 ± 1 mm yr-1 based on the differences between the estimated Gonāve-Caribbean rates and a recent GPS-based estimate of 19.3 ± 1 mm yr-1 of Caribbean-North American Plate motion in Jamaica. These are slower than 15-20 mm yr-1 published estimates of the long-term seafloor spreading rate across the Cayman spreading centre, which records Gonāve-North American Plate motion. Our observations are thus consistent with a model in which Gonāve-Caribbean Plate motion has increased through time while Gonāve-North America Plate motion has decreased, possibly reflecting a progressive transfer of slip from plate boundary structures that accommodate Gonāve-North America motion to faults that carry Gonāve-Caribbean Plate motion. In Jamaica, GPS site directions in both the Caribbean and North American Plate reference frames are parallel to the Caribbean-North America Plate slip direction and are only 10-20° different from the azimuths of the island's major strike-slip faults. The mean P- and T-axes for 48 earthquakes since 1941 have near-horizontal plunges and are oriented 45° from the predicted plate slip direction. The kinematic and seismic data thus indicate that deformation on the island is dominated by left-lateral shear along largely E-W-trending strike-slip faults. Relative to the Caribbean Plate, the Jamaican GPS velocities exhibit a nearly monotonic increase in site velocities from south to north along a transect orthogonal to the island's major E-W faults. This velocity gradient likely reflects the accumulation of elastic strain due to frictional locking of the major E-W faults. Velocities however also increase monotonically from the WSW to ENE along a transect orthogonal to the island's numerous NNW-striking faults, consistent with the accumulation of significant elastic shortening along those faults and supportive of an interpretation of those faults as restraining bends. Seismic hazard assessment based on the GPS-derived deformation budget, likely maximum fault rupture lengths, and the historical seismic record suggests approximate repeat times of one to several centuries for earthquakes of maximum magnitudes of Mw= 7.0-7.3 . The strain that has accumulated since the destructive 1692 earthquake near the capital city of Kingston is presently sufficient to release a Mw= 7.2 earthquake in eastern Jamaica.....