Previous GPS results

The GPS data collected in the norheastern Caribbean over the past decade has been used first to show that the Caribbean plate is currently moving east-northeastward (070$^o$) at a rate of 18 mm/yr with respect to North America (DeMets et al., 2000). This rate is 65% faster than the Nuvel-1A plate motion model of DeMets et al. (1994). The plate motion direction implies maximum oblique plate convergence centered on the topographically elevated and seismogenic island of Hispaniola (Figure 2.1).

GPS velocities in Puerto Rico, St. Croix, and the Lesser Antilles show that these areas move largely as a single block part of the Caribbean plate, consistent with the presence of a little deformed Oligocene-early Pliocene carbonate platform over much of that area (Figure 3.1; Jansma et al., 2000; Mann et al, 2002). Earthquake slip vectors at the Puerto Rico trench are parallel to the Caribbean-North America relative plate motion, indicating oblique subduction without strain partitioning (Figure 2.1).

GPS velocities in central and eastern Hispaniola (Dominican Republic) show a drastically different pattern with significant residuals with respect to the Caribbean plate. This pattern indicates elastic strain accumulation on the major faults described above and full partitioning of the oblique Caribbean-North America plate motion into arc-normal shortening on the North Hispaniola fault and arc-parallel strike-slip on the Septentrional and Enriquillo faults (Calais et al., 2002; Mann et al., 2002). Strain partitioning is further evidenced by the rotation of subduction earthquake slip vectors in a trench-perpendicular direction at the longitude of Hispaniola (Figure 2.1).

Figure 3.1: GPS-derived velocities in the Dominican Republic and Puerto Rico with respect to the Carribean plate. Ellipses are 95% confidence. Black vectors: GAMIT solution, Purdue Univ.; Grey vectors: GIPSY-OASIS solution, Univ. Arkansas. The large uncertainties at most sites is due to the fact that they have only been measured twice 2 years apart.

The analysis of the GPS data collected in the Dominican Republic from 1994 to 2001 show that the oblique convergence between the North American and Caribbean plates is partitioned between 5.2$\pm$2 mm.yr$^{-1}$ of dip-slip reverse motion on the North Hispaniola thrust, and 12.8$\pm$2.5 mm.yr$^{-1}$ and 9.0$\pm$9.0 mm.yr$^{-1}$ of left-lateral slip on the Enriquillo and Septentrional faults, respectively. Integrated with paleoseismology data, these results indicate that seismic hazard is currently high on the Septentrional fault. However, other less well known faults such as the Enriquillo fault in southern Hispaniola, which runs a few kilometers south of the Haitian capital of Port-au-Prince, may also be potential sources of significant earthquakes and merit additional studies.