"Many earthquakes and tsunamis have occurred in the northeastern Caribbean, where the movements of the Earth's surface plates are rapid and complicated. Future such events pose serious hazards to the 3.7 million people who live in Puerto Rico and the U.S. Virgin Islands. The Caribbean is one of the smaller surface plates of the Earth. The approximately rectangular plate extends from Central America on the west to the Lesser Antilles on the east, and from just south of Cuba on the north to South America on the south (fig. 1). Earthquakes occur all around its periphery, and volcanoes erupt on its eastern and western sides. This plate remains at a fixed spot relative to the deep Earth, while the North American plate, which includes the continent of North America and the western North Atlantic ocean basin, is being shoved westward. The motion between these plates is rapid for a geological process, at about 2 centimeters/year (about 1 inch/year)."
U.S. Geological Survey: http://www.usgs.gov/