That the Caribbean is an area of much recent seismicity can be seen in the graphic below from the USGS:
Note that this event occurred on a transform boundary.
NPR describes the tectonics of the region:
... the Caribbean plate is moving east in relation to the North American plate. Large earthquakes frequently occur on these plate boundaries.
The Caribbean plate has been moving about a quarter of an inch per year, relative to the North American plate. But the two plates don't simply glide past one another. Strain builds up along faults at the plate boundaries, until it's released in a sudden burst of energy. That's an earthquake.
There are two major faults along Hispaniola, the island shared by Haiti and the Dominican Republic. This earthquake occurred on the southern fault, the Enriquillo-Plantain Garden fault system.There hasn't been a major quake on this system for about 200 years. That means stress has been building up there for quite some time. When the strain finally grew too large, rock along the fault failed, and released a huge burst of energy in less than a minute.
... it appears that 30 to 60 miles of the fault gave way. That not only triggered the original quake but has also generated more than a dozen aftershocks of magnitude 5 or higher. Those are also strong quakes, and they pose a risk to the buildings that were damaged in the original shock.A ScienceDaily news article from 5 years ago summarizes research on past and inevitable future large quakes and resulting tsunamis in the region:
ScienceDaily (Feb. 8, 2005) — A dozen major earthquakes of magnitude 7.0 or greater have occurred in the Caribbean near Puerto Rico, the U.S. Virgin Islands and the island of Hispaniola, shared by Haiti and the Dominican Republic, in the past 500 years, and several have generated tsunamis. The most recent major earthquake, a magnitude 8.1 in 1946, resulted in a tsunami that killed a reported 1,600 people.
With nearly twenty million people now living in this tourist region and a major earthquake occurring on average every 50 years, scientists say it is not a question of if it will happen but when. They are calling for the establishment of tsunami early warning systems in the Caribbean Sea, Gulf of Mexico and Atlantic Ocean, and better public education about the real tsunami threats in these regions.
In a new study published December 24, 2004 in the Journal of Geophysical Research from the American Geophysical Union, geologists Uri ten Brink of the U.S. Geological Survey in Woods Hole and Jian Lin of the Woods Hole Oceanographic Institution (WHOI) report a heightened earthquake risk of the Septentrional fault zone, which cuts through the highly populated region of the Cibao valley in the Dominican Republic. In addition, they caution, the geologically active offshore Puerto Rico and Hispaniola trenches are capable of producing earthquakes of magnitude 7.5 and higher. The Indonesian earthquake on December 26, which generated a tsunami that killed (to date) an estimated 150,000 people, came from a fault of similar structure, but was a magnitude 9.0, much larger than the recorded quakes near the Puerto Rico Trench.
The Puerto Rico Trench, roughly parallel to and about 75 miles off the northern coast of Puerto Rico, is about 900 kilometers (560 miles) long and 100 kilometers (60 miles) wide. The deepest point in the Atlantic Ocean, the trench is 8,340 meters (27,362 feet) below the sea surface. The Hispaniola Trench parallels the north coast of the Dominican Republic and Haiti, and is 550 kilometers (344 miles) long and only 4,500 meters (14,764 feet) deep...
Hispaniola, Puerto Rico and the U.S. Virgin Islands sit on top of small crustal blocks that are sandwiched between the North American and Caribbean plates. The island of Hispaniola faces a double risk: an earthquake from the Septentrional fault on the island itself as the plates move past each other, and an earthquake deep in the earth in the subduction zone on which the island sits. Both could cause severe damage and loss of life, although the researchers say an earthquake in the subduction zone could be more devastating and has the potential to cause a tsunami...
The Puerto Rico Trench, which is capable of producing earthquakes of magnitude 7 to 8 or greater, faces north and east into the Atlantic Ocean. There are few land areas or islands to block a tsunami generated near the Puerto Rico Trench from entering the Atlantic Ocean. The direction of the waves would depend on many factors, including where in the trench the earthquake occurred.Chris Rowan also has a nice Highly Allochthonous blog entry on this quake.