Puerto Rico and the Virgin Islands are located near the northeastern corner of the Caribbean seismic zone. Numerous large earthquakes have struck these islands, some with disastrous results. The 400-yr-long earthquake record of Puerto Rico describes shocks affecting nearly all portions of the island. The last destructive shock, in 1918 (7.5 MS), did not occur along the main seismic zone, but rather on an intraplate fault near Mona Canyon off the northwest coast. A possible great earthquake in 1787 (8 to 8.25 MS) appears to have occurred along the main seismic zone near the Puerto Rico Trench to the north of the island, but data for the event are scarce.
A disastrous earthquake in the Virgin Islands (1867, 7.5 MS) also occurred on an intraplate fault. This fault is one of a series that bound the Anegada Trough separating Saint Croix from the main chain of the Virgin Islands.
Seafloor morphology, microearthquakes, and the record of historic earthquakes define a zone of deformation extending from the Puerto Rico Trench, northeast of the Virgin Islands, trending southwest along the Anegada Trough and then westerly along the Muertos Trough. The Muertos Trough is the locus of convergence between the floor of the Caribbean Sea and Puerto Rico. Maximum dimensions of future large earthquakes are inferred from sizes of blocks in the Anegada and Muertos Troughs as well as Mona Canyon. Shocks as large as 7.5 can occur on these intraplate faults. Although strain rates on these faults may be an order of magnitude less than on faults in the Puerto Rico Trench, the large number of potential sources suggest that damaging earthquakes in this part of the Caribbean can come from either the Puerto Rico Trench or the intraplate faults with nearly equal probability.