A 23-day marine geophysical expedition in the summer of 1996 revealed a previously unidentified strike-slip fault zone within 60–100 km of the densely populated northern coast of Puerto Rico. The purpose of the expedition, which was held aboard the U.S. research vessel Maurice Ewing, was to map the northeastern portion of the North America Caribbean plate boundary.
The 3000-km-long North America-Caribbean plate boundary stretches from Guatemala to the Lesser Antilles arc. The dominantly left-lateral strike-slip boundary accommodates slow (1–2.6 cm/yr) eastward motion of the Caribbean plate relative to North America. Distributed interplate motion in the region generates large earthquakes (Figure 1, inset). In 1976, an M 7.5 left-lateral, strike-slip earthquake in Guatemala killed approximately 23,000 people and left a quarter of the nation's population homeless.