Figure 1. Bathymetry of the northeast corner of
the Caribbean plate. Click on image for larger view and more
The Puerto Rico Trench is the deepest part of the Atlantic Ocean, with water
depths exceeding 8,300 meters (figure 1). Its depth is comparable to the
deep trenches in the Pacific Ocean. Trenches in the Pacific are located
in places where one tectonic plate subducts or slides under another one.
The Puerto Rico Trench, in contrast, is located at a boundary between two
plates that slide past each other with only a small component of subduction.
The trench is less deep where the component of subduction is larger.
The unusually deep sea floor is not limited to the trench, but also extends
farther south toward Puerto Rico. The Puerto Rico Trench is also associated
with the most negative gravity anomaly on earth, -380 milliGal, which indicates
the presence of an active downward force. Finally, a thick limestone platform,
which was originally deposited in flat layers near sea level, is now tilted
northward at a uniform angle. Its northward edge is at a depth of 4,200 m,
and its southern edge can be found on land in Puerto Rico at an elevation of
a few hundred meters. Many tectonic models have been proposed to explain this
geologically fascinating, tectonically active region; however, none has gained
acceptance, and the region remains poorly understood, largely because its
underwater location makes it difficult to study.
Figure 2. Location of earthquakes as a function of depth and size in
the northeastern Caribbean. Click on image for larger view and more
Puerto Rico, the Virgin Islands to its east, and eastern Hispaniola to
its west, are located on an active plate boundary zone between the North
American plate and the northeast corner of the Caribbean plate (figure 1).
The Caribbean plate is roughly rectangular, and it slides eastward at
about 2 cm/yr relative to the North American plate. Motion along its
northern boundary (in the plate boundary zone region) is dominantly
strike-slip (sideways motion between the plates), with a small component
of subduction (one plate sinks under the other plate). In contrast, the
Caribbean plate farther east overrides the North American plate, creating
the island arc of the Lesser Antilles with its active volcanoes. There
are no active volcanoes in Puerto Rico and virgin islands.
The geologic settings of Puerto Rico and the Virgin Islands have created
or contributed to several pressing societal issues related to human safety,
environmental health, and economic development. Over 4 million U.S. citizens
live on these islands, mostly along the coast. Because the island lies on
an active plate boundary, earthquakes are a constant threat, and the densely
populated coastal areas are vulnerable to tsunamis. Erosion is a concern in
many coastal areas, but is particularly serious to island economies that
rely heavily on the tourist industry.
Figure 3. Probability for damaging ground motion at
San Juan and Mayaguez in comparison to other seismic high-risk areas
in the U.S. (e.g. Seattle). Click on image for larger view and
The region has high seismicity and large earthquakes
(figure 2). Examples include a magnitude 7.5 earthquake centered northwest
of Puerto Rico in 1943, and magnitude 8.1 and 6.9 earthquakes north of
Hispaniola in 1946 and 1953, respectively. Historically, other large
earthquakes have also struck the area, such as one in 1787 (magnitude~8.1),
possibly in the Puerto Rico Trench, and one in 1867 (magnitude~7.5) in
the Anegada Trough (figure 1). A draft U.S. Geological Survey (USGS)
hazard map places equal probability for damaging ground motion for
Mayaguez in western Puerto Rico as for Seattle, Washington. Other Puerto
Rican cities also have substantial risk.
Figure 4. Numerical simulation of the 1918 Puerto
Rico tsunami showing calculated inundation of the Aguadilla coast in
northwest Puerto Rico. Click on image for larger view and more
The hazard from tsunamis is also apparent. Immediately
after the 1946 earthquake, a tsunami struck northeastern Hispaniola
and moved inland for several kilometers. Some reports indicate that
nearly 1,800 people drowned. A 1918 magnitude 7.5 earthquake resulted
in a tsunami that killed at least 40 people in northwestern Puerto Rico
(figure 4). Eyewitness reports of an 1867 Virgin Islands tsunami gave
a maximum wave height of >7 m in Frederiksted, St. Croix, where a large
naval vessel was left on top of a pier. Essentially, all of the known
causes of tsunamis are present in the Caribbean -- earthquakes, submarine
landslides, submarine volcanic eruptions, subaerial pyroclastic flows
into the ocean, and major tsunamis called teletsunamis. Because of its
high population density and extensive development near the coast, Puerto
Rico has a significant risk for earthquakes and tsunamis.