Caribbean Tsunami and Earthquake Hazards Studies

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Puerto Rico and the Virgin Islands are located at an active plate boundary between the North American plate and the northeast corner of the Caribbean plate. Plate movements have caused large magnitude earthquakes and devastating tsunamis, but scientists have so far failed to explain the deformation of this complex region in a coherent and predictable picture, and this has hampered their ability to assess the seismic and tsunami hazards. It is as if we would try to assess earthquake hazards in California without knowing of the existence of the San Andreas Fault system and its rate of motion.

 

Map of the North American - Caribbean tectonic plate boundary

Map of the North American - Caribbean tectonic plate boundary. Colors denote depth below sea level and elevation on land. Bold numbers are the years of moderately large (larger than about M7) historical earthquakes written next to their approximate location. Asterisk - Location of the January 12, 2010 earthquake. Barbed lines- boundary where one plate or block plunges under the other one. Heavy lines with half arrows - faults along which two blocks pass each other laterally. Click on image to return to Introduction page

The risk to life and economic infrastructure is high, because 4 million U.S. citizens live along the coastlines of Puerto Rico and the Virgin Islands. The objective of this project is to provide the understanding needed to approach the problem of assessment, education and mitigation of tectonic hazards in Puerto Rico and the Virgin Islands. By determining the likely hazards and their causative mechanisms and providing this information to government agencies and the public we may aid in such activities as improvement of building codes, encouraging safer zoning, and assisting public education for response to hazards.

In the 20th century alone there have been several very large earthquakes north of Puerto Rico (Ms 7.3 in 1918; Ms 7.8 in 1943; Ms 8.0 in 1946 and four major aftershocks of Ms 7.6, 7.0, 7.3, 7.1 between 1946 and 1953). Large tsunamis have also hit Puerto Rico and Hispaniola, reportedly killing 1800 people in 1946 and 40 people in 1918. Images of the slope north of Puerto Rico disclose massive slope failure scars, as much as 50 km across, that probably generated tsunamis along the north shore of the island. Other margins of the island (west, south, and south west) are also associated with massive tectonic features and may pose addtional hazard.

Location of earthquakes as a function of depth and size in the northeastern Caribbean.

Location of earthquakes as a function of depth and size in the northeastern Caribbean.

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.

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. 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.

Perspective view of the seafloor of the Atlantic Ocean and the Caribbean Sea.

Perspective view of the seafloor of the Atlantic Ocean and the Caribbean Sea.

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.

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.

Bathymetry of the northeast corner of the Caribbean plate

Bathymetry of the northeast corner of the Caribbean plate

 

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