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Earthquakes and Tsunamis in Puerto Rico and the U.S. Virgin Islands
90oW 80oW 70oW 60oW 30oN Atlantic Ocean
o Cuba 20 N WestWest Bahamas Caribbean Hispaniola Sea
10oN Landslide arib Caribbean North Plate Muertos Trough Tilted Platform Figure 1. Morphology and color-shaded North bathymetry of the northeastern Caribbean Puerto P u e region looking toward the west. Digital Rico r to data courtesy of Troy Holcombe and Lisa R i Taylor, National Geophysical Data Center, c Virgin Islands o Boulder, Colo. From the International T Trough r Bathymetric Chart of the Caribbean Sea e North American n and the Gulf of Mexico, sheet 1–09, pub- c h Plate lished by Instituto Nacional de Estadística, Geografía e Informática, Mexico, 1992. The boundary of the Caribbeanouth plate is Anegada Trough outlined in red on the location map. South Arc South Antilles Introduction EastEast Many earthquakes and tsunamis have occurred in the northeastern Caribbean, where the movements of the Earth’s surface plates are rapid and com- plicated. Future such events pose serious westward. The motion between these North of Puerto Rico is the Puerto hazards to the 3.7 million people who plates is rapid for a geological process, Rico Trench, the deepest part of the live in Puerto Rico and the U.S. Virgin at about 2 centimeters/year (about Atlantic Ocean and the place where the Islands. 1 inch/year). pull of gravity at sea level is less than The Puerto Rico/Virgin Islands anywhere else on Earth (free-air gravity Plate Movements region is located at the northeastern cor- anomaly). The stresses are holding part The Caribbean is one of the smaller ner of the Caribbean plate where motions of the North American plate down to surface plates of the Earth. The approxi- are complex. The westward-moving form the trench. The collision has also mately rectangular plate extends from North American plate is being driven caused the tilting of the limestone plat- Central America on the west to the Lesser under the Antilles Arc where volcanism is form deposits of northern Puerto Rico Antilles on the east, and from just south active. On the north side of the plate cor- during the last 3.3 million years. These of Cuba on the north to South America ner, the North American plate slides past formerly horizontal layers, deposited near on the south (fig. 1). Earthquakes occur the Caribbean (fig. 1), but irregularities in the water surface, now are tilted strongly all around its periphery, and volcanoes the plate boundaries cause stresses that to the north and have subsided to depths erupt on its eastern and western sides. result in a complicated underthrusting of of more than 4.5 kilometers (2.8 miles). This plate remains at a fixed spot relative plate fragments. The interaction of plates Sea-floor images collected by the U.S. to the deep Earth, while the North Ameri- causes the volcanism of the Antilles Arc Geological Survey (USGS) show that can plate, which includes the continent of on the eastern boundary of the Caribbean great slabs of limestone, as much as 70 North America and the western North plate and creates major stresses all along kilometers (43 miles) wide, have broken Atlantic ocean basin, is being shoved the northern boundary. off and slid into the Puerto Rico Trench.
U.S. Department of the Interior USGS Fact Sheet FS–141–00 U.S. Geological Survey April 2001 fs141-00 4/12/01 8:26 AM Page 2
Figure 2. Block diagram showing loca- o 20oN tions of earthquake foci beneath the 16.5 N 69 o W northeastern Caribbean from 1963 to pre-
sent. The earthquake foci are projected orthogonally onto the sides and top of 0 North the block. Earthquake locations provided km 50
Puerto Rico by Robert Van der Hilst, Massachusetts 100 Institute of Technology.
63.5 o W 50
NORTH ? 100 Depth (km) Caribbean ? North American Plate Events Plate Events 250 200 150
Figure 3. Conceptual drawing of the geometry Table 1. Large earthquakes near Puerto mate where earthquakes and tsunamis and relative motion of the major plates of the Rico and the Virgin Islands. will be initiated. A problem for Puerto northeastern Caribbean. The microplates that Rico and the Virgin Islands is that sites form Puerto Rico and the Virgin Islands have Earthquake location Date Magnitude of tsunami generation are likely to be been removed for clarity. Hispaniola 1953 6.9 very close to the coast, and so warning Mona Canyon 1946 7.5 time is short, unlike Hawaii, where sites Hispaniola 1946 8.1 are very distant and warnings can be Earthquake Patterns Mona Canyon 1918 7.5 broadcast hours in advance. Therefore, Stresses in the plates cause frequent Anegada Trough 1867 7.5 improved understanding of the geology Puerto Rico Trench 1787 8.1 earthquakes, and so the pattern of deep must be used for public education and earthquakes can be used to map the planning for safer construction and the locations of cold, brittle underthrust Tsunami Hazards proper siting of structures where people plates (fig. 2). Two groups of earth- Tsunami waves form when large congregate. quake foci (locations of initial ruptures pieces of the sea floor undergo abrupt —Jeffrey W. Nealon and William P. Dillon within the Earth) extend diagonally vertical movement due to fault rupture, under the islands, one from the north landslides, or volcanism. Immediately For more information, please contact: and one from the south. Apparently the after the 1946 earthquake, a tsunami William Dillon ([email protected])1 North American plate is bent down to struck northeastern Hispaniola and prop- Uri ten Brink ([email protected])1 the south, and the Caribbean plate is agated inland for several kilometers; Kathryn Scanlon ([email protected])1 bent down to the north; they may be in some reports indicate that nearly 1,800 Arthur Frankel ([email protected])2 contact beneath the islands. The islands people drowned. In 1918, a magnitude are situated on a set of independent 7.5 earthquake resulted in a tsunami that 1U.S. Geological Survey small plate fragments or microplates killed at least 91 people in northwestern 384 Woods Hole Road that are located in the V-shaped trough Puerto Rico. Eyewitness reports of an Woods Hole, MA 02543 between the downbent parts of the 1867 Virgin Islands tsunami give a maxi- Telephone: 508Ð548Ð8700 major plates (fig. 3). mum wave height of more than 7 meters 2U.S. Geological Survey, Mail Stop 966 Large earthquakes have occurred (23 feet) in Frederiksted, St. Croix, Box 25046, Federal Center in the Puerto Rican region (table 1). where a large naval vessel was left on Lakewood, CO 80225 USGS research indicates an equal top of a pier. probability for damaging ground A description of a USGS-sponsored motion for Mayaguez in western USGS Research workshop on tsunami and earthquake Puerto Rico as for Seattle, Washington; The USGS is studying the geology hazards in the region is available online other Puerto Rican cities also have of the region to map earthquakes, under- at: http://woodshole.er.usgs.gov/epubs/ substantial risk. stand how they are generated, and esti- openfiles/ofr99-353