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The Pacific trenches occur where one tectonic plate subducts, or slides, under another one. The Puerto Rico Trench, in contrast, is situated at a boundary between two plates that slide past each other with only a small component of subduction. The trench is shallower 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 milliGals, indicating the presence of an active downward force. Finally, a carbonate platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. Its north edge is at 4,500-m depth, and its south edge is observable on land in Puerto Rico at an elevation of a few hundred meters.
Several tectonic models have been proposed to explain these unusual observations, and marine exploration efforts of the type reported here are needed to discriminate between them. Many earthquakes and tsunamis resulting from these plate-tectonic movements have occurred during historical time in the northeastern Caribbean. As the population in this region continues to grow, future events will pose serious hazards to the 4 million U.S. citizens of Puerto Rico and the Virgin Islands, mainly in the form of submarine faults and landslides.
U.S. Geological Survey (USGS) scientists recently participated in a short exploration cruise to map part of the Puerto Rico Trench. The cruise resulted in the discovery of a major active strike-slip fault system close to the trench, submarine slides on the descending North American tectonic plate, and an extinct mud volcano, which was cut by the strike-slip fault system.
Another strike-slip fault system closer to Puerto Rico that was previously considered to accommodate much of the relative plate motion appears to be inactive. The seaward continuation of the Mona Rift, a zone of extension between Puerto Rico and the Dominican Republic that generated a devastating tsunami in 1918, was mapped for the first time.
These discoveries indicate weak coupling between the descending North American plate and the overlying Puerto Rico block, leading us to suggest that this part of the plate boundary may be capable of generating only moderate earthquakes. An additional 3-week cruise is planned for February 2003 to map the remaining part of the Puerto Rico Trench and provide a base map for further tectonic, environmental, and biological studies.
The cruise, funded by the National Oceanic and Atmospheric Administration (NOAA)'s Office of Ocean Exploration, was carried out September 24-30, 2002, aboard the NOAA ship Ronald H. Brown. Multibeam bathymetry and acoustic-backscatter data were collected over an area of about 25,000 km2, bigger than the State of New Jersey. Participants included Uri ten Brink (chief scientist) and Chuck Worley from the USGS Woods Hole Field Center, and Lt. Shep Smith, NOAA, from the Center for Coastal and Ocean Mapping Joint Hydrographic Center, a University of New Hampshire program operating in partnership with NOAA's National Ocean Service.
in this issue:
Exploring the Puerto Rico Trench
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