Three exploration cruises carried out in the Puerto Rico Trench within the past year have for the first time mapped the morphology of this entire tectonic-plate boundary, stretching from the Dominican Republic in the west to the Lesser Antilles in the east, a distance of 700 km (430 mi).
The Puerto Rico Trench marks a boundary where two tectonic platesthe North American plate and the Caribbean plateslide past each other, with the North American plate also subducting or sliding beneath the Caribbean plate.
Water depths of more than 8 km (5 mi) make the Puerto Rico Trench the deepest part of the Atlantic Ocean. It is less deep near the Lesser Antilles, where the component of subduction is larger.
Many earthquakes and tsunamis resulting from the tectonic-plate motions have occurred in historical time in the northeastern Caribbean. Future such events will pose serious hazards to the growing population in this region, including approximately 4 million U.S. citizens of Puerto Rico and the Virgin Islands. The hazards to these islands are mainly in the form of submarine earthquakes, landslides, and tsunamis.
Observations from the three exploration cruises, coupled with computer modeling and with published global-positioning-system (GPS) results and earthquake focal mechanisms, have completely revised our view of the seismic and tsunami hazard from this plate boundary.
While the seismic hazard appears to be less severe than was previously estimated from generic models, tsunami hazards may be more severe. The occurrence of continuous retrograde slumping (with new slumps occurring progressively farther upslope) on the Caribbean plate, the observation of large cracks on the slope north of Puerto Rico, and the discovery of giant landslides on the downgoing North American plate all highlight the potential for future submarine landslides, which could trigger damaging tsunamis.
The observations collected during these cruises have also contributed to our basic understanding of the mechanisms that govern plate tectonics, in this case, the creation of the Island of Puerto Rico and the deep Puerto Rico Trench to its north.
The three cruises were funded by the National Oceanic and Atmospheric Administration (NOAA)'s Office of Ocean Exploration and were carried out aboard the NOAA ship Ronald H. Brown.
An area the size of the State of Maine was mapped during 21 survey days by the SeaBeam 2112 multibeam system, which was mounted on the keel of the ship. In addition to gathering data, these cruises also served to test and improve the performance of the system for future use by NOAA.
The most recent cruise took place between August 28 and September 4. Participants included U.S. Geological Survey (USGS) scientists Uri ten Brink (cruise chief scientist), Bill Danforth, and Chris Polloni from the USGS Woods Hole Field Center (WHFC).
A uniform morphology data base for Puerto Rico and its surrounding seas at a grid interval of 150 m was produced by Pilar Llanes Estrada, a Ph.D. student visiting WHFC from the University of Madrid, by merging our new multibeam bathymetry data with lidar data around the coast of Puerto Rico (processed by John Brock, USGS, St. Petersburg, FL), the island topography, and a compilation of older low-resolution bathymetry.
in this issue:
Puerto Rico Trench Mapping