• Geologic Setting
  
• Survey Methods
  

Geologic Setting

The Puerto Rico Trench (Figure 2), located north of Puerto Rico and the Virgin Islands, is the deepest part of the Atlantic Ocean. It is also associated with the

Figure 2. Location map of the Puerto Rico Trench. Click for larger image.

most negative gravity anomaly on earth whose origin is presently unknown, at -380 mGal indicating a dynamic disequilibrium within the Earth. The Puerto Rico Trench is a tectonic plate boundary where the North American plate slides by and descends under the Caribbean plate. Although much of the trench lies within the U.S. Exclusive Economic Zone (EEZ), surprisingly few surveys have been conducted within the trench during the past 25 years. The bathymetry of the trench is known only in general terms. The tectonic setting of the trench is poorly understood, but GLORIA (Geologic Long-Ranged Inclined Asdic) side-scan surveys and old seismic surveys suggest that the morphology is shaped by large landslide scars, long east-west oriented faults, subduction of the north American plate, and left-lateral strike-slip motion between the plates. These active elements present significant earthquake and tsunami hazards to the north coast of Puerto Rico where the majority of the island's population of 4 million people is concentrated (ten Brink, U.S., W.P. Dillon, A. Frankel, R. Rodriguez, and C. Mueller, Seismic and tsunami hazards in Puerto Rico and the Virgin Islands, USGS Open-File Report 99-353, 1999.)

In February-March 2003, we conducted a seventeen day multibeam bathymetry survey of the Puerto Rico Trench as part of a three survey effort to provide a complete bathymetric coverage of the Puerto Rico Trench and its vicinity. The 800-km long trench was mapped from north of the Dominican Republic in the west to NE of Anguila in the east at water depths between 2500 meters and 8400 meters. Field work was funded by the National Oceanic and Atmospheric Administration (NOAA) Office of Ocean Exploration. It was carried out aboard the NOAA ship Ron Brown and utilized the onboard SeaBeam 2112 multibeam bathymetry system. The unprecedented detail of the bathymetry data and the accompanied back scatter image resulted in significant scientific discoveries, gave us insights into the forces that shape this plate boundary, and provided important information for assessments of the earthquake and tsunami hazards from the Puerto Rico Trench.  A summary of the scientific results is described in ten Brink, U.S., Danforth, W., Polloni, C., Andrews, B., Llanes Estrada, P., Smith, S., Parker, E., Uozumi, T., New seafloor map of the Puerto Rico trench helps assess earthquake and tsunami hazards in the northwest Caribbean, EOS, v., 85, no., 37, p. 349, 354, 2004.  For additional information about the Coastal and Marine Geology Program Caribbean studies, see the project Web page.