APPLICATION OF SEAFLOOR MAPPING TOOLS TO THE STUDY OF THE PUERTO RICO TRENCH, THE DEEPEST PART OF THE ATLANTIC OCEAN

A 700-km long section of the northeast Caribbean plate boundary was systematically mapped from water depths of 2000 to 8400 m using the SeaBeam 2112 system aboard the NOAA ship Ron Brown. At this plate boundary the North America (NOAM) plate subducts obliquely under the Caribbean plate. The bathymetry data and the associated backscatter images were combined with single-beam and Lidar bathymetry data, seismic profiles, earthquake focal mechanisms, GPS measurements, the gravity field, and kinematic and static stress models to investigate the origin of the deepest part of the Atlantic Ocean and the seismic and tsunamogenic hazards that it poses to Puerto Rico, the Dominican Republic, and the Virgin Islands. Landslides’ scarps and large normal faults on the NOAM plate, an unusually deep forearc (7500 m), and a northward tilted carbonate platform north of Puerto Rico attest to the vertical collapse of the trench area. The collapse of the trench and the uplift that created the island of Puerto Rico are attributed to tearing of the descending NOAM slab under the northeast edge of the Caribbean. A continuous strike-slip fault system was mapped 10-15 km south of the trench and parallel to it. This fault accommodates the large lateral component of relative plate motion. The location of the fault close to the trench and far from the islands is compatible with GPS and earthquake data and indicates reduced seismic hazard to Puerto Rico and the Virgin Islands from earthquakes in the subduction zone and on strike slip faults in the forearc, but heightened hazard to the Dominican Republic. Large retrograde submarine slide scarps and cracks were imaged along the edge of the carbonate platform 35- 45 km north of Puerto Rico. They indicate that submarine slides continue to be active and may generate tsunamis along the north shore of Puerto Rico.