Caribbean Tsunami and Earthquake Hazards Studies

Seafloor Map

(Top image) Shaded relief of the new multibeam bathymetry along the Puerto Rico Trench illuminated from the northwest. Thin contours indicate bathymetry at 500-m intervals. (Bottom image) Combined bathymetry map of the multibeam bathymetry data, single-beam bathymetry compilation around Puerto Rico, Lidar data near shore, and topography of Puerto Rico. Contour interval is 500 m. Thick barbed white lines denote thrust faults; thick white lines denote normal faults; thin white line denotes strike-slip fault; thin black line denotes northern edge of tilted carbonate platform and southern edge on land; dashed lines indicate head scarp of slope failures; dotted line indicates debris toe; blue lines indicate fissures in the seafloor; A - pull-apart basin; B - location of probable extinct mud volcano observed on backscatter images. Large box and a grey arrow show location and viewing direction of the figure shown in the tsunami section. Click on image for larger view.

To help understand the origin of the unusual bathymetry, gravity, and vertical tectonics of the plate boundary and to provide constraints for hazard assessment, the morphology of the entire 770-km-long trench from the Dominican Republic in the west to Anguilla in the east was mapped with multibeam echosounder during three cruises in 2002 and 2003. Parts of the Puerto Rico Trench were previously surveyed with side-scan sonar [Grindlay et al.,1997] and multibeam echosounder [Dillon et al., 1998; Grindlay et al., 1997], often at lower resolution and with line orientation and spacing that did not provide complete bathymetric coverage.

The bathymetry (seafloor topography) data were collected using the SeaBeam 2112 multibeam system aboard the National Oceanic and Atmospheric Administration (NOAA) ship Ron Brown, with sufficient swath overlap and proper line orientation for hydrographic survey.The data were gridded at 150-m grid size following resolution tests.Vertical resolution is estimated to be 0.5-1% of the water depth (10-80 m; L.Mayer, oral communication,2003). Backscatter mosaic images derived from the multibeam bathymetry data aided in interpretation.The total mapped area is 100,000 km2, slightly smaller than the area of the state of Virginia.

The trench can be divided into two parts at about 65-66W.The western part includes the deepest sector of the trench, and is associated with the most oblique convergence.This sector is 10-15 km wide and 8300-8340 m deep relative to mean sea level (figure). It is remarkably flat and is covered by nonreflective pelagic sediments.Seismic profiles show it to be underlain by rotated blocks of the NOAM plate that indicate trench subsidence.The unusual depth extends southward over parts of the forearc. The trench floor narrows to the west and abruptly shallows to 4700 m as it turns into the Hispaniola Trench, where convergence is more perpendicular. The eastern part of the trench is shallower by 700 m and more rugged than the deep western part.The subducting NOAM plate is observed in seismic lines to be broken into blocks, but the descending blocks are not rotated.

Detailed seafloor mapping of complete geological provinces, such as the one reported here, provide critical perspective on their origin and development and provide base maps for studies in other disciplines. Here and in related publications, the maps are used to investigate the causes of the subsidence and deformation of this unusually deep part of the Atlantic Ocean and to identify earthquake and tsunami hazards. Earthquake hazard from strike-slip motion in the forearc may be small, although other potential sources of earthquakes in the region may exist.Tsunami hazard to the northern coast of Puerto Rico and the Virgin Islands from submarine slope failures appears to be high.

To see a complete description about this section click HERE to download the paper (1.77MB)

Download Adobe Acrobat Reader