Tracklines of the multibeam bathymetry cruises

Tracklines of the multibeam bathymetry cruises. Green indicates data collected during September 2002 and red indicates data collected during February-March 2003. Click image for larger view and image credit.

Cruise Summary and Results

Leg I: September 24-30, 2002

Leg II: February 18-March 7, 2003

Uri ten Brink, Chief Scientist
U.S. Geological Survey

Despite its fascinating and mysterious origin and the potential hazard it poses to Puerto Rico and the Virgin Islands, the Puerto Rico Trench has not undergone much study during the past 25 years. Our technical capabilities to explore the oceans have improved tremendously during that period. NOAA's Office of Ocean Exploration identified the Puerto Rico Trench, the deepest part of the Atlantic Ocean, as a prime target for exploration and funded two expeditions to map the trench and its vicinity. Mapping was conducted by the U.S. Geological Survey (USGS) aboard the NOAA Ship Ronald H. Brown in cooperation with the NOAA/University of New Hampshire Center for Coastal and Ocean Mapping and the National Ocean Service's Office of Coast Survey.

Mapping data over the course of several days

These mapping data, acquired and processed during the first six days of Leg II of the expedition, demonstrate the time involved in mapping a very deep region of the ocean. Click image for larger view and image credit.

Multibeam bathymetry and acoustic backscatter data were acquired for 27 days. The expedition mapped the Caribbean plate, an entire tectonic province of the Earth, encompassing approximately 22,700 sq nautical miles. The survey was unusual in both its scope and the depth at which it was conducted. Only a thin layer of sediments covers most of the trench floor and its vicinity; therefore, the picture that emerged from these maps indicated a deformation created by large plate tectonic forces. The pattern of deformation was then used to assess the seismic and tsunami hazards of the area.

The major discoveries made during the expedition included the following:

Presenting Dr. Bunce with images of the Bunce fault

Dr. Uri ten Brink presents Dr. Elizabeth (Betty) Bunce with a colorful poster with maps and images of the fault bearing her name during a small ceremony earlier this year. Also present were Hartley Hoskins, Dave du Bois and Dicky Allison of Woods Hole Oceanographic Institution, and Dan Blackwood of the USGS. Click image for larger view and image credit.

These exciting discoveries will form the basis for additional expeditions that will investigate specific features of the sea floor by means of submersibles and remotely operated vehicles.

3-D view of scarp looking south toward Puerto Rico

Figure 1. 3-D view of the scarp looking south toward Puerto Rico. Click image for larger view and image credit.

Submarine Slides North of Puerto Rico: Implications for Tsunami Hazard

By Uri ten Brink, Chief Scientist
U.S. Geological Survey

A large amphitheater-shaped scarp (a line of cliffs produced by faulting or erosion), 55 km across, is located on the northern insular slope of Puerto Rico, 37 km north of the city of Arecibo. The USGS first discovered the scarp in the 1980s while mapping the U.S. Exclusive Economic Zone (EEZ), utilizing the GLORIA system, a long-range side-scan sonar. It was interpreted as a giant submarine slide that removed 1,500 sq km of the Tertiary layer, which was covered by a thick (1,300-1,500 m) sequence of carbonate rocks. Figure 1 is a 3-D view of the scarp looking south toward Puerto Rico. In 1990, it took one month to generate this single image. Much higher resolution images, created in near real-time from this multibeam bathymetry survey (Figure 2), show the morphology of this scarp failure in greater detail.

Gray scale image rotated to mimic the viewing angle and lighting direction in Figure 1
Regional 3-D view with colors representing depth shows the scarp from a different angle

Figures 2a and 2b. Two views of the same slide imaged by the multibeam bathymetry method. Click image for larger view and image credit.

The scarp was interpreted to represent a large-scale slope failure that was probably caused by the tectonic oversteepening of the insular shelf. The carbonate layers, which were deposited horizontally near sea level until 3.3 million years ago, are now tilted approximately 4.5 degrees to the north, reaching a depth of almost 4,000 m near the edge of the scarp. The uppermost surface of these carbonate layers appears as a relatively featureless sloping sea floor south of the scarp (Figure 2b). The slope maintains its angle as it continues upward across the northern shore of Puerto Rico and into the hills above San Juan.

Until now, it was unclear whether the northern insular slope of Puerto Rico continued to tilt and if submarine slides were still active; however, the newly acquired multibeam data and imagery suggest that the slope failure was an initial phase in the landslide process. The top portion of the carbonate platform and underlying strata had simply collapsed downslope, causing the foot of the slope to steepen. This interpretation is supported by the lack of large quantities of slide debris farther north. A crack can also be seen continuing eastward below the edge of the carbonate platform (Figure 2b), and smaller slumps are observed below this crack.

Additional cracks on the sea floor on the upper left side of Figure 2b appear to isolate a 7x3 km polygon of sea floor at the head of the scarp, which is slightly sunken relative to the surrounding area. This polygon may be an initial slump, which may cause a large part of the slope to fail in the future. These observations suggest that slumping along the northern slope of Puerto Rico is an active and ongoing process.

Submarine slides are known sources of tsunamis; the sudden vertical displacement of a large volume of the sea floor during slope failure displaces the overlying water mass, generating a "tidal" wave. One such slide in Papua New Guinea, the result of an earthquake in 1998, caused a massive tsunami with more than 2,100 casualties. The discoveries made during this expedition make clear the potential tsunami hazard to the densely populated northern coast of Puerto Rico if another massive slope failure were to occur during an earthquake.