New Bathymetric Map of Mona Passage, Northeastern Caribbean, Aids in Earthquake- and Tsunami-Hazard Mitigation


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New Bathymetric Map of Mona Passage, Northeastern Caribbean, Aids in Earthquake- and Tsunami-Hazard Mitigation

By Uri ten Brink
May 2007

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The Mona Passage between Puerto Rico and the Dominican Republic in the northeastern Caribbean is an area long known for its strong and shifting currents, abundant marine mammals and fish, and pirates and smugglers. All are there because the Mona Passage is an area of shallow banks over which a vigorous exchange of waters takes place between the Atlantic Ocean and the Caribbean Sea. The Mona Passage is also the site of a devastating earthquake and tsunami that hit western Puerto Rico in 1918, and the site of frequent small earthquakes. Prompted by the likelihood of further tsunamis and earthquakes, the U.S. Geological Survey (USGS), the Woods Hole Oceanographic Institution, and the National Oceanic and Atmospheric Administration (NOAA) undertook the task of mapping in detail the sea floor under the waters of the Mona Passage, to identify active faults and submarine landslides and to better understand their underlying causes.

Above: Perspective view of the bathymetry of the Mona Passage, looking eastward toward Puerto Rico. Depths indicated by color, from red (shallowest) to purple (deepest); black indicates sea floor not mapped during this study. Small islands are outlined in white to make them more visible. Vertical exaggeration, 6:1; illumination from northeast. [larger version]

Mapping of the sea floor was carried out aboard the NOAA ship Nancy Foster between March 14 and 26, 2007. The data were collected by using a Simrad EM-1002 multibeam-sonar system in water depths of 50 to 1,000 m. Multibeam-sonar systems emit acoustic (sound) energy in a fan shape that sweeps over a swath of sea floor as the ship moves forward. The time it takes for echoes to return to the system is used to calculate the depth to closely spaced points within overlapping swaths. During the March cruise, data were processed in near-real time to create a 30-m. grid (in which all the measurements within each 30- by 30-m cell were averaged to a single value) over about 4,200 km² of the sea floor. These data were added to multibeam bathymetric data collected previously by the USGS and NOAA in deeper waters to produce an image of sea-floor topography over a broad area (totaling approximately 164,200 km²) around Puerto Rico and the Puerto Rico Trench. (Information about some of the earlier mapping is available in Sound Waves article "Mapping of the Puerto Rico Trench, the Deepest Part of the Atlantic, is Nearing Completion") To aid with interpretation, the data were also entered into a geographic-information-system (GIS) database, which includes additional types of data, and were displayed as three-dimensional surfaces.

Above left: Scientists returning to the ship after a visit to Mona Island. The island, with its steep cliffs and flat top, is in the background. Photograph by Kelly Carignan. [larger version]

Above right: Iguana, a common inhabitant of Mona Island. Photograph by Uri ten Brink. [larger version]

First results show a rift zone that extends westward from southern Puerto Rico and overprints an older and partly eroded tilted-block structure. This rift and an additional fault system extending westward from northwestern Puerto Rico are probably the only currently active faults. The map also unexpectedly revealed abundant evidence for concentrated water flow through certain parts of the passage and erosion of the underlying rocks. The important contribution of ocean currents to shaping the sea-floor topography of the passage through massive erosion of the carbonate platform and creation of conspicuous flow marks and sand waves could only be appreciated through these new high-resolution data.

Above: Caribbean region, showing location of the Mona Passage. Read about additional sea-floor mapping in this region in "Mapping of the Puerto Rico Trench, the Deepest Part of the Atlantic, Is Nearing Completion," Sound Waves, October 2003. [larger version]

The scientists and crew got an intimate look at the sea floor under the Mona Passage by hiking, swimming, and diving around Mona Island, a pristine nature reserve on an uplifted piece of the sea floor in the middle of the passage. The scientific party on board included Uri ten Brink, Bill Danforth, Brian Andrews, and Claudia Flores from the USGS Woods Hole Science Center; Jason Chaytor from the Woods Hole Oceanographic Institution; Chris Chamberlin from NOAA's Pacific Marine Environmental Laboratory; Brooke McMahon from NOAA's Office of Coastal Survey; and Kelly Carignan from NOAA’s National Geophysical Data Center. Special thanks are due to Commander James Verlaque and the crew of the Nancy Foster for their professional support and friendly conduct, to Melissa (Missy) Partyka and Lieutenant Junior Grade Tracy Hamburger for their scientific support, and to Lieutenant Commander Alan Hilton for his logistical support.