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Fieldwork

Marine Geophysical Survey of the Virgin Islands Platform Aids in Tsunami-Hazard Assessment


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The November 18, 1867, Virgin Islands earthquake and tsunami are an example of a natural disaster that changed the course of local history. At the time of the event, the United States was engaged in the purchase of the Virgin Islands from Denmark and had sent three Navy ships to explore the islands. The devastating tsunami, which caused loss of lives and damage to the Navy ships and the islands' settlements, was among the reasons the purchase was postponed for another 50 years. The location of the rupturing fault has not been determined because the earthquake predated instrumental recording.

Fishing vessel Tiki XIV Chuck Worley photographs a suspected fault scarp.
Above left: Fishing vessel Tiki XIV used for recent research. [larger version]

Above right: Chuck Worley photographs a suspected fault scarp. Photograph from video footage by Drex Harrington. [larger version]

The U.S. Virgin Islands are now densely populated, and some of the harbors and bays affected by the tsunami have tourist and fueling facilities. Therefore, the identification of potential faults in the vicinity of the islands is important for assessing earthquake and tsunami hazards. So, too, is the identification of ongoing tectonic deformation that could trigger fault rupture. Data from a global-positioning-system (GPS) station recently installed by the U.S. Geological Survey (USGS) on the island of Anegada, at the northeast end of the Virgin Islands, indicate that Anegada is moving westward with respect to St. Thomas, suggesting that tectonic deformation is occurring north of the Virgin Islands.

A marine geophysical survey to study potential tectonic deformation of the shallow seafloor, or "platform," surrounding the Virgin Islands was conducted by Brian Andrews, Emile Bergeron, Bill Danforth, Uri ten Brink, and Chuck Worley of the USGS Woods Hole Coastal and Marine Science Center onboard the 80-ft-long fishing vessel Tiki XIV between March 24 and April 10, 2009. The scientists collected 1,311 linear km of swath bathymetric (depth) data with a SwathPlus interferometric sonar and 821 km of high-resolution shallow seismic profiles (cross-sectional views of sediment layers beneath the seafloor) with a minisparker seismic system and hydrophone (underwater microphone). The ship's captain, Drexel (Stormy) Harrington, his son, Drexel Jr., and Chuck Worley made a scuba dive on a suspected fault scarp at water depths of 25 to 35 m southwest of St. Thomas to collect photographs and video footage.

Survey area around the Virgin Islands.
Above: Survey area around the Virgin Islands. Colored areas, multibeam bathymetry at water depths ranging from approximately 6 to 44 m (red, shallow; blue, deep). [larger version]

High-resolution bathymetry (with a grid resolution of 5 m) and seismic profiles (with a 2.5-m trace interval and penetration as deep as 50 m below the seafloor) show that the platform region southwest of St. Thomas is covered mostly by dense coral reefs alternating with sandy patches (see inset on map, above). The scuba dive provided a direct view of the corals and showed that they consist mostly of Agaricia agaricities (lettuce coral)  and possibly some Agaricia humilis. North of the Virgin Islands, the seafloor is likely covered by carbonate sand with only a few patches of reefs. Faults were not detected on the shallow platform either north or south of the islands; in particular, no evidence was found for a fault southwest of St. Thomas in either the seismic data or the dive observations, despite visible bathymetric relief on the seafloor. Such a fault had been suggested by Thomas W. Donnelly in a 1965 paper titled "Sea-Bottom Morphology Suggestive of Post-Pleistocene Tectonic Activity of the Eastern Greater Antilles" (Geological Society of America Bulletin, v. 76, no. 11, p. 1291-1294). The absence of evidence for tectonic deformation indicates either that deformation is not occurring in this area or that its average rate is slower than the growth rate of Agaricia corals, which can reach a height of 1 to 1.5 m in 100 years.

Pole with interferometric-sonar transducers at near end and two global-positioning-system antennas at far end, used to collect bathymetric (depth) data. Chuck Worley trims the edges of the copper wires of the minisparker.
Above left: Pole with interferometric-sonar transducers (yellow) at near end and two global-positioning-system (GPS) antennas at far end, used to collect bathymetric (depth) data. During operation, the pole swings over the rail and is attached to the side of the boat so that the transducers are 2 m below the waterline. The transducers emit fan-shaped beams of sound energy that reflect from a swath of seafloor as the vessel moves forward; depths are calculated from the time it takes the sound energy to travel from the transducers to the seafloor and back. (Read more about bathymetry systems.) [larger version]

Above right: Chuck Worley trims the edges of the copper wires of the minisparker. Towed behind the vessel, this device produces an electric spark that vaporizes a small volume of water. Rapid expansion of the vapor bubble generates a sharp pulse of sound that penetrates beneath the seafloor and is reflected from boundaries between sub-seafloor sediment layers. The returning echoes are picked up by a hydrophone (underwater microphone). (Read more about seismic-profiling systems.) [larger version]

The multibeam bathymetric data collected in this survey will contribute to construction of an accurate bathymetric grid by the National Oceanic and Atmospheric Administration (NOAA) and the USGS, which is needed to model tsunami propagation into the Virgin Islands. For example, data from north of the island of Anegada are being incorporated by NOAA's Pacific Marine Environmental Laboratory (PMEL) into a model of tsunami runup on Anegada from the 1755 Lisbon earthquake. Evidence for overwash of the island within the past 500 years, likely from a tsunami, was recently discovered by a USGS team headed by Brian Atwater (view abstract).

High seas and technical challenges slowed data collection at the start of the cruise, but calm seas allowed continuous and efficient operation during the rest of the cruise. The downtime was used to invite representatives from local institutions to visit the ship, learn about the USGS work, and exchange data and experience. Professors Roy Watlington and Tyler Smith and diving supervisor Steve Prosterman from the University of the Virgin Islands visited the vessel in Charlotte Amalie, St. Thomas; and Senior Technical Planning Manager Cindi Rolli-Kelley from the Department of Disaster Management of the British Virgin Islands visited the vessel in Road Town, Tortola. The professional and dedicated efforts of captain Drexel (Stormy) Harrington, his son, Drexel Jr., and Stormy's wife, Bev, not only assured the success of the survey but also made the work very pleasant.


Related Sound Waves Stories
Earthquake Swarms in the Puerto Rico Trench Monitored by Ocean-Bottom Seismometers
December 2007
Could It Happen Here? Tsunamis That Have Struck U.S. Coastlines
January 2005

Related Web Sites
Sea-bottom morphology suggestive of post-Pleistocene tectonic activity of the eastern Greater Antilles
Geological Society of America Bulletin
Geologic evidence northeast of Puerto Rico for an Atlantic tsunami in the last 500 years
American Geophysical Union, Fall Meeting 2008, abstract

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Fieldwork
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Geophysical Survey Aids Virgin Islands Tsunami-Hazard Assessment

Drop in California Sea Otter Numbers

Geologic Mapping of Massachusetts Seafloor

USGS Maps Faults Offshore of Los Angeles

Research Beach Sand and Stomach Aches

Meetings Workshop on Data-Preservation Techniques

Publications September 2009 Publications List


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