Submarine Landslides as Potential Triggers of Tsunamis That Could Strike the U.S. East Coast


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Submarine Landslides as Potential Triggers of Tsunamis That Could Strike the U.S. East Coast

First results from systematic sea-floor mapping of the continental slope from Cape Hatteras to Georges Bank

By Uri ten Brink
August 2009

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Above: General shape of continental shelf, slope, and rise. Modified from U.S. Office of Naval Research. [larger version]

A recent assessment of tsunami hazard along the U.S. east coast, carried out by the U.S. Geological Survey (USGS) and funded by the U.S. Nuclear Regulatory Commission, has identified submarine landslides along the submerged continental margin as the primary potential source of dangerous tsunamis to this coast. The seriousness of this hazard was demonstrated by the 1929 Grand Banks submarine landslide, which produced a tsunami 3 to 8 m high that killed 28 people along the sparsely populated Newfoundland coast.

Most submarine landslides on the continental margin occur on the continental slope and upper rise; a lack of detailed maps for parts of this region has hampered efforts to produce a quantitative assessment of tsunami hazard. During May 11-25, 2009, a team of scientists conducted a 15-day survey aboard the National Oceanic and Atmospheric Administration (NOAA) ship Ronald H. Brown to provide a complete sea-floor map of the continental slope and upper rise from Cape Hatteras in the south to the eastern end of Georges Bank in the north, a distance of 1,200 km (750 mi). Uri ten Brink, David Twichell, Bill Danforth, and Elizabeth Pendleton from the USGS Woods Hole Science Center; LTjg. Lindsey Waller from NOAA's Pacific Marine Environmental Laboratory; and Leslie Sautter, Brian Kennedy, Emily Allen, Shannon Hoy, and Will Sautter from the College of Charleston used the Ronald H. Brown's hull-mounted SeaBeam 2112 multibeam echosounder to carry out the mapping. Before the survey, Jason Chaytor (USGS Woods Hole Science Center) compiled available multibeam bathymetric data to help focus new data collection on areas that were lacking coverage, particularly along the southern New England and Georges Bank margins. With the newly collected bathymetric data, more than 99 percent of the sea floor deeper than 400 m (and some areas to depths as shallow as 150 m) has now been mapped with a multibeam echosounder, allowing us to draw accurate maps of submarine canyons and the remains of past landslides along the entire U.S. east coast continental margin.

Above: Shaded-relief map of U.S. Atlantic margin (gray), gridded from single-beam bathymetric soundings, and new bathymetric data (color-coded for depth) collected with a multibeam echosounder during the May 2009 cruise. Rectangles outline approximate areas of oblique views accompanying this article (below and next page). [larger version]

These new data provide the first detailed understanding of the morphology of the source areas for many of the largest landslides. Bathymetric data were collected along nearly 5,500 km of survey tracklines, and these data show that the largest landslides have occurred along the southern New England continental slope and upper rise between Hudson Canyon and Atlantis Canyon. This section of the continental slope is characterized by numerous steep cliffs, some of them exceeding 100 m in relief. These cliffs are the headwall scarps of landslides—the surfaces that remain after the landslide material has fallen away. Some of the headwall scarps can be traced along the slope for 15-22 km, indicating that huge volumes of sediment were removed during single events. The height of a tsunami generated from a submarine landslide is highly dependent on the landslide volume, so the evidence for high-volume offshore landslides suggests that the southern New England coast may be prone to landslide-generated tsunamis. Farther east, offshore of Georges Bank, the continental slope is incised by numerous submarine canyons, and the headwall scarps of past landslides there are smaller and more widely spaced.

Above: Oblique view of the continental slope of southern New England, showing a convex profile and numerous landslide scarps (see rectangle near center of above map for approximate location). Most deeply incised canyon in this view is the meandering Block Canyon. Image in color (top) shows depth, and image in gray tones (bottom) shows details of topography. [larger version]

Above: Oblique view of the continental slope of Georges Bank, showing numerous canyons incising the slope and relatively few landslide scarps (see rectangle at upper right of map for approximate location). Image in color (top) shows depth, and image in gray tones (bottom) shows details of topography. [larger version]

The newly collected multibeam echosounder data, coupled with available seismic-reflection data, fill a key gap in our information on submarine-landslide source areas. This new information is central to refining our assessment of tsunami hazard along the densely populated U.S. Atlantic coast.

Additional photographs showing life at sea and the rich marine life encountered during the survey of the continental margin can be found at the U.S. East Coast Continental Slope Seafloor Mapping Cruise Web site.

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