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High-Resolution Multibeam Mapping of Mid-Atlantic Canyons to Assess Tsunami Hazards
For the past 5 years, U.S. Geological Survey (USGS) scientists at the Woods Hole Coastal and Marine Science Center in Woods Hole, Massachusetts, have been studying submarine canyons and landslides to assess the potential for landslide-generated tsunamis along the U.S. east coast. This study was requested and funded by the U.S. Nuclear Regulatory Commission (NRC), which is concerned about the potential impact of tsunamis on new and existing nuclear power plants. Recent devastating tsunamis in Samoa (2009), Chile (2010), and Japan (2011) offer sober reminders of the importance of accurately identifying and characterizing the natural events, or "sources," that can generate tsunamis.
In a study funded by the NRC in 2009, the USGS identified submarine landslides along the submerged continental margin as the primary potential source of dangerous tsunamis to the U.S. east coast. Such landslides fall into two categories: (1) those that originate in submarine canyons and (2) those that originate on the continental slope and rise between submarine canyons ("open slope" landslides). Because submarine landslides and canyons are closely related, the USGS scientists studying these phenomena expect their data to shed light on such fundamental questions as "How do submarine canyons form?" as well as to provide vital information for assessing the risk posed by landslide-induced tsunamis.
Critical to the USGS evaluation of submarine landslides and canyons has been the availability of high-resolution bathymetric (seafloor depth) data along the shelf edge, slope, and rise of the Atlantic continental margin. Many of these data were collected in support of the establishment of the U.S. Extended Continental Shelf (where the United States can exercise sovereign rights over seabed and subseabed resources; see http://continentalshelf.gov/). Additional data were collected by the USGS in 2009 on the upper slope between Cape Hatteras and Georges Bank to support submarine-landslide studies (http://soundwaves.usgs.gov/2009/08/fieldwork.html).
Although the U.S. Atlantic continental margin is now one of the best-mapped margins in the world, significant gaps still remain along the upper slope and shelf, where evidence of the margin's dynamic evolution is recorded. The USGS Woods Hole science center recently took the opportunity to fill some of these gaps by collaborating with a group of scientists on the multiyear Deep-Water Mid-Atlantic Canyons Project, a systematic exploration of submarine canyons off the U.S. mid-Atlantic coast. This effort, which is being funded by the Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE), the USGS, and the National Oceanic and Atmospheric Administration (NOAA) office of Ocean Exploration and Research (OER), involves multiple investigators from academic and governmental institutions. Project scientists are studying the ecology, geology, and oceanography of the canyons, focusing on sensitive deep-water corals and cold-seep communities. (An associated study on "Microbial Ecology of Deep-Water Mid-Atlantic Canyons" is described in this issue). Another important aspect of the project is the identification and investigation of historically significant artifacts in the study area, particularly shipwrecks.
The first field effort of this project was a multibeam bathymetric mapping cruise conducted aboard the NOAA Ship Nancy Foster from June 4 to June 16, 2011. The cruise was headed by Steve Ross (University of North Carolina Wilmington) and Sandra Brooke (Marine Conservation Institute and Oregon Institute of Marine Biology) and included Jason Chaytor and Daniel Brothers (USGS Woods Hole Coastal and Marine Science Center), Rod Mather (University of Rhode Island), Caitlin Casar (intern of Amanda Demopoulos at the USGS Southeast Ecological Science Center), and students Veronica Holton (College of Charleston) and Megan Prescott (University of Washington). The sites of primary interest were Baltimore, Washington, and Norfolk Canyons and selected regions of the continental shelf between the canyons.
Using a combination of hull-mounted Kongsberg-Simrad EM1002 and Reson 7125 multibeam echosounders installed on the Nancy Foster, the science team mapped canyons and shelf regions at high resolution (horizontal resolutions of 5 m in shallower water and 10 m in deeper water, vertical resolution of approximately 1 m), providing an unprecedented view of the study sites' morphology. Both bathymetric and backscatter data (related to seafloor texture and composition) were collected along a total of 2,520 km of survey trackline (exceeding 1,000 km2 of seafloor coverage) extending from south of Cape Hatteras to Baltimore Canyon.
Preliminary analysis of these new data reveal the presence of sharp, stepped erosional escarpments rimming the upper slope around each of the mapped canyons, which may be submerged paleoshorelines cut during periods of lower sea level. Furthermore, each of the canyons displays markedly different sedimentary processes, from canyon-filling sedimentation to formation of sediment waves, maintenance of channel thalwegs, and incision of canyon walls by gullies. Several submarine landslides, some previously unknown, were also partly mapped. In some places this mapping provided the data needed to attain complete coverage of the landslides, allowing them to be fully evaluated as part of the tsunami-source-analysis project. Future plans call for collecting sediment cores that will be used to date large landslides, a critical step in determining landslide "recurrence intervals"—that is, determining how often large landslides, and associated tsunamis, are likely to occur.
To learn more about the June research cruise, visit NOAA's Ocean Explorer Web site at http://oceanexplorer.noaa.gov/explorations/11midatlantic/welcome.html. Informative background essays and logs from sea are posted on the Web site hosted by the North Carolina Museum of Natural Sciences at http://deepwatercanyons.wordpress.com/.
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