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USGS Atlantic Margin Expedition Combines Submarine-Landslide Studies with Law of the Sea Mapping
What do big submarine landslides capable of triggering tsunamis have in common with U.S. sovereignty over resources on and beneath the seafloor? Answer: the 2014 Atlantic margin expedition aboard the research vessel (R/V) Marcus G. Langseth. This research cruise, which began in Brooklyn, New York, on August 20 and ended in Norfolk, Virginia, on September 13, combined objectives of the USGS Law of the Sea project to map sediment thickness for identifying the outer limits of the U.S. Extended Continental Shelf with objectives of the USGS Atlantic hazards project to study submarine landslides capable of generating dangerous tsunamis.
Under international law as reflected in the Law of the Sea, every coastal country automatically has a Continental Shelf out 200 nautical miles from its shore, where it may exercise sovereign rights over resources on and beneath the seabed. (This definition is different from the geographic definition of “continental shelf” as the relatively flat, submerged edge of a continent.) In some cases, a country can have a Continental Shelf beyond 200 nautical miles, or an “Extended Continental Shelf.” The Law of the Sea allows a country to use calculations based on the shape of the seafloor and (or) the thickness of sub-seafloor sediment to determine the edge of its Extended Continental Shelf.
Both objectives of the Langseth expedition—submarine-landslide studies and delineation of Extended Continental Shelf—required the use of seismic (sound) energy to map the seafloor and image layers of sediment beneath the seafloor at high resolution, as well as to measure the velocity of sound through sub-seafloor sediment in order to correctly calculate its thickness. Likewise, both objectives required exploration of the same region: the Atlantic continental margin, which encompasses the continental shelf, slope, and rise—that is, all the seafloor from the shoreline to the deep ocean basin. The landslide-hazards objectives were met by collecting data along ship’s tracks perpendicular to the margin while transiting to and from tracks along the deep margin where data were collected to help delineate the Extended Continental Shelf.
Delineating the edge of the U.S. Extended Continental Shelf is the focus of the U.S. Extended Continental Shelf (ECS) Interagency Task Force, which has been working since 2007 to identify all parts of the U.S. margins beyond 200 nautical miles where the nation can potentially exert its sovereign rights over seabed resources such as deep-water corals or mineral crusts and nodules, and sub-seabed resources such as oil and gas. (Read about this group and its work in “Department of State Recognizes U.S. Extended Continental Shelf Project Team with Superior Honor Awards,” Sound Waves, May/June 2013.) Only after the Extended Continental Shelf is delineated can it be evaluated and designated for conservation, management, resource exploitation, or other purposes. Unless the Extended Continental Shelf is delineated as part of the Continental Shelf of the United States, it could be explored and exploited outside of the U.S. regulatory system.
The ultimate determination of the outer limits of the Extended Continental Shelf on the U.S. Atlantic margin will depend in part on the data collected during the survey last August–September, as well as a second survey planned for 2015. The first survey acquired reconnaissance data along tracks parallel to the margin in order to assess variability in sediment thickness and depth to the igneous rocks that lie below the sediments. The second, follow-up expedition is planned to use knowledge of sediment-thickness variability to position tracks that allow the full extent of the Extended Continental Shelf to be identified.
The submarine-landslide mapping is part of assessing geologic hazards that could generate tsunamis along the Atlantic seaboard. (See “Submarine Landslides as Potential Triggers of Tsunamis That Could Strike the U.S. East Coast,” Sound Waves, August 2009.) Since the 2004 Indian Ocean tsunami and the 2010 Tohoku tsunami in Japan, the U.S. Nuclear Regulatory Commission has contracted with the USGS to evaluate the potential tsunami threat to nuclear power plants along U.S. margins. Tsunamis could threaten other infrastructure as well, such as coastal cities, industrial centers, and port facilities, and so additional agencies have requested USGS input and assessments for their tsunami-preparedness planning. These agencies include Federal Emergency Management Agency (FEMA) offices in several coastal states and the City of Boston Office of Emergency Management.
Tsunamis on passive margins that are far from active tectonic-plate boundaries, such as the Atlantic seaboard, pose a challenge to regulators because these events are rare (they have low probability) but potentially devastating (they pose high risk). Three lines of evidence demonstrate that the Atlantic margin is not immune to potential tsunami hazards: (a) the 1929 Grand Banks tsunami, generated by a submarine landslide triggered by a magnitude 7.2 earthquake, killed 28 people along the sparsely populated Newfoundland coast; (b) scientists have measured and modeled abnormally high fluid pressure in sediment on the New Jersey margin, which can cause slope failure; and (c) seafloor mapping has revealed evidence of enormous submarine landslides, such as the Cape Fear slide off North Carolina.
As part of its research into submarine landslides, the USGS has used a multipronged approach, employing, for example, analytic and numerical models, geomorphic (related to the shape of the feature) analysis, regional assessments using existing data, geotechnical (related to physical properties of materials) analysis, and laboratory studies. Until now, no single landslide on the Atlantic margin had been mapped and imaged in the subsurface from the location of its origin high on the continental slope, where the headwall, or rupture surface, occurs, to its run-out position on the lower continental rise or abyssal plain. Seismic-reflection images provide important information about subsurface structures that determine where and why landslides occur. The lack of such information prevents further modeling of the processes associated with these landslides and evaluating the potential tsunami-generating risks they have posed or could pose along the Atlantic margin. The 2014 Langseth survey offered the opportunity to study the internal structures of two major landslide regions on the U.S. margin: the Southeast New England landslide complex offshore New Jersey and the Cape Fear submarine landslide offshore North Carolina.
In total, the expedition collected more than 2,700 kilometers of multichannel seismic-reflection data that will be used to create images of rock layers below the seafloor, more than 4,000 kilometers of multibeam bathymetry (seafloor depth) and backscatter data (amplitude of reflections from the seafloor) that will be used to map seafloor features and composition, and chirp data that provide high-resolution images of shallow sediments. The expedition collected additional data from deployment of 34 sonobuoys that will provide information about speed of sound through the deeper crust and improve images of structures at greater depths, as well as 101 expendable bathy thermographs, or XBTs, which provide information about speed of sound through water, required for calculation of water depths. The total data volume was about 1.5 terabytes. All of the multichannel seismic-reflection data were processed at sea with preliminary geometric corrections to get initial images of the subsurface. The multibeam bathymetry and backscatter data were edited and gridded, and the high-resolution chirp data were gathered into lines coinciding with the multichannel seismic-reflection profiles. This at-sea work facilitated on-the-fly decisions about minor adjustments to tracks during the cruise and subsequent decisions about post-cruise processing and interpretation.
R/V Langseth is a 234-foot-long vessel owned by the National Science Foundation (NSF), operated by Lamont-Doherty Earth Observatory (LDEO), and specially designed for both 2D and 3D multichannel seismic-reflection data acquisition. Langseth was acquired by NSF in 2008 and was used in USGS Extended Continental Shelf studies in 2011 in the Bering Sea and the Gulf of Alaska (see “Three-Week Expedition Images Sediments Beneath the Gulf of Alaska”, Sound Waves, August 2011.) The multichannel streamer is a solid-state, 8-kilometer-long streamer that is towed behind the vessel (yes, that’s 8 kilometers, or 5 miles, long). Langseth also tows an airgun array that provides the sound source for seismic-reflection operations.
The cruise was an outstanding success. The Extended Continental Shelf objectives of mapping sediment thickness were fully achieved on all lines acquired for that purpose (see map, above). Several other sedimentary features were spectacularly imaged, including contourite deposits formed by deep, southward-flowing currents (see Hatteras Outer Ridge cross section below). Major unconformities, or surfaces where sediment layers have been truncated by erosion; seamounts; variations in basement roughness and relief; faults; possible fluid-flow features; and other anomalies that disturb the sedimentary layering are evident in the data at scales that range from the resolution of the multichannel seismic-reflection data (10s of meters to kilometers) to much higher resolution in the chirp and multibeam data (meters to 10s of meters). The seismic lines crossed two boreholes drilled by the Deep Sea Drilling Program in 1970 and 1983, enabling identification and dating of stratigraphy imaged by the seismic data.
Hazards objectives were also achieved. A full transect of the Cape Fear submarine landslide was collected in the southern part of the study area, and these data show multiple deformation events, faults, slide surfaces, and disturbed stratigraphy. The landslide is continuous for more than 375 kilometers from its multiple headwalls near 2,500-meter water depth to its toe in 5,400-meter water depth. Remarkably, failed blocks that are several kilometers long have been imaged near the headwall (see Cape Fear submarine landslide images above). One block is clearly shown in the multichannel seismic-reflection data, in the chirp image, and along the multibeam-bathymetry track. This is the first time that a landslide on the U.S. Atlantic margin has been mapped with multiple imaging techniques along its entire length, providing a rich dataset for studying the multiple failures evident in the data. To the north, offshore New Jersey, the same data types were collected across smaller landslides of the Southeast New England landslide complex. The transects acquired to study both submarine landslide regions show salt diapirs, basement beneath much of the sedimentary section, and clearly delineated stratigraphy that will enable these data to be tied to known Atlantic margin chronologies.While the cruise ended as a complete success, it did not begin that way. The starting date was delayed by a week because of mechanical issues with the ship and permitting delays. After only 3½ days of profiling, the scientists were forced to haul gear and run back toward land to avoid Hurricane Cristobal (see map above). While the hurricane may have been “safely out to sea” for onshore weathermen, her path could not have more perfectly crossed the expedition’s field area. The scientists lost 2½ days avoiding Cristobal and had just resumed collecting data for 3 hours when the captain informed them that they needed to break work for a medical emergency. That took another 2 days of down time, though the researchers got to witness some impressive maneuvering by the Coast Guard helicopter during the medical evacuation. They wondered when the next shoe would drop, but for the rest of the expedition they were treated to flat seas, calm winds, and near-perfect conditions for data acquisition. These were also ideal conditions for the Protected Species Observers to observe wildlife, yet only a single detection of unidentified dolphins occurred during seismic-data collection, and seismic operations were interrupted for 12 minutes. Although the total kilometers of data were slightly less than planned, LDEO allowed the scientists two extra days to make up for the medical emergency. The hard work of reprocessing, analyzing, interpreting, and publishing the results is the next big challenge!
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