William W. Danforth

Evidence for faulting related to dissociation of gas hydrate and release of methane off the southeastern United States

Abstract An irregular, faulted, collapse depression about 38 × 18 km in extent is located on the crest of the Blake Ridge offshore from the south-eastern United States. Faults disrupt the sea floor and terminate or sole out about 40–500 m below the sea floor at the base of the gas hydrate stable zone, which is identified from the location of the bottom simulating reflection (BSR). Normal faults are common but reverse faults and folds also are widespread. Folds commonly convert upward into faults. Sediment diapirs and deposits of sediments that were erupted onto the sea floor are also present. Sea-floor depressions at faults may represent locations of liquid/gas vents. The collapse was probably caused by overpressures and by the decoupling of the overlying sediments by gassy muds that existed just beneath the zone of gas hydrate stability.

New seafloor map of the Puerto Rico Trench helps assess earthquake and tsunami hazards

The Puerto Rico Trench, the deepest part of the Atlantic Ocean, is located where the North American (NOAM) plate is subducting under the Caribbean plate (Figure l). The trench region may pose significant seismic and tsunami hazards to Puerto Rico and the U.S.Virgin Islands, where 4 million U.S. citizens reside. Widespread damage in Puerto Rico and Hispaniola from an earthquake in 1787 was estimated to be the result of a magnitude 8 earthquake north of the islands [McCann et al., 2004]. A tsunami killed 40 people in NW Puerto Rico following a magnitude 7.3 earthquake in 1918 [Mercado and McCann, 1998]. Large landslide escarpments have been mapped on the seafloor north of Puerto Rico [Mercado et al., 2002; Schwab et al., 1991],although their ages are unknown.

Acoustic mapping as an environmental management tool: I. detection of barrels of low-level radioactive waste, Gulf of the Farallones National Marine Sanctuary, California

Abstract The oceans have been and will continue to be disposal sites for a wide variety of waste products. Often these wastes are not dumped at the designated sites or transport occurs during or after dumping, and, subsequent attempts to monitor the effects the waste products have on the environment are inadequate because the actual location of the waste is not known. Acoustic mapping of the seafloor with sidescan sonar is a very effective technique for locating and monitoring dredge-spoil material and other debris. Sidescan sonar provides an acoustic image or sonograph of the sea floor that is similar to a satellite image of the Earths land surface. In effect sidescan sonar allows the water column to be stripped from the sea floor, thereby providing a clear, unobstructed view of the sea bed. An example of the potential of this technique is summarized herein for the Gulf of the Farallones region. More than 47 800 drums (55 gallon) and other containers of low-level radioactive waste were dumped on the continental margin offshore the San Francisco Bay between 1946 and 1970. These drums now litter a large area (1200 km 2 ) of the sea floor within the Gulf of the Farallones National Marine Sanctuary (GFNMS). The exact location of the drums and the potential hazard the drums pose to the environment are unknown. To evaluate the risk, samples of the sediment, biota and water must be collected near and distant from the concentrations of barrels. To do this the exact location of the barrels must be known prior to sampling. The USGS, through a cooperative research agreement with GFNMS, used sidescan sonar to map two areas within the sanctuary. Total sea-floor coverage was obtained and computer-processed sonographic mosaics were constructed on board ship. Many small nongeologic targets were distributed throughout the survey areas that covered about 70 km 2 on the shelf and 120 km 2 on the slope. Analysis of the sidescan data suggests that the targets are 55-gallon drums. This interpretation was confirmed at one site with an underwater video and 35-mm camera system. Data were collected with both a 30-kHz and a 120-kHz sidescan system within a 15-km 2 area on the shelf. We found that the barrels were more easily detected with the mid-range 30-kHz system than with the higher resolution 120-kHz system. Maps of barrel distribution derived from the sonographs are being used to design sampling schemes to evaluate the risk that the radioactivity may have on the biota and environment.

Bathymetry and backscatter intensity of the sea floor of the Sandy Hook artificial reef, offshore of New Jersey

The Sandy Hook artificial reef, located on the sea floor offshore of Sandy Hook, New Jersey was built to create habitat for marine life. The reef was created by the placement of heavy materials on the sea floor; ninety-five percent of the material in the Sandy Hook reef is rock. In 2000, the U.S. Geological Survey surveyed the area using a Simrad EM1000 multibeam echosounder mounted on the Canadian Coast Guard (CCG) ship Frederick G. Creed. The purpose of this multibeam survey, done in cooperation with the U.S. Army Corps of Engineers when the Creed was in the New York region in April 2000, was to map the bathymetry and backscatter intensity of the sea floor in the area of the Sandy Hook artificial reef. The collected data from this cruise are bathymetry, backscatter intensity, and navigation trackline.

Bathymetry, backscatter intensity, and geomorphology of the sea floor of the Hudson Canyon and adjacent slope and rise

The Hudson Canyon begins on the outer continental shelf off the east coast of the United States at about 100-meters (m) water depth and extends offshore southeastward across the continental slope and rise. A multibeam survey was carried out in 2002 to map the bathymetry and backscatter intensity of the sea floor of the Hudson Canyon and adjacent slope and rise. The survey covered an area approximately 205 kilometers (km) in the offshore direction, extending from about 500 m to about 4,000 m water depth, and about 110 km in the alongshore direction, centered on the Hudson Canyon. The sea floor was mapped using a SeaBeam Instruments 2112 multibeam echosounder aboard the National Oceanic and Atmospheric Administration (NOAA) ship Ronald H. Brown. Maps derived from the multibeam observations show sea-floor bathymetry and backscatter intensity (a measure of sea floor texture and roughness), geomorphic provinces, and sea-floor environments (Butman and others, 2006). The sea floor was mapped by the U.S. Geological Survey in cooperation with Rutgers University and with support from NOAA.

Shaded relief and sea floor topography of Quadrangle 2 in western Massachusetts Bay offshore of Boston, Massachusetts

Location map showing mapped Quadrangle 1 shaded in blue. The western Massachusetts Bay map series comprises quadrangles 1–3 (outlined in black); the maps for Quadrangle 1 include this map and Butman and others (2003a,b). The shaded relief and sea floor topography of Quadrangle 2 are shown at scale 1:25,000 in Butman and others (2003c). Quadrangles 1–18 (outlined in gray) compose the companion Stellwagen Bank National Marine Sanctuary (SBNMS) map series. The shaded relief and sea floor topography of the entire area of quadrangles 1–18 is shown at scale 1:60,000 in Valentine and others (2001, 2003b); it is also shown by quadrangle at scale 1:25,000 as U.S. Geological Survey Geologic Investigations Series Maps I–2701 (for quadrangle 1) through I–2718 (for quadrangle 18). The SBNMS boundary is shown as a dashed line. Selected bathymetric contours are labeled in meters. 25