Potential for Large-Scale Submarine Slope Failure and Tsunami Generation Along the U.S

NRC038 Potential for large-scale submarine slope failure and tsunami generation along the U.S. mid-Atlantic coast Neal W. Driscoll Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Jeffrey K. Weissel Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA John A. Goff Institute for Geophysics, University of Texas, Austin, Texas 78759, USA ABSTRACT SUBMARINE LANDSLIDES AND The outer continental shelf off southern Virginia and North Carolina might be in the initial MARGIN MORPHOLOGY stages of large-scale slope failure. A system of en echelon cracks, resembling small-offset normal Coastal regions face increasing threats from a faults, has been discovered along the outer shelf edge. Swath bathymetric data indicate that about variety of natural hazards as their populations 50 m of down-to-the-east (basinward) normal slip has occurred on these features. From a societal grow and urban areas expand. Development of an perspective, we need to evaluate the degree of tsunami hazard that might be posed by a major sub- improved understanding and forecasting of these marine landslide, such as the nearby late Pleistocene Albemarle-Currituck slide, if it nucleated on hazards must be considered a priority in order to the newly discovered crack system. Toward this goal, a tsunami scenario is constructed for the mitigate future threats. Media coverage and nearby coastal zone based on the estimated volume and nature of the potential slide. Although a storm tracking has increased public awareness of maximum tsunami height of a few to several meters is predicted, the actual extent of flooding the damage to coastal communities caused by would depend on the tidal state at the time of tsunami arrival as well as the details of the hinterland severe storms. Tsunamis resulting from offshore topography. The Virginia–North Carolina coastline and lower Chesapeake Bay would be most at earthquakes, landslides, and volcanic activity are risk, being nearby, low lying, and in a direction opposite to potential slide motion. equally destructive but rarer events. As a result, public awareness of tsunamis is limited and our Keywords: slope failure, tsunami, submarine canyons, continental margin morphology. capability to forecast when or where they will 38°N strike is still underdeveloped (Synolakis et al., 1 1997; Kawatat et al., 1999). Marine geologists A. have long appreciated the importance of submarine landslides in the development of conti- 50 Mature Canyons nental margin seascapes (Booth et al., 1993; Baltimore-Accomac Slide Evans et al., 1996; Embley and Jacoby, 1986), but until recently little attention has been given to 100 the potential hazards that large submarine slides 500 1000 c. pose to coastal communities in terms of asso- Washington ciated tsunamis (Synolakis et al., 1997). The deva- Canyon 1500 stating tsunami that struck northern Papua New b. Guinea in July 1998, killing about 2000, brought 2000 the problem of tsunami hazards triggered by seafloor deformation into sharp focus (Kawatat et al., 2500 1999; Tappin et al., 1999). It is often difficult to differentiate tsunamis generated by landslides en echelon cracks from those generated by earthquakes on the basis Norfolk Young Canyons of teleseismic records (Hasegawa and Kanamori, Canyon 1987; Julian et al., 1998). Tsunamis are com- ° 37 N Recent a. 3000 monly much larger than predicted from earth- Faults Hudson quake magnitude, suggesting that landslides play Canyon 100 500 an important role in tsunami generation (Tappin B. 250 1500 50 1000 et al., 1999). Most researchers agree on the need 2000 2500 for more and better bathymetric surveys to find 3000 3500 evidence for past landsliding, and to identify areas Wilmington Canyon 3500 Young Canyons of seafloor susceptible to future slope failure Baltimore (Synolakis et al., 1997). Canyon In this paper we draw attention to a system of 1 2 en echelon cracks recently discovered along a 40-km-long section of the outer continental shelf Albemarle-Currituck Slide 4000 3 Failure Recent Major Figure 1. System of en echelon cracks, resembling small offset normal faults, recently discovered off Virginia and North Carolina between Norfolk Canyon and Albemarle- Currituck submarine slide using NOAA Cape gridded bathymetry. Inset A shows enlarge- Hatteras 36°N ment of en echelon cracks and inset B shows 75°W 74°W 73û location map. Geology; May 2000; v. 28; no. 5; p. 407–410; 3 figures. 407 off southern Virginia and North Carolina. These slides nucleating on this en echelon crack system conduits for many small-scale events, and the features are located in water depths of 100– might trigger a tsunami that poses a danger to large-scale landslides to be temporally infre- 200 m between the Norfolk Canyon and the populations along the adjacent coast. quent, and spatially isolated events. When these Albemarle-Currituck submarine slide (Figs. 1 The swath data illustrate how large submarine infrequent large-scale slope failures occur, they and 2). The asymmetric shape of the features in slides and canyon systems shape the morphology effectively reset or smooth the continental slope vertical cross section suggests that about 50 m of of the U.S. Atlantic continental margin (Figs. 1–3). by undermining and removing canyon systems, down-to-the-east normal slip of the continental We recognize two different categories of slope which are forced to regenerate over time follow- shelf edge has already occurred on a failure failure: (1) smaller scale failures that either form ing the landslide event. Evidence for this is surface subparallel to the upper continental slope canyons, or occur within and are channeled by shown by the small canyons that are starting to (Fig. 3). This asymmetry is best explained by the existing canyon systems, regardless whether the regenerate within the Albemarle-Currituck slide existence of a normal fault with collapse and mode of failure was progressive (top down), or scar at depths of ~1500 m (Figs. 1 and 3). rollover of the hanging wall into the fault trace. retrogressive (bottom up); and (2) larger scale, The two scales of slope failure proposed here There are two reasons for studying these features. catastrophic failures like the Albemarle-Currituck predict that the morphology of the resulting debris First, knowledge about these incipient submarine slide (Figs. 1–3) that undermine large areas of deposits downslope from the failure scarp should landslides will lead to a better understanding of canyons and effectively erase preexisting canyon also vary as a function of failure mode. Funneled, how cycles of large-scale slope failure, canyon morphology. Because work on land has shown coalescing slide deposits like the Baltimore- cutting, and sedimentation interact to create the that landslide size cumulative frequency statistics Accomac slide (Figs. 1 and 2) reflect failures that observed margin morphology. Second, it is follow power-law or fractal distributions (e.g., form canyons or occur within canyon systems, important to understand the hazard implications Hovius et al., 1997), we expect the canyon sys- and their downslope morphology is affected by of these features. Any future submarine land- tems to be steady-state features because they are the evolving and/or preexisting canyon relief. In 38°N 2 A. 50 100 500 Baltimore-Accomac Slide Washington 1000 Canyon 1500 2000 Figure 2. GLORIA sidescan imagery collected by U.S. 2500 Geological Survey (EEZ- SCAN 87 Scientific Staff, 1991). Slide deposits ap- pear to vary as function of Norfolk failure mode and produce Canyon (1) funneled, coalescing ° slide deposits (e.g., Balti- 37 N 3000 more-Accomac slide) and (2) large blocky slide deposits (e.g., Albemarle- Currituck slide). Contours en echelon cracks from Figure 1 are coregis- tered on GLORIA sidescan. Inset A shows enlargement 3500 of en echelon cracks. en echelon cracks Albemarle-Currituck Slide 4000 36°N 75°W 74°W 73°W 408 GEOLOGY, May 2000 the GLORIA sidescan data (Fig. 2), mass-wasting continental slope and gas hydrate decomposition TSUNAMI GENERATION AND HAZARD debris deposits can be recognized because of their may occur in slope sediments (Kvenvolden, Extremely large landslides are a feature of generally brighter backscatter compared to sea- 1993). We propose that secular warming of mid-ocean volcanic islands like the Hawaiian floor covered by hemipelagic finer grained sedi- bottom water during interglacials caused by com- Islands (Moore et al., 1989; Normark et al., 1993) ments (Schlee and Robb, 1991). Debris channeled petition and deflection of water masses could and the Canary Islands (Watts and Masson, in and by canyon systems tends to show as narrow also cause gas hydrate decomposition and conse- 1995). Passive continental margins are also sites ribbons of bright backscatter, as for the Norfolk quent slope failure. During glacial periods, intensi- of slope instability and major landslides (Embley and Washington Canyons and the coalescing fied North Atlantic Intermediate Water (Boyle and Jacobi, 1986). If a large submarine landslide deposits associated with the Baltimore-Accomac and Keigwin, 1987) could deflect the warmer were to occur on a margin adjacent to a popu- slide (Fig. 2). Conversely, large sheet-like areas Gulf Stream seaward away from the U.S. conti- lated coastal area, it could be catastrophic be- of bright backscatter are associated with the nental slope. As the production of North Atlantic cause of tsunami generation. Only 71 yr ago, a blocky debris fields of large slope failures like the Intermediate Water diminishes during inter- tsunami from the landslide associated with the Albemarle-Currituck slide that undermine several glacials (Boyle and Keigwin, 1987), the Gulf magnitude 7.2 Grand Banks earthquake of 1929 canyon systems. Stream would return to its present position. Be- left 51 dead along the south coast of Newfound- Prior et al. (1986) proposed that the Albemarle- cause of their extreme sensitivity to temperature land (NOAA, 1999).

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