Geology concepts and applications

North Carolina Tsunami Risks tsunami is a wave train, or series of waves, generated in a body of water by a sudden disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and meteorites impacts, can generate tsunamis. It was generally believed until a few years ago that only earthquakes and shockwave-generating disasters such as nuclear blasts and meteorites could generate a tsunami. However, scientists have uncovered a new culprit: underwater landslides. These can be precipitated by underwater topography and vary according to its shape. As deadly as they are, tsunamis have generally been limited to areas of the Pacific Rim that are susceptible to catastrophic seismic activity. Recent discoveries about the nature of the continental shelf off the coast of Cape Hatteras have lead scientists to re-consider their likelihood.

The last major tsunami to hit the United States occurred in 1964 when an earthquake occurred in Prince William Sound in Alaska. The earthquake had a surface-wave magnitude of 8.4, somewhat higher than the San Francisco Earthquake of 8.25 that had leveled many neighborhoods of the city. When the earthquake hit the Aleutian Islands, it took the lives of more than 106 people and caused 84 million in damage. The effects of the earthquake were felt as far away as Hawaii, and took 16 other lives in addition to those in Alaska. Tsunamis ravage coastlines and can be deadlier than hurricanes; whereas a hurricane is identified weeks in advance, a tsunami can strike without warning. Costal residents prepared for the possibility of an earthquake would be caught unawares in the event of a tsunami.

In a paper published in the May 2000 issue of the journal Geology, Neal Driscoll of the Woods Hole Oceanographic Institution and colleagues Jeffrey Weissel of Columbia University's

Lamont-Doherty Earth Observatory and John Goff of the University of Texas at Austin, said that recently discovered cracks along the edge of the continental shelf could be an early warning sign that the seafloor is unstable in these areas. The most recent example of a tsunami that happened due to an event in an unstable sea floor was in 1998 when a tsunami struck the coast of Paupa New Guinea, leaving 3000 dead.(Neal W. Driscoll et all, 2000) Scientists haven't developed a consensus on the cause of the event, because of the difficulty involved in differentiating tsunamis generated by landslides from those generated by earthquakes on the basis of teleseismic records. A particularly volatile undersea topography can exacerbate the effect of a tsunami. In response, scientists have attempted to develop a methodology by which to determine areas where the underwater geography might be conducive to tsunami. According to an article in the May 7, 2000 edition of Geology magazine, researchers have come to a consensus on the need for more and better bathymetric surveys to find evidence for past land sliding, and to identify areas of seafloor susceptible to future slope failure. (Synolakis, 1997)

The paper draws specific attention to a system of en echelon cracks recently discovered along a 40-km-long section of the outer continental shelf off southern Virginia and North Carolina. These cracks occur in water that is 100-200M deep between geological features known as the Norfolk Canyon and the Albamarle-Currituck submarine slide. The asymmetric shape of these features, that lie parallel to the Eastern Seaboard, has lead scientists to believe that they are already experiencing landslides of up to 50 meters. The Woods Hole Oceanographic Institution recognizes two types of failure: (1)smaller scale failures that either form canyons, or occur within and are channeled by canyons, or occur within and are channeled by mode of failure was progressive (top down), or retrogressive (bottom up); and (2) larger scale, catastrophic failures like the Albemarle-Currituck slide that undermine large areas of canyons and effectively erase preexisting canyon morphology. (Neal W. Driscoll et all, 2000) The institution contrasts the frequency of the former events with the relative infrequency of the other. Although WHOI is interested in being able to track the process of costal change over the course of years, of particular and ominous interest is this second risk. When the large-scale events occur, the ocean's bed smoothes out, eliminating the large canyons that typify the places where river basins empty into the ocean.

The reason that North Carolina is particularly susceptible when compared to any given point along the coastline lies in the shape of the deposit thatthe river makes. The deposit from the Susquehanna River and Chesapeake Bay area assumes a funnel-shaped canyon, where deposits are more even and the sea floor is much more static.

Although a major tsunami has not hit the Eastern Seaboard since 1929, simulations have been generated using hydrodynamic shallow water equations. It was found that the most dangerous variety of tsunami, in which a large, single wave inundates a coast, is most likely to result from landslides that occur rapidly and accelerate to a high velocity. Therefore, it is reasoned that a small, fast landslide is much more dangerous than a larger landslide that happens more slowly. The largest waves occur in the direction of the slope failure; they loosely correspond to the shape of the seabed.

This is why continental slope failure off the coast of Canada between Nova Scotia and Newfoundland affected the two areas differently; whereas Newfoundland was hit dead-on by the tsunami, which ranged from 4-12 meters, Nova Scotia was only awash in waves that were a meter above their normal height. It is estimated that should a tsunami hit the coast of North Carolina, it would carry the same power as a level 3 to level 4 hurricane. (Neal W. Driscoll et all, 2000)