TSUNAMIS AND TSUNAMI-LIKE WAVES OF THE EASTERN UNITED STATES

Abstract

The threat of tsunamis and tsunami-like waves hitting the eastern United States is very real despite a general impression to the contrary. We have cataloged 40 tsunamis and tsunami-like waves that have occurred in the eastern United States since 1600. Tsunamis were generated from such events as the 1755 Queen Anne’s earthquake, the Grand Banks event of 1929, the Charleston earthquake of 1886, and the New Madrid earthquakes of 1811-1812. The Queen Anne tsunami was observed as far away as St. Martin in the West Indies and is the only known teletsunami generated in this source region. Since subduction zones are absent around most of the Atlantic basin, tsunamis and tsunami-like waves along the United States East Coast are not generated from this traditional source, but appear, in most cases to be the result of slumping or landsliding associated with local earthquakes or with wave action associated with strong storms. Other sources of tsunamis and tsunami-like waves along the eastern seaboard have recently come to light including volcanic debris falls or catastrophic failure of volcanic slopes; explosive decompression of underwater methane deposits or oceanic meteor splashdowns. These sources are considered as well. Science of Tsunami Hazards, Volume 20, Number 3, page 120 (2002) BACKGROUND INFORMATION Notable Historical Events Traditionally the East Coast of the United States has been thought of as an area that has been almost entirely free of tsunamis. Unlike the Pacific Ocean, the Atlantic Ocean is not surrounded by marked subduction zones where earthquakes with a relatively large vertical offsets are likely to occur. Classic thought about tsunamis has considered such subduction zone earthquakes to be the major generators of large tsunamis. However, in the last five years this idea has begun to change world-wide. For example, in 1998 a Mw 7.1 earthquake occurred in Papua New Guinea that generated a 15-meter tsunami. This tsunami resulted in about 2,200 deaths. This was a surprisingly large tsunami to have resulted from the Mw 7.1 magnitude earthquake. It was later determined that this tsunami was only indirectly related to the earthquake. An offshore earthquaketriggered landslide had generated this locally very-large tsunami. One of the most notable earthquakes on the East Coast of North America occurred off the Burin Peninsula of Newfoundland in November 1929 caused much property damage and 29 deaths along the coast of Newfoundland. This 7.2 Ms earthquake caused a turbidity current that cut twelve transatlantic telegraph cables. It generated a local tsunami (perhaps a landslide-tsunami) that was recorded at Atlantic City, New Jersey, and Charleston, South Carolina, and possibly other places on the East Coast. The tsunami was recorded on tide gauges as far afield as the Azores and the coast of Portugal. The three to seven meter waves resulted in 29 deaths, but none of these fatalities were in the United States. (Ruffman, 1989) The most destructive tsunami ever reported in the Atlantic Ocean was generated off the coast of Portugal on November 1, 1755. In addition to the local tsunami damage in Portugal, damage was reported at Funchal, Madeira Islands; Cadiz, Spain; Safi, Morocco; Praia da Vitoria, Azore Islands; Durham, England; and as far distant as La Martinique in the West Indies, and Santiago, Cuba. Modeling done by Mader (1997) has given us an appreciation of what may have happened on the United States East Coast in that event. Tsunami waves of three meters may have affected portions of the southeast coast of the United States in that event. A number of smaller earthquakes (some of which were accompanied with cable breaks) have occurred in the Atlantic Ocean. De Smitt (1932) includes a list of 33 earthquakes most of which occurred in the Mid Atlantic Ridge area between 1755 and 1929. This table also includes a list of cable breaks that occurred during the same period. Although most of the cable breaks were not associated with earthquakes, the cable breaks show that slumping and landsliding is a frequent occurrence on the continental slope and shelf. Any of these events could have resulted in a tsunami. Methane and Pressurized Water Deposits Recent discoveries along the East Coast of the United States’ continental slope have demonstrated the existence of pressurized hydrates and pressurized water layers in the continental shelf, and have produced speculation on possible triggers that could cause sudden and perhaps violent releases of this compressed material, along with the resultant landslides and tsunamis. Two off-shore areas are under close investigation. Enormous cracks northeast of Cape Hatteras could be an underwater landslide in the making. This area off the coast of North Carolina and Virginia along with a second area of mysterious submarine canyons about 150 kilometers east of Atlantic City, New Jersey, have been areas of intense scrutiny. (Driscoll et al., 2000, Flemings and Dugan, 2000) It has been suggested that large scale submarine slope failure is possible along the East Coast of the United States’ continental slope. “The outer continental shelf off southern Virginia and North Carolina might be in the initial stages of large-scale slope failure. A system of en echelon cracks, resembling small offset normal faults, has been discovered along the outer shelf edge. Swath bathymetric data indicate that about 50m of down-to-the-east (basinward) normal slip has occurred on these features.” (Driscoll et al., 2000). Further investigation has shown that the cracks are in areas of large deposits of methane hydrate and pressurized water. The sudden release of the water or methane may have produced the cracks and slope failures. One possible cause for the sudden release of hydrates would be a quick warming of the waters. Another might be the sudden lowering of pressure with the passage of a hurricane. Should the passage of a hurricane trigger the