USGS studies tsunamis in the Atlantic

By Environmental News Network staff

Adapted from a wood-block print, the Great Wave off the Coast of Kanagawa by Hokusai, a famous late 18th- and early 19th-century Japanese artist, is often used in tsunami literature. It is somewhat misleading because tsunamis do not always manifest themselves as the huge breaking waves depicted in the print.

(ENN) -- The very word tsunami conjures pictures of a giant wall of water crashing onto land, wiping out everything in its path.

We don't normally worry too much about tsunamis in the Atlantic, but as part of its mission to evaluate site hazards, the U.S. Geological Survey has completed some initial reports as part of an interagency effort to develop a manual on design and construction standards strong enough to guard against tsunami and flood wave damage in the Atlantic.

Tsunamis are mainly restricted to the Pacific basin, an area surrounded by volcanic island arcs, mountain chains and subduction zones, earning it the nickname the "ring of fire." It is the most geologically active area on the planet. In fact, tsunamis are so closely identified with Japan that scientists worldwide use the Japanese name. However, while the tsunami threat to the Atlantic and Gulf Coast states is small, it can't be ignored when hazards are being investigated for critical facilities such as nuclear power plants.

For instance, Puerto Rico and the Virgin Islands lie along the eastern boundary of the Caribbean tectonic plate and earthquakes along this subduction zone can generate local tsunamis.

The Virgin Islands were hit in 1867 and 1868 by tsunamis; the wall of water in 1867 that swept the harbors of St. Thomas and St. Croix was 20 feet high and moved inland a distance of 250 feet in St. Thomas. This particular tsunami also affected southeastern Puerto Rico, where the tide was reported to have retreated from shore about 150 yards, and then moved returned and moved an equal distance inland.

In more recent times, an earthquake and resulting tsunami in November 1918 killed 116 people in Puerto Rico and produced damage reported in excess of $4 million.

As part of the interagency team examining the cause of these events, USGS scientist William Dillon has concluded that they resulted from landslides that have been detected by satellite on the nearby sea floor.

"These disturbances can generate long-period waves that begin to pile up into a wall of water as they approach shallow water near shorelines. Our data indicate that the Puerto Rico tsunamis may have been caused by the landslides that appear on the nearby sea floor," said Dillon.

Tsunamis are more commonly known by the misnomer "tidal waves." They are not, however, related to tides, which are caused by gravitational forces from the moon and the sun. Scientists have known for many years that tsunamis are generated by disturbances of the sea floor from events such as underwater landslides, earthquakes and volcanic eruptions. The waves generated spread outward in all directions and travel across the oceans at speeds between 425 and 500 miles per hour.

The waves travel across the ocean with extremely long wavelengths (up to hundreds of miles between wave crests in the deep ocean). When these waves approach shore, the speed of the wave decreases as they begin to "feel" the bottom. It is at this time that the height of the wave drastically increases. As the waves strike shore they may inundate low-lying coastal areas resulting in mass destruction and loss of life.

Dillon concluded that the hazard in the Caribbean from distantly generated tsunamis is likely to be less than the hazard from locally generated tsunamis or hurricane surges. Dillon presented his evidence at the American Geophysical Union meeting last month in Boston.

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