In 6,000 BC, 8,000 years before the 2004 Indian Ocean tsunami, a paleotsunami whose waves reach taller than the Statue of Liberty allegedly was generated by a landslide from Mount Etna and ravaged the coasts of the Mediterranean, devastating ancient villages and killing untold numbers. A team of scientists pieces together evidence of this megatsunami and reveals the face of this ancient tsunami for the first time. 3-D computer generated animation recreates the massive waves that may have changed the course of history. Megatsunami is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis. Unlike usual tsunamis -- which originate from tectonic activity and the raising or lowering of the sea floor -- known megatsunamis have originated from a large scale landslide, collision, or volcanic eruption event. Concept A megatsunami is meant to refer to a tsunami with an initial wave amplitude (height) measured in several tens, hundreds, or possibly thousands of meters. Normal tsunamis generated at sea result from movement of the sea floor. They have a small wave height offshore, and a very long wavelength (often hundreds of kilometers long), and generally pass unnoticed at sea, forming only a slight swell usually of the order of 30 cm (12 in) above the normal sea surface. When they reach land the wave height increases dramatically as the base of the wave pushes the water column above it upwards. By contrast, megatsunamis are caused by giant landslides and other impact events. This could also refer to a meteorite hitting an ocean. Underwater earthquakes or volcanic eruptions do not normally generate such large tsunamis, but landslides next to bodies of water resulting from earthquakes can, since they cause a massive amount of displacement. If the landslide or impact occurs in a limited body of water, as happened at the Vajont Dam (1963) and Lituya Bay (1958) then the water may be unable to disperse and one or more exceedingly large waves may result. Two heights are sometimes quoted for megatsunamis -- the height of the wave itself (in water), and the height to which it washes when it reaches land, which depending upon the locale, can be several times larger. History of the hypothesis Geologists searching for oil in Alaska in 1953 observed that in Lituya Bay, mature tree growth did not extend to the shoreline as it did in many other bays in the region. Rather, there was a band of younger trees closer to the shore. Forestry workers, glaciologists, and geographers call the boundary between these bands a trim line. Trees just above the trim line showed severe scarring on their seaward side, whilst those from below the trim line did not. The scientists hypothesized that there had been an unusually large wave or waves in the deep inlet. Because this is a recently deglaciated fjord with steep slopes and crossed by a major fault, one possibility was a landslide-generated tsunami. On 9 July 1958, an earthquake of magnitude 7.7--8.3 (on the Richter scale) caused 90 million tonnes of rock and ice to drop into the deep water at the head of Lituya Bay. The block fell almost vertically and hit the water with sufficient force to create a wave up to 1720 feet high. Howard Ulrich and his son, Howard Jr., were in the bay in their fishing boat when they saw the wave. They both survived and reported that the wave carried their boat "over the trees" on one of the initial waves which washed them back into the bay.

• published: 12 Jan 2014
• views: 670436