Moderate earthquakes can sometimes launch huge tsunamis. An example is the 1946 earthquake that occurred off Unimak Island in the eastern Aleutian Island arc, just west of the tip of the Alaskan Peninsula. The earthquake's magnitude was "only" somewhere between 7.3 and 7.8, but a large submarine block slide triggered by the earthquake set off a Pacific-wide tsunami. A subduction quake acting alone to generate waves of comparable size would have needed to have a magnitude of nearly nine.

When in 1883 the island volcano Krakatau was blasted into pieces in the Straits of Sunda, Indonesia, a gigantic volcanic caldera formed on the seafloor, while the volcanic debris thrown into the air came down onto the surface of the sea surrounding the former island. This double motion from below and above set off a tsunami crashing onto the shores across the entire Indian Ocean, in many places substantially exceeding the tsunami heights from the December 26, 2004, earthquake off northern Sumatra. Volcano- or earthquake-induced slides of portions of the Hawaiian Islands into the Pacific, or from the Canary Islands into the Atlantic are feared to put at risk most of the coasts in and across each of these two ocean basins. While such events have low probability, they could have catastrophic consequences.

Humanity has not experienced a major global tsunami from a meteorite impact into an ocean. But probably one of the most extreme tsunamis in the geological history of the earth was caused by the impact of a meteorite near what is now known as the Chicxulub structure, on the Yucatan Peninsula in Mexico. This event occurred some 65 million years ago and caused the extinction of many species, including the dinosaurs, most likely from a global drop of temperatures due to volcanic debris and dust in the atmosphere shielding the earth's surface from the sun's energy. But the meteor impact and associated ballistic debris fallout also caused a global tsunami estimated to be on the order of 1,000 feet high. Evidence of this great wave continues to be found at the coasts of many continents, including the U.S. states facing the Gulf of Mexico.

A very local, but still deadly tsunami occurred on Dec 12, 1917, in the harbor of Halifax, Nova Scotia. The explosion of a World War I-ammunition-laden freighter in the harbor set off a local tsunami, causing a 60-foot high wall of water to rush into town.

When enclosed water bodies such as lakes, reservoirs or fjords are subjected to sudden displacements of their waters by rock or landslides, they can also cause deadly and often spectacular upward surges of water on the opposing shorelines. The declared record in historic time is held by a rockslide in Lituya Bay in southeast Alaska, triggered by the 1958 Fairweather Fault strike-slip earthquake with an approximate magnitude of eight. A huge rockslide had gained high speed as it descended into the narrow fjordlike Lituya Bay, which made the displaced water run up on the opposing rock wall of the fjord to heights of 1,700 feet (520 m). The surging water stripped millions of forest trees and all soil from the entire rock wall up to this height.

Comments