Tsunami: Overview

A tsunami is a succession of waves generated by rapidly displaced water on a massive scale. The term has its origins in the Japanese language – "tsu" means harbor and "nami" means wave.

Earthquakes, landslides, volcanic eruptions and large meteorite impacts are the main causes of tsunamis. Of these, the most common cause is an undersea earthquake. Sometimes, a chain of events resulting from an earthquake-triggered landslide can also lead to devastating tsunamis. At plate boundaries, vertical movements of the plates can cause massive water displacement leading to a tsunami. Subduction earthquakes, a result of denser oceanic plates slipping under continental plates, can also generate tsunamis.

Once the water is displaced by earthquakes, landslides, volcanic eruptions or large meteorites, the water radiates across the ocean and forms giant ripples. Tsunamis have small amplitudes and very long wavelengths (up to hundreds of kilometers long). These characteristics often result in tsunamis going unnoticed at sea. Tsunamis often suffer from the misnomer of tidal waves, but they actually have no relation to tides and hence the term is not technically correct.

In the 1950s, larger tsunamis than previously thought possible called mega-tsunamis were discovered. Dr. Edward Bryant, author of Tsunami: The Underrated Hazard, indicates that impact events such as giant landslide and comic impacts are the most probable cause of mega-tsunamis. Specifically, he examines the case of the Australian mega-tsunami struck the south east Australian coast around 1500 A.D. The continental shelf drops off steeply to depths of 4 kilometers in some places allowing sediments periodically tumble down its slope. The other explanation of the Australian mega-tsunami is based on that largest recorded death toll from a meteorite that fell in the Chinese city of Ch'ing-yang Shansi around 1490 A.D. – ten years before the mega-tsunami.