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On Earthquakes and Tsunamis

Sam Vaknin, Ph.D. - 9/1/2005

Tsunami - a seismic sea wave - means in Japanese "harbor-wave". It is also misleadingly called "tidal wave". It is an ocean wave caused by an earthquake of magnitude 6.5 on the Richter scale (or greater) that occurs less than 50 kilometers beneath the seafloor. Tsunamis can also be caused by volcanic eruptions and by landslides.

Tsunami waves are followed by three to five oscillations of the continental shelf waters. These convulsions may last up to a week. If the initial wave reaches the shore at its trough phase, the water recede and expose the seafloor. This happened in Lisbon Port on November 1, 1755. A few minutes later, the displaced waters return with energetic vengeance.

In the ocean, tsunami waves are merely 0.5-2 meters high with a wavelength of up to 200 kilometers. Consequently, they are virtually impalpable though they move at speeds of up to 700 kilometers per hour. As the waves near the shoreline, friction with the shallow bottom reduces their velocity, shortens their wavelength, increases their amplitude and their height.

The tsunami wave that swept across the coasts of Indonesia, Sri Lanka, Thailand, India, and Africa on December 26, 2004 was 10-12 meters high. It traveled almost 6000 kilometers. It killed almost 300,000 people. An earthquake in the fjord-like Lituya Bay, Alaska, on July 9, 1958, generated a tsunami wave 524 meters (1719 feet) high, moving at a speed of 160 kilometers per hour. Luckily, the area was largely uninhabited.

Other notable tsunamis:

In 1703 at Awa, Japan or, more likely, as a result of the Genroko (Kanto) earthquake. It claimed close to 10,000 lives.

On April 24, 1771, a tsunami caused by an underwater earthquake struck the Japanese island of Ishigaki (in the Ryuku chain). It was 85 meters high. It was so powerful that it hurled a 750 ton piece of coral to a distance of 2.5 kilometers inland.

Again in Japan, 27,000 people drowned in 1896, in a giant tsunami.

In the wake of the underwater volcanic eruptions that obliterated the island of Krakatau (Krakatoa) on August 26-27, 1883, a wave 35 meters high swept across the East Indies killing in excess of 36,000 people.

Triggered by a submarine landslide, a tsunami at least 375 meters high struck the island of Lanai in Hawaii about 105,000 years ago.

The 1960 earhquake in Chile created tsunami waves that traveled more than 10,000 kilometers to Hilo, Hawaii. The 12 meters high water wall killed 61 people and destroyed many buildings.

The Seismic Sea Wave Warning System (SSWWS), based in Honolulu, is an early warning system covering the entire, tsunami-prone, Pacific Ocean.

Dr. Catherine M. Petroff of the University of Washington editor and contributing author to the website "Tsunami!" at the
University of Washington, adds:

"Other more disastrous tsunamis include South China Sea 1782 (40,000 dead), Krakatau 1883 (36,500 dead) and Nankaido 1498 (31,201 dead). Even higher death tolls are associated with other events such as the 1755 Lisbon earthquake and the ca 1400 B.C. eruption of Santorini, but in these cases it is difficult to find accurate numbers for strictly tsunami related fatalities."