Earthquake Hazards: Tsunami

 A tsunami is a high sea wave. It can travel hundreds, even thousands, of miles across the ocean. While over deep water, they can be difficult to detect due to their large wavelengths, which can take 20 to 30 minutes for it to cycle and may only have a few feet of height. As the tsunami passes through shallower water, wave shoaling compresses the wave, making it slower and taller.

Tsunami wave rises high and quick over steep coastline
Tsunami wave rises high and quick over steep coastline


Tsunami wave rises as it moves over coast

Tsunamis can be caused by vertical displacement of the seabed due to an earthquake. They can also be caused by a landslide crashing into the water, and that landslide could have been caused by an earthquake.

A December 2004 Indian Ocean earthquake of magnitude 9.1–9.3 caused tsunamis along the coasts of the Indian Ocean. The tsunami waves killed 230,000 to 280,000 people, with heights up to 100 feet. The earthquake was the third largest since recording started in 1900, and it caused one of the deadliest disasters in modern history. It was also the longest earthquake ever recorded on a seismograph, lasting from 8.3 to 10 minutes, depending on the location of the recording station.

2004 tsunami in Ap Nang, Krabi province in Thailand. Image linked from https://commons.wikimedia.org/wiki/File:2004-tsunami.jpg
2004 tsunami in Ap Nang, Krabi province in Thailand. Image linked from https://commons.wikimedia.org/wiki/File:2004-tsunami.jpg

A magnitude 9.0–9.1 earthquake off the coast of Tōhoku, Japan, on March 11, 2011, caused tsunami waves that reached 133 feet in Miyako. Some of the waves travelled six miles inland in the Sendai area. This earthquake was the fourth strongest since 1900.

While earthquakes of these magnitudes are historically rare, it is interesting that these two occurred a little more than six years apart.

The two largest earthquakes, a 9.4–9.6 in Valdivia, Chile, in 1960 and a 9.2 in Alaska in 1964, also caused tsunamis. The 1960 Great Chilean earthquake lasted about 10 minutes. It was a tsunami from this earthquake that struck Japan and motivated Japan’s tsunami preparations.

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