A simulation by OpenHazards' Steve Ward of the tsunami waves from two volcanic explosions — Krakatoa in 1883 and Santorini (or Thera) in 1628 BC.
Tsunamis are huge, destructive waves caused by the sudden displacement of large volumes of water. Most tsunamis are caused by earthquakes, but they can also be caused by landslides, volcanoes, glacier collapse, or more rarely by meteorites and large underwater explosions (e.g. early U.S. nuclear testing[1]). Not every earthquake causes a tsunami, but for those that do, the size of the tsunami is proportional to how far the earthquake causes the sea floor to move vertically.

Tsunamis travel quickly in deep water, moving up to 800 miles per hour (1300 kph) — as fast as jet planes! In deep water, they are often only a few feet tall with wave crests up to 300 miles (500 km) apart. (The distance between crests is called the wavelength.) When a tsunami enters shallower water, it slows down, but does not lose energy nor change its wavelength. As a result, the waves build up in height, sometimes reaching as tall as 100 feet (30 meters) above sea level.[2]

Warning signs

A simulation by OpenHazards' Steve Ward of the 1958 Lituya Bay, Alaska landslide and tsunami.

The first part of a tsunami to hit land is the trough (lowest point), not the crest of the wave. Beach observers often notice a drawback, or an extreme recession of the shoreline, sometimes exposing hundreds of meters of ocean bottom. It looks almost like an unusually low tide, which is likely why tsunamis have mistakenly been called "tidal waves". A knowledgeable observer will not explore this newly exposed beach, but instead quickly head the other direction towards higher ground. Pacific beaches in highly affected areas like California often have signs for the tsunami escape route, marking the fastest path to safety. The time between the drawback and arrival of the crest of the first tsunami wave varies depending on the tsunami. Beach observers may have anywhere from 5 minutes to an hour to safely evacuate a beach. The wave itself can arrive as a breaking wave or large wall of turbulent water, but more frequently looks like an extreme rising tide.

Once on higher ground, you should not venture back to the shore even after the first wave has passed. The wave you see first may not be the only wave or even the largest wave of the tsunami. Because tsunami crests can arrive hours apart, observers should wait several hours after normal shoreline behavior resumes before returning to the beach.

Governments with affected coastlines have developed sophisticated warning systems for tsunamis that can track a tsunami’s progress across the ocean floor and blast announcements to communities at risk. These early warning systems prevented loss of life in Hawaii, for instance, from the eastward counterpart of the major 2011 tsunami to hit Japan. Unfortunately, even the best warning systems today cannot detect a tsunami before its initial formation in the ocean. Japan, much closer to the initiating event, was hit by the 2011 tsunami just minutes after it was first detected.


Tsunamis often cause significant flooding and destruction. When a tsunami wave arrives, it creates a forceful impact on shoreline structures and landscapes. When the wave recedes, it tears at the land and pulls debris, plants, loose objects, and even people out to sea. Flooding and destruction of infrastructure often cause secondary damage in the form of fires and hazardous spills.

The Tsunami that hit Japan on March 11, 2011, is a tragic example of the destructive power of a tsunami. First, a magnitude 9 earthquake occurred about 45 miles (70 km) from the coast of Japan, causing an astounding 5 minutes of powerful shaking in some urban areas. The Fukushima nuclear power plant survived the earthquake and seismic sensors automatically shut down the nuclear reactors, but the plant had to turn to its backup generators to power the cooling system. The coastline of Japan dropped 3 feet (1 m) in this tremendous earthquake, which only increased the flooding from the coming tsunami. The earthquake formed a massive tsunami that traveled at over 500 miles per hour (800 kph) towards Japan’s coastline. Although many Japanese cities have built tsunami walls up to 12 meters tall, this major tsunami passed over a large number of these barriers. The destruction was devastating, killing approximately 20,000 people and washing away entire towns. Damaged oil pipelines and infrastructure created devastating fires across Japan. The tsunami also overtopped the seawalls protecting the Fukushima nuclear plant, destroying the backup power system and resulting in several large explosions and significant radioactive leaks that continued to affect local food and water supplies for months.[3]

Largest wave recorded

In 1958, a "mega-tsunami" occurred in a fjord in the Gulf of Alaska called Lituya Bay. An earthquake caused 1.9 billion cubic feet (55 million m3) of land and ice wall to fall into the bay, and the displaced water formed a short-lived but incredibly tall tsunami wave. This mega-tsunami was measured at 1,720 ft (524 meters) high. There was even an eye witness on a fishing boat who survived to recount the event.[4]

[1] Nuclear Testing Review. Nuclear Film Declassification Project. US Department of Energy. http://wn.com/nuclear_testing_review_nuclear_test_film . (Retrieved 8.15.2012)

[2] Prof. Nelson, Stephen A. “Tsunami.” Tulane University. http://www.tulane.edu/~sanelson/geol204/tsunami.htm (Retrieved 8.15.2012)

[3] “Japan's Killer Quake: An eyewitness account and investigation of the epic earthquake, tsunami, and nuclear crisis.” Aired February 29, 2012 on PBS. http://www.pbs.org/wgbh/nova/earth/japan-killer-quake.html (Retrieved 8.15.2012)

[4] “1958 Lituya Bay Megatsunami.” Wikipedia. http://en.wikipedia.org/wiki/1958_Lituya_Bay_megatsunami#cite_note-t14web.lanl.gov-2 (Retrieved 8.15.2012).

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