Tsunami

A tsunami is a series of ocean waves caused when large bodies of water are suddenly moved or displaced. The word comes from Japanese, meaning "harbor wave". The most common cause is an earthquake beneath the seafloor, when tectonic plates shift suddenly, but tsunamis can also be triggered by underwater volcanic eruptions, landslides on the ocean floor, and landslides on ice.

Tsunamis differ from ordinary wind-driven waves. Wind waves move only the surface layer of water, whereas a tsunami moves the entire water column from surface to seafloor. For this reason, a tsunami can carry enormous energy and travel great distances without rapidly losing strength.

How it travels

In deep water, a tsunami travels at very high speeds—several hundred kilometers per hour—but the wave height above normal sea level may be only a few centimeters to a meter. Ships on the open ocean typically do not notice it. When the wave approaches the coast, the water becomes shallower, the speed decreases, and the wave height increases. This is where the danger to coastal areas begins.

Warning signs

A well-known warning sign is when the sea suddenly retreats unexpectedly and the seafloor becomes visible. Many people also report hearing a loud roaring sound from the ocean. If an earthquake is felt near the coast, people are advised to move to higher ground immediately without waiting for an official warning, because the time between the earthquake and the arrival of the wave can be only minutes.

Warning systems and preparedness

Warning systems use seismic monitoring instruments and ocean-bottom measuring devices (such as DART buoys) to confirm that a real tsunami has occurred. Data is analyzed at warning centers, which then alert coastal nations. The effectiveness of the system depends on whether the warning reaches the local population, whether evacuation routes are marked, and whether training has been conducted.

High-risk areas

The Pacific Ocean is the most frequent location for such events because of the concentration of volcanoes and earthquakes around its rim. The Indian Ocean and the Caribbean Sea also have histories of such occurrences. The coasts of Somalia on the Indian Ocean were affected by a tsunami caused by an earthquake in the central Indian Ocean in 2004, with noticeable damage reported particularly to small ports and fishing vessels in the eastern regions.