What is a tsunami?

How large are tsunami?

In the open ocean, tsunami may have wave-lengths of up to several hundred miles and travel at speeds up to 450 mi per hr (720 km per hr). Despite these remarkable features, a tsunami may have wave heights of less than 3 feet (1 meter) that pass unnoticed beneath a ship at sea.

The period between wave-crests varies from 5 minutes to about 1 hour. When tsunami approach shallow water along a coast, their speed slows, causing their length to shorten and their height to rise sometimes to as high as 100 feet (30 meters). When they break, they often destroy anything in their path - piers, buildings and beaches - and take human life. The wave height as they crash upon a shore depends almost entirely upon the underwater topography offshore. Waves tend to rise to greater heights along gently sloping shores, along ridges or in coastal embayments.

Is it possible to predict a tsunami?

There is little warning of a tsunami's approach. When a "train" of tsunami waves approaches a coastline, the first indication is often a sharp swell, similar to an ordinary storm swell, followed by a sudden outrush of water that often exposes offshore areas as the first wave trough reaches the coast. After several minutes, the first huge wave crest strikes, inundating the newly exposed beach and rushing inland to flood the coast. Generally, the third to eighth wave crests are the largest.

Since tsunami principally occur in the Pacific Ocean following shallow-focus earthquakes over magnitude 6.5 on the Richter scale, one of the best means of prediction is the detection of such earthquakes on the ocean floor with a seismograph network (see seismology).

Tsunami may be detected by wave gauges, such as those emplaced as part of the Tsunami Warning System operating in the Pacific regions. Measurement of sudden sea level changes from satellites can also warn of potential tsunami.

When was the largest tsunami recorded?

Probably the most destructive tsunami occurred following the explosive eruption of the volcano Krakatoa in the East Indies on August 27, 1883, when over 36,000 people were killed as a result of the wave. Waves were up to 100 feet (30 meter) high with speeds between 350 and 450 miles per hour (560-720 km per hr). Its passage was traced as far away as Panama. It is believed that a 0.6-mile-wide (1-kilometer-wide) asteroid that struck the ocean southwest of New Zealand about AD 1500 created tsunami that reached heights of more than 425 feet (130 meters).