A tsunami is a vast volume of seawater in motion, extending from the surface to the ocean floor. It is a series of long-period waves created by an abrupt disturbance that displaces a large amount of water. Tsunamis can travel at speeds of up to 600 mph (jet speed) in deep water, and the characteristics of the ocean floor affect them. In shallow water, as they approach coastlines, they slow down, bunch up, and can get enormously tall. They are more like rivers or walls of flooding water than like waves.
Due to their long wavelenth, the distance from one wave crest to the next can be 100 miles (distance from Hilo to Kona across the island of Hawai'i!). Out in the deep open ocean the waves are only one to two feet tall, and if you were on a ship in deep water you would not even notice them. But when the waves encounter shallow water, they slow down and the wave heights increase dramatically.
Offshore and coastal features affect the size and impact of tsunami waves. Reefs, bays, river mouths, undersea features, and beach slope all modify the tsunami as it hits the coastline. When the tsunami strikes as a wall of water, sea levels can rise many meters. Water level has risen to more than 50 feet (15 meters) for tsunamis of distant origin and over 100 feet (30 meters) for tsunami waves generated near the epicenter*. The first wave may not be the largest in the series. One coastal community may experience no damaging wave activity, while another nearby community might experience very large, destructive waves. The flooding (inundation) can extend inland by .5 mile (1 kilometer) or more and cover large tracts of land with water filled with debris.
Tsunamis can occur any day of the year and any time of the day. There is no tsunami season. They have nothing to do with the weather or the tides. Click on causes of tsunamis to read about triggering events.
*Intergovernmental Oceanographic Commission. 2008. Tsunami, the Great Waves, Revised Edition. Paris, UNESCO, 16pp., illus. IOC Brochure 2008-1 (English).
Last Revised August 2013