Tsunamis are ocean waves triggered by:
- Large earthquakes that occur near or under the ocean
- Volcanic eruptions
- Submarine landslides
- Onshore landslides in which large volumes of debris fall into the water
Scientists do not use the term "tidal wave" because these waves are not caused by tides. Tsunami waves are unlike typical ocean waves generated by wind and storms, and most tsunamis do not "break" like the curling, wind-generated waves popular with surfers.
Tsunamis typically consist of multiple waves that rush ashore like a fast-rising tide with powerful currents. When tsunamis approach shore, they behave like a very fast moving tide that extends much farther inland than normal water. If a tsunami-causing disturbance occurs close to the coastline, a resulting tsunami can reach coastal communities within minutes. A rule of thumb is that if you see the tsunami, it is too late to outrun it. Even small tsunamis (6 feet in height, for example) are associated with extremely strong currents, capable of knocking someone off their feet. As a result of complex interactions with the coast, tsunami waves can persist for many hours.
As with many natural phenomena, tsunamis can range in size from micro-tsunamis detectable only by sensitive instruments on the ocean floor to mega-tsunamis that can affect the coastlines of entire oceans, as with the Indian Ocean tsunami of 2004. If you hear a tsunami warning or if you feel strong shaking at the coast or see very unusual wave activity (e.g., the sea withdrawing far from shore), it is important to move to high ground and stay away from the coast until wave activity has subsided (usually several hours to days).