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Kinds of Tsunamis

A tsunami is a powerful naturally-generated phenomenon consisting of a series of large waves which inundate a coastline. There are three kinds of tsunamis, based upon the distance between the point of generation and the point of impact. To understand the differences between the types of tsunamis, it helps to have a basic working knowledge of why, how and where tsunamis occur.

The Facts
- Unrelated to tidal waves, tsunamis form when a significant volume of water in an ocean, or less frequently a lake, is rapidly displaced. Underwater or shoreline earthquakes are almost always the cause of tsunamis, but on rare occasions, landslides, undersea volcanic eruptions or even meteors can generate them. Most tsunamis occur in the Pacific Ocean, although devastating tsunamis have occurred elsewhere in the world, as in the case of the earthquake-fueled 2004 Indian Ocean event. Deep waves extending to the sea floor, tsunamis travel across bodies of water extremely rapidly, at average speeds of 400 to 500 miles per hour (250 to 310 kilometers per hour). Like earthquakes, tsunamis cannot be predicted. Once an earthquake occurs, however, tsunami warnings are transmitted to vulnerable areas, and at this point it can be ascertained what, if any, type of tsunami will strike.
Local Tsunamis
- A local tsunami is one that strikes within approximately 60 miles (100 kilometers) of its source. There is often little to no warning of a local tsunami; it will impact the shoreline in less than one hour, sometimes striking within minutes of the event that generates it, which doesn̵7;t allow authorities time to issue warnings. However, people in an area being approached by a local tsunami likely felt shaking from the earthquake or other natural force that generated it. Hence, upon sensing earth movements, canny coastal residents and visitors head for higher ground.
Regional Tsunamis
- Regional tsunamis impact locations about 60 to 600 miles (roughly 100 to 1,000 kilometers) from their source of generation. Due to the increased distance, the travel time for regional tsunamis is longer -- typically one to three hours. This usually provides sufficient time for authorities to issue warnings and sound tsunami warning sirens, giving people a greater chance to escape to safety.
Distant Tsunamis
- Also known as tele-tsunamis, distant tsunamis stem from a source more than 600 miles (approximately 1,000 kilometers) away. Tele-tsunamis take at least three hours to arrive at the point of impact, allowing authorities to give ample notice for residents to evacuate. Tele-tsunamis can travel across entire ocean basins. For instance, an earthquake in Chile can result in a tsunami in New Zealand, or an earthquake in Japan may send a tsunami to California.

Nature