Not all earthquakes generate tsunamis. To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. All oceanic regions of the world can experience tsunamis, but in the Pacific Ocean there is a much more frequent occurrence of large, destructive tsunamis because of the many large earthquakes along the margins of the Pacific Ocean. Any disturbance that displaces a large water mass from its equilibrium position can generate a tsunami. Generally tsunamis caused by landslides or volcanic eruptions dissipate more quickly than Pacific-wide tsunamis caused by some earthquakes and rarely affect coastlines distant from the source.
Tsunami waves are shallow-water waves with long periods and wave lengths.
As the tsunami wave reaches the shallower water above a continental shelf, friction with the shelf slows the front of the wave. As the tsunami approaches shore, the trailing waves pile onto the waves in front of them, like a rug crumpled against a wall creating a wave that may rise up to 30 feet before hitting the shore. Although greatly slowed, a tsunami still bursts onto land at freeway speeds, with enough momentum to flatten buildings and trees and to carry ships miles inland. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life. U.S. coastal communities are threatened by tsunamis that are generated by both local earthquakes and distant earthquakes.