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. Local tsunamis give residents only a few minutes to seek safety. Tsunamis of distant origins give residents more time to evacuate threatened coastal areas but increase the need for timely and accurate assessment of the tsunami hazard to avoid costly false alarms. Thus, U.S. residents in Alaska can experience a local earthquake and tsunami while residents of Hawaii and the west coast may experience this disaster as a distant tsunami. Similarly, west coast residents can experience a local tsunami that may also have an impact on the distant states of Alaska and Hawaii.