Tsunamis reaching heights of more than 100 feet have been recorded. As the waves approach the shallow coastal waters, they appear normal and the speed decreases. Then as the tsunami nears the coastline, it may grow to great height and smash into the shore, causing much destruction. Tsunamis are caused by an underwater disturbance usually an undersea earthquake. Landslides, volcanic eruptions, and even meteorites can also generate a tsunami.
Tsunamis can originate hundreds or even thousands of miles away from coastal areas.
Local geography may intensify the effect of a tsunami. Areas at greatest risk are less than 50 feet above sea level and within one mile of the shoreline. People who are near the seashore during a strong earthquake should listen to a radio for a tsunami warning and be ready to evacuate at once to higher ground. Rapid changes in the water level are an indication of an approaching tsunami. These successive crests and troughs can occur anywhere from 5 to 90 minutes apart.