Tsunami - The Great Wave

Tsunamis (often erroneously called tidal waves) are an infrequent yet serious hazard in the Pacific. A tsunami is a series of traveling ocean waves of great length and long period, generated by disturbances associated with earthquakes in oceanic and coastal regions. As the tsunami crosses the deep ocean, its length from crest to crest may be a hundred miles or more, its height from trough to crest only a few feet. Tsunamis cannot be felt aboard ships in deep water, and cannot be seen from the air. But in deep water, tsunami waves may reach speeds exceeding 600 miles per hour.

As the tsunami enters the shoaling water of coastlines in its path, the velocity of its waves diminishes and wave height increases. It is in these shallow waters that tsunamis become a threat to life and property, for they can crest to heights of more than 100 feet, and strike with devastating force.


When a large earthquake occurs near the coast of Alaska or the West Coast, an automated system and geophysicist at the Alaska Tsunami Warning Center (ATWC) rapidly determine its location (epicenter) and magnitude. If the earthquake is considered great enough to generate a tsunami, the ATWC will issue an immediate TSUNAMI WARNING for a limited area near the epicenter. This warning is issued through the Military, State Emergency Management, Coast Guard, National Weather Service and other agencies. A TSUNAMI WATCH is issued to adjacent areas of Alaska, Canada and West Coast states, as appropriate, alerting them to a possible tsunami threat. If a significant tsunami is detected by tide stations near the epicenter, the WARNING is expanded to the entire coastline. If no wave was generated the WARNING will be canceled.


Washington State Military Department, Emergency Management Division