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.

WARNING AND WATCH

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.

SAFETY RULES

• A strong earthquake felt in a low-lying coastal area is a natural warning of possible, immediate danger. Keep calm and quickly move to higher ground, away from the coast.
• All large earthquakes do not cause tsunamis, but many do. If the quake is located near or directly under the ocean, the probability of a tsunami increases. When you hear that an earthquake has occurred in the ocean or coastline regions, prepare for a tsunami emergency.
• A tsunami is not a single wave, but a series of waves. Stay out of danger until an "all
• Approaching tsunamis are sometimes heralded by noticeable rapid rise or fall of coastal water. This is nature's tsunami warning and should be heeded.
• A small tsunami at one beach can be a giant a few miles away. Don't let the modest size of one make you lose respect for all.
• If you are at sea when a tsunami warning is issued do not return to port. Tsunami wave activity is imperceptible in the open ocean.
• High, multi-story, reinforced concrete hotels are located in many low-lying coastal areas. The upper floors of these hotels can provide a safe place to find refuge if you cannot move quickly inland to higher ground.
• Homes and small buildings located in low lying coastal areas are not designed to withstand tsunami impacts. Do not stay in these structures should there be a tsunami warning.
• Never go down to the beach to watch for a tsunami. WHEN YOU CAN SEE THE WAVE YOU ARE TOO CLOSE TO ESCAPE. Tsunamis can move faster than a person can run.
• Stay tuned to your radio, NOAA Weather Radio, Marine Radio, or television stations during a tsunami emergency bulletins issued through your local Emergency Management office and National Weather Service offices can save your life.

Washington State Military Department, Emergency Management Division