What: A tsunami (soo-NAH-mee) -- a Japanese word for "harbor wave" -- is a series of traveling ocean waves. Sometimes incorrectly called "tidal waves," they are unrelated to tides.

Cause: they primarily are caused by ocean floor earthquakes where one side of a fault thrusts upward, displacing huge volumes of water. Not all coastal or near coastal quakes produce tsunamis. The waves also can be caused by undersea landslides caused by quakes or submarine volcanic eruptions.

Speed: The energy generated by the earthquake is transmitted through the deep oceans, where the waves can travel at speeds reaching 600 mph and go virtually unnoticed by ships.

Size: When a tsunami approaches shallower coastlines, the wave energy is compressed into a shorter distance, its speed diminishes and its height increases. In extreme cases, tsunamis can be more than 50 feet high for quakes of distant origin and more than 100 feet high for quakes near a coastline.

In Oregon: Research indicates that magnitude 8 or 9, tsunami-producing earthquakes strike off the Northwest coast every 300 to 600 years. The last one occurred in 1700. Within 5 to 30 minutes after the start of an earthquake, the first of several high-velocity waves will hit the coast. Tsunamis from distant undersea earthquakes off Alaska and Japan take several hours to reach Oregon, allowing time for warnings and evacuations. 

What to do when you're at the coast:

• Duck, cover and hold on if you feel an earthquake.
• When the shaking stops, quickly go inland and uphill if you are in a low-lying area. Tsunamis can move faster than a person can run.
• If a tsunami warning is issued, never go down to the beach towatch the waves. Often the first wave may be the least dangerous.
• Return to low-lying areas only when the official all-clear is given.

Where to get tsunami hazard maps: