A tsunami is a series of ocean waves produced by sudden movements of the ocean floor, landslides, or volcanic activity. In the deep ocean, tsunami waves may travel across the ocean as fast as a jet plane (more than 500 miles per hour) but look like ripples perhaps only a few inches high—likely not noticeable to someone on a boat or ship. For more information on how tsunamis form and travel across the ocean, including animated graphics, visit the NOAA Tsunami website.
As tsunami waves approach shore, they begin to slow down and build up in height. The shape of the waves is changed by the depth of the water and the slope of the coast.
To someone standing on the shore watching a tsunami approach (not recommended!), it might look like a fast-moving (20–30 mph) wall of water 10 feet high or more. Large tsunamis are rare, but when they happen they carry great destructive potential and pose a serious threat to coastal communities in their path.
Tsunamis have no “season,” and can strike at any time of day or night, in good weather or bad. They occur in every ocean and sea, but most tsunamis (about 85 percent) happen in the Pacific Ocean. Tsunamis cannot be prevented; however, their impacts on coastal communities can be mitigated through planning, preparedness, timely warnings, and effective response. Keep in mind: any tsunami today happens on top of the 8-inch rise in average global sea level since 1900. By the year 2100, tsunamis will happen on top of another 8 inches to 6.6 feet of sea level rise.1
NOAA’s National Geophysical Data Center (NGDC) and co-located World Data Center for Geophysics and the International Tsunami Information Center, a UNESCO/IOC-NOAA partnership, have collaborated to produce a map showing tsunami sources. The information comes from the NGDC Historical Tsunami Database that includes information on tsunami source events throughout the world that range in date from 1410 B.C. to A.D. 2011. The tsunami definitions are from the Tsunami Glossary 2008 published by UNESCO.
This table provides a qualitative tsunami hazard assessment for U.S. states and territories:2
|Region||Hazard Based on Runups||Hazard Based on Frequency||Hazard Based on Local Earthquakes||Number of Reported Deaths Since 1900|
|Hawaii||Very High||Very High||High||326|
|Alaska||Very High||Very High||High||222|
|Puerto Rico and the Virgin Islands||High||High||High||172|
|U.S. West Coast||High||High||High||25|
|U.S. Pacific Island Territories||Moderate||High||High||1|
|U.S. Atlantic Coast||Very Low to Low||Very Low||Very Low to Low||None|
|U.S. Gulf Coast||Very Low||Very Low||Very Low||None|
- 1. Parris, A., P. Bromirski, V. Burkett, D. Cayan, M. Culver, J. Hall, R. Horton, K. Knuuti, R. Moss, J. Obeysekera, A. Sallenger, and J. Weiss, 2012: Global Sea Level Rise Scenarios for the United States National Climate Assessment. NOAA Tech Memo OAR CPO-1. 37 pp.
- 2. Dunbar, P.K., and C.S. Weaver, 2008: U.S. States and Territories National Tsunami Hazard Assessment: Historical Record and Sources for Waves. National Tsunami Hazard Mitigation Program, 59 pp.