NWS/West Coast and Alaska Tsunami Warning Center

Tsunami Characteristics

Last updated: 7/2005

What is a tsunami?

• A tsunami is a series of waves with a long wavelength and period (time between crests) generated by a large, impulsive displacement of sea water.
• Time between crests of the wave can vary from a few minutes to over an hour.
• Tsunamis are often incorrectly called tidal waves; they have no relation to the daily ocean tides.

How are tsunamis generated?

• Tsunamis are generated by any large, impulsive displacement of the sea level.
• Tsunamis are also triggered by landslides into or under the water surface, and can be generated by volcanic activity and meteorite impacts.

How often do tsunamis occur?

• On the average, two tsunamis occur per year throughout the world which inflict damage near the source.

• Approximately every 15 years a destructive, ocean-wide tsunami occurs.

Can strike-slip (horizontal motion) earthquakes trigger tsunamis?

• Yes, approximately 15% of all damaging tsunamis were triggered by strike-slip earthquakes.
• This type of earthquake is less likely to trigger a tsunami than one with vertical motion.

• The waves are likely generated by associated landslides or motion of a sloping bathymetric feature.
• Tsunamis generated by strike-slip earthquakes normally affect regions near the source only.

What does the word “tsunami” mean?

• Tsunami (soo-NAH-mee) is a Japanese word meaning harbor wave.

How fast do tsunamis travel?

• Tsunami velocity depends on the depth of water through which it travels (velocity equals the square root of the product of the water depth times the acceleration of gravity).
• Tsunamis travel approximately 475 mph in 15,000 feet of water. In 100 feet of water the velocity drops to about 40 mph.
• A tsunami travels from the central Aleutian Is. to Hawaii in about 5 hours and to California in about 6 hours, or from the Portugal coast to North Carolina in about 8.5 hours.

How big is a tsunami?

• Tsunamis range in size from inches to over a hundred feet.
• In deep water (greater than 600 feet), tsunamis are rarely over 3 feet and will not be noticed by ships due to their long period (time between crests).
• As tsunamis propagate into shallow water, the wave height can increase by over 10 times.
• Tsunami heights vary greatly along a coast. The waves can be amplified by shoreline and bathymetric (sea floor) features.
• A large tsunami can flood low-lying coastal land over a mile from the coast.

What does a tsunami look like when it reaches shore?

• Normally, a tsunami appears as a rapidly advancing or receding tide.
• It some cases a bore (wall of water) or series of breaking waves may form.

How is a tsunami different from a wind-generated wave?

• Wind-generated waves usually have periods (time between crests) between 5 and 20 seconds. Tsunami periods normally range from 5 to 60 minutes.
• Wind-generated waves break as they shoal and lose energy offshore. Tsunamis act more like a flooding wave. A twenty foot tsunami is a twenty foot rise in sea level.