General Information-Tidal Waves and Earthquakes

Tsunamis: Frequently Asked Questions*

* What is a tsunami?

- A tsunami (pronounced “soo-nah-mee”) is a  series of waves of extremely long wave length and long period generated in a body of water by an impulsive disturbance that vertically displaces the water.  The term tsunami was adopted for general use in 1963 by an international scientific conference.  Tsunami is a Japanese word represented by two characters: "tsu" and "nami."  The character "tsu" means harbor, and the character "nami" means wave. In the past, tsunamis were often referred to as "tidal waves." The term "tidal wave" is a misnomer. Tides are the result of gravitational influences of the moon, sun, and planets. Tsunamis are not caused by the tides and are unrelated to the tides; although a tsunami striking a coastal area is influenced by the tide level at the time of impact.  See also:  FEMA Backgrounder on Tsunamis  Tsunami! at the University of Washington, The Physics of Tsunamis from the West Coast/Alaska Tsunami Warning Center, or Tsunami the Great Waves

* What causes a tsunami?

- There are many causes of tsunamis but the most prevalent is earthquakes.  In addition, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis.

* How do earthquakes generate tsunamis?

- Tsunamis can be generated when the sea floor abruptly shifts and vertically displaces the overlying water from its equilibrium position. Waves are formed as the displaced water mass attempts to regain its equilibrium.  The main factor which determines the initial size of a tsunami is the amount of vertical sea floor deformation.  Not all earthquakes generate tsunamis. To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. All oceanic regions of the world can experience tsunamis, but in the Pacific Ocean there is a much more frequent occurrence of large, destructive tsunamis because of the many large earthquakes along the margins of the Pacific Ocean.  See also: Tsunami! at the University of Washington

* How do landslides, volcanic eruptions and cosmic collisions generate tsunamis?

- Any disturbance that displaces a large water mass from its equilibrium position can generate a tsunami. Generally tsunamis caused by landslides or volcanic eruptions dissipate more quickly than Pacific-wide tsunamis caused by some earthquakes and rarely affect coastlines distant from the source. See also: Tsunami the Great Waves or Tsunami! at the University of Washington

* How do landslides, volcanic eruptions and cosmic collisions generate tsunamis?

- Any disturbance that displaces a large water mass from its equilibrium position can generate a tsunami. Generally tsunamis caused by landslides or volcanic eruptions dissipate more quickly than Pacific-wide tsunamis caused by some earthquakes and rarely affect coastlines distant from the source. See also: Tsunami the Great Waves or Tsunami! at the University of Washington

* How do tsunamis differ from other water waves?

- Tsunami waves are shallow-water waves with long periods and wave lengths. (A wave is classified a shallow-water wave when the ratio between the water depth and its wavelength gets very small. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (32ft/sec/sec or 980cm/sec/sec) and the depth of the water.)  Shallow water waves are different from wind-generated waves (the waves many of us have observed on the beach). Wind-generated waves usually have period (time between two succesional waves) of five to twenty seconds and a wavelength (distance between two successional waves) of about 50 to 600 feet (15 to 200 meters) A tsunami can have a period in the range of 10 minutes to 1 hour and a wavelength in excess of 700 km (430 miles). See also:  Tsunami! at the University of Washington or Tsunami the Great Waves

* What happens to a tsunami as it approaches the shore?

- "As the tsunami wave reaches the shallower water above a continental shelf, friction with the shelf slows the front of the wave. As the tsunami approaches shore, the trailing waves pile onto the waves in front of them, like a rug crumpled against a wall creating a wave that may rise up to 30 feet  before hitting the shore.  Although greatly slowed, a tsunami still bursts onto land at freeway speeds, with enough momentum to flatten buildings and trees and to carry ships miles inland." (From: Waves of Destruction by Tim Folger, Discover Magazine, May 1994, pp. 69-70). See also: Tsunami! at the University of Washington or Tsunami the Great Waves

* What are the impacts of a tsunami?

- Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life. For more information, see Tsunami Damage Photos from the National Geophysical Data Center  USC Photos from the Papua New Guinea Tsunami

* Are tsunamis all the same?

- No. U.S. coastal communities are threatened by tsunamis that are generated by both local earthquakes and distant earthquakes. Local tsunamis give residents only a few minutes to seek safety. Tsunamis of distant origins give residents more time to evacuate threatened coastal areas but increase the need for timely and accurate assessment of the tsunami hazard to avoid costly false alarms. Thus, U.S. residents in Alaska can experience a local earthquake and tsunami while residents of Hawaii and the west coast may experience this disaster as a distant tsunami. Similarly, west coast residents can experience a local tsunami that may also have an impact on the distant states of Alaska and Hawaii. Of the two, local tsunamis are more devastating.

* What are some NOAA services related to tsunamis?

NOAA participates in the Tsunami Warning System, operating two Tsunami Warning Centers. The Alaska/West Coast Tsunami Warning Center (ATWC) in Palmer, Alaska, serves as the regional Tsunami Warning Center for Alaska, British Columbia, Washington, Oregon, and California. The Pacific Tsunami Warning Center in Ewa Beach, Hawaii, serves as the regional Tsunami Warning Center for Hawaii and as a national/international warning center for tsunamis that pose a Pacific-wide threat.

* Citizen Responsibility - what should you do?

- FEMA Factsheet   WC/ATWC Tsunami Safety Rules   National Tsunami Hazard Mitigation Program Tsunami Awareness page

* Can Tsunamis be Predicted?

Since science cannot predict when earthquakes will occur, they cannot determine exactly when a tsunami will be generated. But with the aid of historical records of tsunamis and numerical models, scientists can get an idea where tsunamis are most likely to be generated. 

* Where can I find more information about tsunamis?

Good sources of information about tsunamis are the  Tsunami links from the NOAA/PMEL Tsunami page, the NOAA/PMEL National Tsunami Hazard Mitigation Program page, and the NWS FAQ page

Graphics Credits: the Great Wave off the Coast of Kanagawa, by the Japanese artist Hokusai, from the University of Washington Tsunami Page