What a Tsunami Is

Tsunamis, commonly called tidal waves by the general public, are large sea waves or surges. These waves can carry a lot of energy from one side of the globe to the other, reeking havoc where ever they make landfall, and as shown by the December 26, 2004 SE Asian event, tsunami's can claim thousands of lives and cause millions of dollars worth of damage to property.

The United States Geological Survey defines tsunami as: "A sea wave of local or distant origin that results from large-scale sea floor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands" http://earthquake.usgs.gov/4kids/eqterms.html#tectonic

  • Many people picture large, breaking waves when they hear the word tsunami. This is usually not the case, however.
  • Most tsunamis make landfall as little more than a gigantic surge, as if the tide just moved in way too far way too fast.
  • This surging nature of tsunamis is mostly due to the extremely long wavelength, generally on the order of 100-200km.
  • A tsunami can turn into a locally, large and breaking wave if the wave energy is concentrated, shortening the wavelength and increasing the amplitude.
  • This often happens if the wave enters a bay, fjord or similar feature.
  • Tsunamis can be regional, like the recent tsunami in SE Asia, or localized, like the megatsunami in Lituya Bay, Alaska in 1958.
  • Regional scale tsunamis are general caused by crustal rebound after a large earthquake, usually associated with a subduction zone
  • Localized tsunamis are also generally associated with earthquakes, but the physical cause of the wave is usually due to a landslide or pyroclastic flow.

July 9, 1958 Lituya Bay, SE Alaska Megatsunami


December 26, 2004 Tsunami