Tsunamis – Types and Classifications

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Lituya Bay after Tsunami – August 1958[2]
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Tsunamis are usually ranked by damage and lives lost, but they also can be categorized by the distance to the location of the triggering event and the time it takes the waves to reach a given site:[1]

  • Far-Source-Generated
    A far-source-generated tsunami (teletsunami) is one that originates from a source that is far away from the site of interest and takes 2 hours or longer after the triggering event to arrive. The originating earthquake or landslide will likely not be felt before the first wave arrives, thus the warning will come from the tsunami warning center. The warning will generally give a population time to evacuate to safe, high ground but the tsunami can still cause significant damage. In the December 2004 Indian Ocean Tsunami, Sri Lanka suffered major damage despite being located 1,000 miles from the earthquake that triggered the tsunami.
  • Mid-Source-Generated
    A mid-source-generated tsunami is one in which the source is somewhat closer to the site of interest, but not close enough for the effects of the triggering event to be felt at the site. Mid -source- generated tsunamis would be expected to arrive between 30 minutes and 2 hours after the triggering event. The tsunami warning center can give a timely warning that needs to be responded to almost immediately in order to provide enough time for evacuation. In general, a community at risk from mid-source-generated tsunamis should use the same considerations as those at risk from a near-source- generated tsunami.
  • Near-Source-Generated
    A near-source-generated tsunami is one that originates from a source that is close to the site of interest, and can arrive within 30 minutes or less. The 1993 tsunami that hit Okushiri, Hokkaido, Japan, for example, reached the shoreline within 5 minutes after the earthquake, and resulted in 202 fatalities.

Megatsunamis

Megatsunami is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis. Unlike usual tsunamis – megatsunamis have originated from a large scale landslide, volcanic eruption, or impact event.

On July 8, 1958, a 7.5 Magnitude earthquake occurred along the Fairweather fault that triggered a landslide that caused 30 million cubic metres of rock and ice to fall into the narrow inlet of Lituya Bay, Alaska. A megatsunami wave was generated and washed out trees to a maximum altitude of 516 meters (1,720 feet) in height and much of the rest of the shoreline of the bay was denuded by the tsunami from 30 to 200 meters. This is the highest recorded megatsunami and the largest known in modern times.[3]

 

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