Tsunami (Tidal Wave) Hazard
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Tsunami (Tidal Wave) Hazard




Tsunami Definition

Definition:  A tsunami is a series of long-period sea waves, of either local or distant origin, resulting from a sudden and massive displacement of ocean water. Most often generated by a large-scale seafloor displacement associated with a large-magnitude earthquake, a tsunami can also result from a major submarine landslide, the catastrophic explosion of a volcanic island, or a significant impact event.


Tsunami, a concatenation of two Japanese words, translates as “harbor wave”. Tsunamis are often colloquially referred to as tidal waves; this description is inaccurate because their origins are not related to tides, although tidal conditions such as diurnal variation (daily fluctuation of high versus low tide), lunar maximums and wind- or storm-induced surge) can amplify their magnitude and destructive capability. A tsunami may also be called a seismic sea wave; this usage is appropriate if the tsunami has been produced by an undersea earthquake.


Tsunami Facts & Info

Some typical tsunami facts and information:
  • A tsunami can only occur if there is a massive displacement of ocean water. A major earthquake occurring onshore cannot produce a tsunami unless a portion of the earthquake fracture zone results in an offshore seafloor displacement or a submarine landslide is triggered by the earthshaking.
  • Not all undersea earthquakes produce tsunamis. An earthquake must exhibit a vertical component of seafloor movement in order to produce a tsunami. A thrust earthquake is the type of quake most likely to produce a tsunami.
  • Two major characteristics of an earthquake that may lead to the issuance of a tsunami warning are its location and magnitude. A critical characteristic which cannot be fully factored into a tsunami warning is the type of earthquake because that information is not immediately available. (The type of earthquake most likely to occur on a particular fault system is often known, however.)
  • It is important to take all tsunami warnings seriously, even when prior warnings may not have been followed by a (visible) tsunami. Remember that a tsunami warning alerts the public to the potential for a dangerous tsunami; therefore, every warning should be heeded. Numerous “baby” tsunamis occur which are never noticed by beachgoers or coastal residents.
  • A tsunami is always a series of waves; these waves radiate outward from the source until they intersect a landmass such as an island or a coastline. Because the waves are of long period, a given location experiencing a tsunami event may remain at risk for repeated strikes for many hours. The first wave in the series is not necessarily the largest wave.
  • Most tsunami waves in the deep water of the open ocean do not present a threat because the wave height is generally under one meter. Even a tsunami produced by a great earthquake, while possibly achieving an open-ocean height of 15 meters, should not be very hazardous in deep water because the long period of its waves will result in large swells rather than bores or breaking waves. Tsunamis are most dangerous near coastlines.
  • As tsunami waves encounter a coastline or shallow water, they undergo a process known as shoaling. A shoaling wave slows down; as it does so, the distance between the wave and the incoming wave behind it (the wavelength) shortens, but the volume of water carried by the waves remains constant. This “bunching” of waves, the rising shoreline, and the momentum of the wave all serve to force the water in a tsunami wave forward and upward as it approaches land. Wave heights of large tsunamis typically range from 7 to 15 meters (21 to 45 feet), but can reach heights of over 33 meters (100 feet).
  • It is important to note that, because tsunamis are a series of waves, there arrival may result in a permanently elevated sea level for the duration of the event. This phenomenon is similar to the storm surge associated with tropical storms.
  • Tsunami waves are of greatest risk along coastlines, but may also pose a significant risk in inlets and harbors and are capable of propagating upriver from a coastline for appreciable distances. Low-lying areas away from the beach may be at special risk because the water deposited by a tsunami wave may have no way of returning to its source, leaving such areas flooded by seawater.
Tsunami-like events can occur in smaller inland bodies of water such as fiords or dammed lakes. These are not labeled as tsunamis, but possess characteristics of a tsunami on a smaller scale (though the resultant wave heights may be much greater). Typically, such events are singularly impact events caused by landslide or glacial calving. In reality, they resemble mega-tsunamis, described below.


Mega-Tsunamis

Historically, giant tsunamis have resulted from significant impact events such as the massive flank failure of an island stratovolcano or the collision with the ocean surface of an extraterrestrial body such as a large meteor, small asteroid or comet. These types of events may comprise both impact and seafloor displacement components. The impact component is of special interest because it can generate a shock wave capable of displacing a far greater volume of water than would be expected solely from a comparable displacement event. Such giant waves, which have in the past been documented to have reached heights of at least 350 meters (over 1000 feet), are known as mega-tsunamis.

While there are at least two geologic areas of concern today that possess a potential to create a mega-tsunami, potential does not necessarily translate into immediate risk. Mega-tsunamis are geologically rare events, occurring on the order of tens of thousands to millions of years. The locations in question, in the Canary Islands in the Atlantic and the Hawaiian Islands in the Pacific, present some compelling evidence as to why a collapse could occur in the near-future, but present equally-compelling reasons as to why it will not occur. These locations are being actively monitored, so risk can be considered minimal at the present time. For an excellent discussion of the potential mega-tsunami hazard posed by the island stratovolcanoes of La Palma in the Canaries and on the Big Island of Hawaii, read the paper by George Pararas-Carayannis.


Tsunami Hazard Summary

In summary, tsunamis are hydrokinetic events that take place routinely around the globe and present a hazard to anyone dwelling on or near a coastline, especially around the Pacific Rim. Recent events have shown that regions outside the Pacific Basin are not immune to the effects of tsunamis. Large tsunamis are infrequent events, but the risk of a dangerous tsunami is real and ever-present. Coastal inhabitants should be aware of tsunami risk and should know the proper safety precautions to take if a tsunami warning is issued.


Authored by Kenneth L. Anderson.  Original article published 18 February 2005, updated 19 February 2005.


Follow links to the right to learn more about tsunamis, what causes a tsunami, tsunami articles, tsunamis and earthquakes, mega-tsunamis, tsunami history and tsunami effects. At the left margin, Related Links address topics of interest pertaining to geologic hazards and other security issues. View the Security & Consumer Protection SiteMap for a complete list of security and consumer protection topics.


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