Tsunami Definition

Tsunami Facts & Info

• 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 Hazard Summary