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a brief note on tsunami
A tsunami is a very long-wavelength wave of water that is generated by sudden displacement of the seafloor or disruption of any body of standing water.  Tsunami are sometimes called "seismic sea waves", although they can be generated by mechanisms other than earthquakes.  Tsunami have also been called "tidal waves", but this term should not be used because they are not in any way related to the tides of the Earth.  Because tsunami occur suddenly, often without warning, they are extremely dangerous to coastal communities.

How Tsunami are Generated

There is an average of two destructive tsunami per year in the Pacific basin. Pacific wide tsunami are a rare phenomenon, occurring every 10 - 12 years on the average.  Most of these tsunami are generated by earthquakes that cause displacement of the seafloor, but, as we shall see, tsunami can be generated by volcanic eruptions, landslides, underwater explosions, and meteorite impacts.

Earthquakes

Earthquakes cause tsunami by causing a disturbance of the seafloor. Thus, earthquakes that occur along coastlines or anywhere beneath the oceans can generate tsunami.  The size of the tsunami is usually related to the size of the earthquake, with larger tsunami generated by larger earthquakes.  But the sense of displacement is also important.  Tsunami are generally only formed when an earthquake causes vertical displacement of the seafloor.  The 1906 earthquake near San Francisco California had a Richter Magnitude of about 7.1, yet no tsunami was generated because the motion on the fault was strike-slip motion with no vertical displacement.  Thus, tsunami only occur if the fault generating the earthquake has normal or reverse displacement.  Because of this, most tsunami are generated by earthquakes that occur along the subduction boundaries of plates, along the oceanic trenches.  Since the Pacific Ocean is surrounded by plate boundaries of this type, tsunami are frequently generated by earthquakes around the margins of the Pacific Ocean.

Volcanic Eruptions

Volcanoes that occur along coastal zones, like in Japan and island arcs throughout the world, can cause several effects that might generate a tsunami.  Explosive eruptions can rapidly emplace pyroclastic flows into the water, landslides and debris avalanches produced by eruptions can rapidly move into water, and  collapse of volcanoes to form calderas can suddenly displace the water. 

Landslides

Landslides moving into oceans, bays, or lakes can also generate tsunami.  Most such landslides are generated by earthquakes or volcanic eruptions.  As previously mentioned, a large landslide or debris avalanche fell into Lituya Bay, Alaska in 1958 causing a wave with a run-up of about 60 m as measured by a zone completely stripped of vegetation.

Underwater Explosions

Nuclear testing by the United States in the Marshall Islands in the 1940s and 1950s generated tsunami.

Meteorite Impacts

While no historic examples of meteorite impacts are known to have produced a tsunami, the apparent impact of a meteorite at the end of the Cretaceous Period, about 65 million years ago near the tip of what is now the Yucatan Peninsula of Mexico, produced tsunami that left deposits all along the Gulf coast of Mexico and the United States.


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