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What Causes a Tsunami?

Tsunamis are a series of extremely long waves that are created after a large volume of water is displaced.  But what is a large volume?  And how is it displaced?

Although there is no calculated volume of water that constitutes a standardized quantity that will generate a tsunami, the amount displaced must be significant enough to create waves underwater in the vast ocean.  Waves are formed as the displaced water mass moves under the influence of gravity to regain its equilibrium and radiates across the ocean like ripples on a pond.  The larger the displacement, the larger the wave generated.       

A tsunami can be generated by four general ways: (1) an undersea earthquake; (2) landslide; (3) volcanic eruption; or (4) an extraterrestrial collision. 

(1)An Undersea Earthquake - is the most common form of tsunami formation, typically generating the most destructive tsunamis.  The earth is constantly moving on large tectonic plates.  When these tectonic plates move past each other, collide and/or slide under one another (subduction), an earthquake results.  This is what happened with the recent tsunami that devastated Southern Asia.  Here, a massive earthquake in the Indian Ocean measuring 10.0 on the Richter scale jolted the seabed causing the sudden displacement of a very large volume of water.  The earthquake temporarily produces a fluctuation in the mean sea level of a specified area.  Waves quickly form as the displaced water tries to recapture equilibrium by filling the vacuum that was created.  It should be noted that not all earthquakes generate tsunamis.  Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.

(2)Landslides – resulting in rockfalls, icefalls, or underwater (submarine) landslides or slumps can generate displacement of water to create a tsunami.  More often than naught, submarine landslides are often caused by earthquakes, large and small, therefore strengthening the force of an earthquake induced tsunami.  The most notable example of a landslide-induced tsunami can be traced to Southern France in the 1980’s where the movement of a significant amount of earth for the construction of an airport triggered an underwater landslide, which resulted in destructive tsunami waves hitting the harbor of Thebes.       

(3)Volcanic Eruption - Although relatively infrequent, violent volcanic eruptions represent also impulsive disturbances, which can displace a great volume of water and generate extremely destructive tsunami waves in the immediate source area.    Volcanic disturbances can generate waves by the sudden displacement of water caused by a volcanic explosion, by a volcano's slope failure, or more likely by a phreatomagmatic explosion and collapse and/or engulfment of the volcanic magmatic chambers.  The majority of tsunamis that occur in the Pacific Ocean happen around the “Ring of Fire” Area surrounding the Hawaiian Islands.  The periphery has also been dubbed the 'Ring of Fire' because of the extraordinarily high number of active volcanoes and seismic activity located in the region.  Since 1819, over 40 tsunamis have struck the Hawaiian Islands.  One of the largest and most destructive tsunamis ever recorded was generated in August 26, 1883 after the explosion and collapse of the volcano of Krakatoa (Krakatau), in Indonesia.  This explosion generated waves that reached 135 feet, destroyed coastal towns and villages along the Sunda Strait in both the islands of Java and Sumatra, killing 36, 417 people.  

(4)Extraterrestrial Collision – Tsunamis caused by extraterrestrial collision (i.e. asteroids, meteors) are an extremely rare occurrence.  Although no meteor/asteroid induced tsunami have been recorded in recent history, scientists realize that if these celestial bodies should strike the ocean, a large volume of water would undoubtedly be displaced to cause a tsunami.  Scientists have calculated that if a moderately large asteroid, 5-6 km in diameter, should strike the middle of the large ocean basin such as the Atlantic Ocean, it would produce a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States.  On both sides of the Atlantic, coastal cities would be washed out by such a tsunami.  An asteroid 5-6 kilometers in diameter impacting between the Hawaiian Islands and the West Coast of North America, would produce a tsunami which would wash out the coastal cities on the West coasts of Canada, U.S. and Mexico and would cover most of the inhabited coastal areas of the Hawaiian islands.