Earthquake-caused tsunamis are not a recent phenomenon.  This image of a copper engraving, from 1755, depicts the impact of an earthquake and tsunami on the city of Lisbon.  The "great quake" occurred on November 1, 1755. Thucydides, the ancient Greek historian, describes receding sea water which returns in massive waves, inundating coastal areas.  He attributes the cause of such waves to earthquakes. Original engraving maintained at the Museu da Cidade, Lisbon.  Image online, courtesy Wikimedia Commons.


When we think about earthquakes which occur underground, we envision falling buildings, buckling freeways and observable fault lines (like the San Andreas). 

When we think about earthquakes which occur undersea, we worry about tons of displaced water racing toward shorelines (both near and far).

That racing seawater is called a Tsunami.  It can develop when an under-ocean earthquake vertically jolts the seabed, transferring energy from below the ocean (caused by the quake) to the surface (producing a tsunami).                     

Think of it like this.  If you were underwater, and you suddenly and forcefully thrust your fist upwards, you would displace the water on all sides of your moving fist. That is a simplified analogy, on a significantly smaller scale, of what happens when a tsunami is born.

When a thrust earthquake occurs underwater, the upward movement of the seabed - sometimes by many meters - displaces hundreds of cubic kilometers of water.  That water has to go someplace, once it's been disturbed in such a dramatic fashion.

Just like ripples forming around a stone, which someone has thrown into the water, large waves begin to move away from the earthquake's center.  The difference between stone-throw ripples and earthquake-caused waves, however, is profound.  The ripples quickly die-out, but tsunami waves keep moving.

In deep water, a tsunami moves incredibly fast.  In shallow water, as the tsunami approaches coastal areas, the waves slow down but grow taller.   

Sometimes the trough of a tsunami wave reaches the shoreline before its crest arrives.  When a leading-depression wave occurs, seawater suddenly draws-back from the shore, exposing hundreds of meters of seabed. 

People unfamiliar with tsunamis can misunderstand why fish are suddenly flapping around on a drying seabed.  Drawn to the shore, to investigate, they put themselves in harm's way since the crest of the wave will soon arrive. 

Tsunamis often have several waves which overwhelm the shoreline.  They arrive in intervals, sometimes five to ninety minutes apart.  Even this phenomenon confuses people who think that if they have survived the first wave, they have survived the tsunami. 

On the 27th of December, 2004 - the day after the officially named "Great Sumatra-Andaman Earthquake" - the waves of the resulting tsunami were still reaching shorelines thousands of miles from the quake's epicenter.  About 29 hours after its birth, the tsunami reached both American coasts.

Much closer to the epicenter, the island of Phuket - with its hard-working local families, lovely beaches and vacationing tourists - was seriously at-risk for a tsunami strike.

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Author: Carole D. Bos, J.D. 5190stories and lessons created

Original Release: Jan 01, 2013

Updated Last Revision: Jul 15, 2019

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"CAUSE of INDIAN OCEAN TSUNAMI" AwesomeStories.com. Jan 01, 2013. Jan 26, 2020.
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