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Deadly Tsunami Triggered By Two Earthquakes

August 18, 2010

New research finds that a deadly tsunami that hit several South Pacific islands last year was spawned by two monstrous earthquakes.

It was originally thought that a single powerful magnitude-8.1 jolt triggered the tsunami last September 29 that killed over 200 people in Samoa, American Samoa and Tonga.

Two teams used different research techniques that have separately concluded that the disaster was because of a rare double whammy that hit within minutes of each other.  The two earthquakes had magnitudes that were greater than 8.

They said that what is notable was that the quakes occurred along separate fault lines and ruptured differently.

Although the scientists are not sure which struck first, their discovery of two earthquakes hitting solves a mystery that has baffled the scientific community since the disaster.

The findings will be published in Thursday’s issue of the journal Nature.

Scientists were confused last year when the South Pacific sea floor rumbled and tsunami waves did not arrive at the predicted times and the aftershocks did not cluster around the main quake. 

However, a group led by geophysicists John Beavan of the New Zealand geological agency GNS Science used GPS data and deep-ocean tsunami wave observations to determine that the tsunami was actually generated by two powerful quakes. 

A separate team led by Thorne Lay of the University of California, Santa Cruz concluded that the magnitude-8.1 quake unleashed the megathrust jolt.  Megathrust quakes are generally triggered by other jolts.  Ground vibrations from the first were so strong that they masked the energy released by the second quake.

Lay told The Associated Press that the second tremor “does show up clearly on seismic records, but only once you look very hard.”

Scientists not involved in the latest study said the findings shed light on what happened in the South Pacific.

“It is difficult to say how typical this behavior is in the region,” U.S. Geological Survey geophysicist Paul Earle told AP. He said that is because there is a long time between earthquakes and modern instruments were not available for previous massive earthquakes.

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