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The September 29, 2009, quake in Samoa, American Samoa and Tonga was in fact a triple-quake.
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Image Courtesy of Keith Koper, University of Utah

Deadly Tonga Earthquake Revealed as Three Big Quakes

Geologists have learned that a magnitude-8.1 earthquake and tsunami that killed 192 people on September 29, 2009 in Samoa, American Samoa and Tonga was in fact a triple-whammy. The 8.1 "great earthquake" concealed and triggered two major quakes of magnitude 7.8.

"At first, we thought it was one earthquake," says paper co-author Keith Koper, director of the University of Utah Seismograph Stations. "When we looked at the data, it turned out it wasn't just one great earthquake, but three large earthquakes that happened within two minutes of one another. The two quakes that were hidden were responsible for some of the damage and tsunami waves."

The earthquake created tsunami waves that varied in height depending upon where they struck.  In some places the water reached more than 49 feet above sea level. It is the first time that scientists know that a large "normal" fault earthquake (the 8.1 quake) occurred on a sea-floor tectonic plate and then triggered major "thrust" quakes (the 7.8 quakes) in the "subduction zone" where the oceanic plate is diving or "subducting" beneath a continental plate of Earth's crust.

"This is the first time a large normal-faulting quake has been shown to trigger large thrust-faulting earthquakes," says Koper.

Scientists know of only three previous cases of great earthquakes--those measuring magnitude 8 or more--that happened due to pull-apart or normal faulting within a diving seafloor plate.

Last modified September 13, 2010 by Becca Hatheway.

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