Double-Whammy Earthquake Caused Tsunami

By Laura Sanders, Science News August 19, 2010


A giant earthquake that triggered a deadly southwest Pacific tsunami was actually two great temblors, finds a pair of new studies in the Aug. 19 Nature. These results uncover an unusual sequence of geological events that is the first of its kind to be observed by scientists, the study authors say.

The earthquakes, which likely struck within two minutes of each other on Sept. 29, 2009, spawned a tsunami that killed nearly 200 people in Samoa, American Samoa and Tonga. Scientists assumed that a single quake under the ocean floor had caused the devastation, but the pattern of far-flung aftershocks, aberrant tsunami waves and the inexplicable movement of a Tongan island cast doubt on that simple explanation.

“We knew right off the bat that something was weird about this earthquake,” says geophysicist Eric Geist of the U.S. Geological Survey in Menlo Park, California. Geist wasn’t involved in the current studies but has puzzled over the anomalous signs produced by the quake. “This is a very complicated event, and these studies, for me, really helped explain a lot.”

The earthquake that everybody knew about was a whopper: a magnitude-8.1 quake in which the ground was pulled apart along a fault. The hidden quake was a different type. It happened about 70 kilometers from its predecessor on a thrust fault where the west-moving Pacific plate dives under the east-bound Tonga block of the Australia plate, an event called subduction. These two plates scrape past each other 24 centimeters each year, “the fastest plate tectonics on the planet,” says study co-author John Beavan of GNS Science in Lower Hutt, New Zealand.



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