Contact: Lee Siegel
University of Utah
Caption: A new study shows that a “great earthquake” and tsunami that killed 192 people in Samoa, American Samoa and northern Tonga on Sept. 29, 2009, actually was triple whammy. University of Utah seismologist Keith Koper says the main quake of magnitude 8.1 concealed and triggered two major quakes of magnitude 7.8 that occurred within two minutes of the main shock. Stars on map show epicenters of the three quakes. Dashed lines show boundaries between Earth's crustal plates, and arrows show plate motions. The 8.1 quake occurred when the Pacific plate cracked as it dived slowly westward beneath the Tonga block of the Australia plate, triggering the two 7.8 quakes on the boundary between the Pacific plate and Tonga block. Such a pattern never had been observed previously.
Credit: Credit: Keith Koper, University of Utah Seismograph Stations.
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Related news release: A seismic triple whammy