High magnitude quakes can cause minor jolts worlwide

Definitive study shows large earthquakes routinely trigger smaller jolts worldwide, including on the opposite side of the planet and in areas not prone to quakes. Until 1992, when California's magnitude-7.3 Landers earthquake set off small jolts as far away as Yellowstone National Park, scientists did not believe large earthquakes sparked smaller tremors at distant locations. "Previously it was thought seismically active regions or geothermal areas were most vulnerable to large earthquake triggers," says Kris Pankow, a seismologist at the University of Utah Seismograph Stations and a co-author of the new study.
But Pankow and colleagues analyzed 15 major earthquakes stronger than magnitude-7.0 since 1992, and found that at least 12 of them triggered small quakes hundreds and even thousands of miles away, according to the findings published online Sunday, May 25, 2008 in the journal Nature Geoscience.

"We conclude that dynamic triggering is a ubiquitous phenomenon," they wrote. Pankow conducted the study with seismologist Aaron Velasco and undergraduate student Stephen Hernandez, both at the University of Texas at El Paso; and seismologist Tom Parsons, of U.S. Geological Survey in Menlo Park, Calif. They analyzed data from more than 500 seismic recording stations five hours before and five hours after earthquakes that registered more than 7.0 on the "moment magnitude" scale, which scientists say is the most accurate scale for large earthquakes. (The frequently cited Richter scale measures only relatively small, nearby quakes). Univ. of Utah.

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