Seismic waves trigger bursts of small temblors far away

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SAN FRANCISCO -- When powerful earthquakes strike anywhere in the world, their seismic waves go rippling through the ground and trigger smaller quakes thousands of miles away where no one had ever expected them, scientists have discovered.Geophysicists reported that they have studied records on the aftermath of 15 major quakes during the past 18 years and found evidence that they regularly caused small temblors on distant, obscure faults in 12 of those events.But just what causes those clearly detectable smaller quakes in areas touched by seismic waves from far more powerful temblors remains an unsolved mystery, the scientists say.Tom Parsons, a research geophysicist at the U.S. Geological Survey in Menlo Park, Calif., and his colleagues in Utah and Texas call the phenomenon dynamic triggering. They say the triggered temblors can occur even in stable ground where faults have long been inactive, as well as in volcanic ground and hot springs where the earth is always unstable.Parsons is on the team headed by Kris L. Pankow, assistant director of the University of Utah's Seismographic Stations, that reported the findings Sunday in the London journal Nature Geoscience.The group studied records from hundreds of small temblors recorded at 500 seismographic stations worldwide after the 15 major quakes, including the enormously powerful one beneath Sumatra and the Andaman Islands in 2004. That quake struck with a magnitude of 9.2 and caused the tsunami that killed 225,000 people around the Indian Ocean.Every earthquake sends many types of seismic waves coursing through the Earth -- some deep within the Earth's crust and others across the surface. Two types of surface waves are known as Rayleigh and Love waves, and the scientists focused on the impact of those.Rayleigh waves move the ground from side to side like a snake and cause most of the shaking in any earthquake, while Love waves move up and down as they travel, very much like waves on the ocean.The deadly quake in Indonesia, for example, sent both Love and Raleigh waves across the Earth's surface and they triggered small quakes halfway around the world at Mount Wrangell in Alaska nearly two hours later, the scientists said. The Indonesian quake also triggered a cluster of small temblors with magnitudes around 2.5 near the Long Valley Caldera on the Nevada-California line, according to the report.The Denali quake of 2002 that struck in Alaska with a magnitude of 7.7 and California's 7.5 magnitude Landers quake of 1992 in the Mojave Desert were two others whose seismic waves triggered bursts of tremors halfway across the world.Seismologists have long known that seismic waves travel all around the world, but the fact that they could cause temblors far away and sometimes hours later was unknown. "When earthquakes strike, the entire Earth rings like a bell" has been a common saying among earthquake scientists.Californians are accustomed to modest earthquakes because they happen on the San Andreas and on all the smaller faults throughout the state. But most people aren't aware that every five minutes about 600 smaller seismic events are recorded by instrument networks all over the world.The researchers first examined the record of Love waves during the five hours after each of the 15 major quakes they studied, and calculated where and when small quakes would have been detected as those waves passed. The number of small quakes worldwide increased by 37 percent within each five-hour period, the scientists found. And when Rayleigh waves followed the Love waves, the number of small quakes triggered worldwide shot up by 60 percent, the group reported."This seems to happen everywhere and almost every time there's a major quake anywhere in the world," Parsons said. "We still don't understand how those surface waves apparently increase the stress on faults so far away, but what we've found should help us understand the physics of what's happening."(E-mail Chronicle Science Editor David Perlman at dperlman(at)sfchronicle.com)(Distributed by Scripps Howard News Service, www.scrippsnews.com.)