An earthquake swarm - a steady drumbeat of moderate, related seismic events - over hours or days, often can be observed near a volcano such as Mount St. Helens in Washington state or in a geothermal region such as Yellowstone National Park in Wyoming. New research led by a University of Washington seismologist shows, however, that such swarms can occur anywhere that is seismically active, not just near volcanoes or geothermal regions.

"In our research we saw swarms everywhere and we could see the broad characteristics of how they behaved," said John Vidale, a UW professor of Earth and space sciences and director of the Pacific Northwest Seismograph Network.

Vidale and two colleagues, Katie Boyle of Lawrence Livermore National Laboratory and Peter Shearer of the University of California, San Diego, examined data from 83 Japanese earthquake swarms over about 2