Earthquake Swarms Not Just Clustered Around Volcanoes, Geothermal Regions*

The researchers found that, contrary to expectations, swarms occurring
within 30 miles of Japan's volcanoes lasted perhaps twice as long as
swarms in other types of geological formations.

Seattle WA (SPX) Oct 27, 2006

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 2½ years.
Their findings confirmed work they published earlier this year that
looked at data from 72 events in southern California during a 19-year span.

Both studies examined data collected from swarms in which at least 40
earthquakes were recorded in a few-mile radius over two weeks. The
swarms did not follow the well-recognized pattern of an earthquake burst
that begins with a main shock and is followed by numerous smaller
aftershocks.

"We saw a mix of the two kinds of events, swarms or earthquakes and
aftershocks, wherever we looked," Vidale said. "It confirms what people
have suspected. There are earthquake swarms and they are responses to
factors we can't see and don't have a direct way to measure."

The Japanese research is being published tomorrow in the online edition
of Geophysical Research Letters.

The scientists suspect that "swarminess" in volcanic and geothermal
zones might be driven by hot water or magma pushing fault seams apart or
acting to reduce friction and enhancing the seismic activity in those areas.

Away from volcanic and thermal regions, it is unclear what triggers
swarms that don't include main shocks and aftershocks, Vidale said. It
is possible that swarms are driven by tectonic movements so gradual that
they take many minutes to weeks to unfold but still are much more rapid
than normal plate tectonic motions.

The researchers also found that, contrary to expectations, swarms
occurring within 30 miles of Japan's volcanoes lasted perhaps twice as
long as swarms in other types of geological formations. It was expected
that earthquake episodes would have been briefer in hotter rock formations.

The results help to provide a clearer picture of how seismic swarms are
triggered and give a better means of assessing the danger level for
people living in tectonically active regions where earthquake swarms
might occur, Vidale said.