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This story originally appeared as a Web Extra, Izu
Islands' stresses, on Sept. 18. Visit that story for a related story and
links to more information and graphics.
During the summer of 2000, more than 7,000 earthquakes rumbled beneath the
Pacific Ocean, south of Tokyo amidst the Izu Islands. One of the most dense
networks of GPS (Global Positioning System) receivers and seismometers in the
world registered each centimeter of earth shifting and shaking.
A team of geophysicists capitalized on this rich data set to test a theory about
what causes earthquakes, especially prolonged swarms associated with volcanism.
Their results help validate the seismicity-rate theory developed by Jim Dieterich
of the U.S. Geological Survey (USGS) in 1994. The theory rests on first principles
and laboratory experiments but has rarely been tested in the field.
“This is the best test of Dieterich’s theory,” says Chris Marone,
an expert in earthquake mechanics at Pennsylvania State University.
One of the theory’s key predictions is that earthquake frequencies increase
as stress rates on Earth’s crust increase. The study, published in
the Sept. 5 issue of Nature, finds this relationship in the Izu Islands
volcanic chain.
The swarm of quakes began when a conduit linking magma at depth to Miyake volcanic
island ruptured. Magma rushed out through the rupture and into a blade-shaped
crevice in the crust. Continuing magma flow expanded the blade like a bladder
filling with water and exerted an ever-increasing pressure on the surrounding
crust.
“The blade expanded for seven weeks and constantly stressed the crust up
to 1,000 times normal. It sped up the seismic clock,” says Ross Stein,
a USGS geophysicist and co-author of the study.
The team used slight deformations in Earth’s surface to infer the geometry
of the magma intrusion and how it changed stress rates within the crust.
Layering a map of the observed earthquake frequencies on top of the calculated
changes in stress rates yields the predicted relationship between frequencies
and stressing.
However, the relationship is not perfect. Almost half of the variation in quake
frequencies cannot be explained by stress rates, suggesting the seismicity-rate
theory does not capture all the complexities of quake mechanics in the field.
Even if incomplete, the seismicity-rate theory could improve earthquake predictions
in volcanic areas like Hawaii and the Pacific Northwest. Quickly generating
a map of stressing rates after a magma intrusion could give people days or even
weeks notice that they are living in a danger zone.
Greg Peterson
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