Web Extra  Wednesday, September 18

Izu Islands' stresses

During the summer of 2000, more than 7,000 earthquakes rumbled beneath the Pacific Ocean, 75 miles 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 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 the Earth crust increase. The study, published in the September 5 issue of Nature, finds this relationship in the Izu Islands volcanic chain.

During the summer of 2000, magma intrusion into the Earth's crust triggered a swarm of 7,000 earthquakes in the Izu Islands south of Tokyo. The colors indicate the changes in stress rates caused by the intrusion (warm colors indicate increases in stress rates, cool colors indicate decreases). Earthquakes, as marked by the black dots, occurred much more frequently in areas with high increases in stress rates. Image by Serkan Bozkurt, USGS.

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 U.S. Geological Survey geophysicist and co-author of the study.

The team used slight deformations in the 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 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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Animations of magma intrusion and earthquakes in the Izu Islands