Evidence from the AD 2000 Izu Islands swarm that stressing rate governs seismicity

Authors in Japan (can be interviewed in English) Author in California
Shinji Toda
Active Fault Research Center
AIST (Geological Survey of Japan)
Higashi 1-1, Tsukuba, Japan 305-8567
Phone: 81-298-61-3743 Fax: 81-298-52-3461
Ross S. Stein
Earthquake Hazards Team
U.S. Geological Survey, MS 977
345 Middlefield Road, Menlo Park, CA 94025
Tel: 1 650 329 4840 Fax: 1 650 329 4876
Takeshi Sagiya
Geography and Crustal Dynamics Research Center
Geographical Survey Institute
1 Kitasato, Tsukuba, Ibaraki, 305-0811 Japan
TEL: +81-298-64-6939 FAX: +81-298-64-2655
English Summary, Commentary about the paper in Nature by Chris Marone

World Record
During the summer of 2000, a burst of 7,000 M>3 shocks--including five damaging M>6 shocks--occurred 75 miles south of Tokyo, beneath the Pacific Ocean. We believe the total energy release during this two-month-long 2000 Izu Islands swarm is the highest ever recorded. The energy release is almost 10 times larger than the 1980-82 Long Valley, California, swarm (near Mammoth Lakes, California). The rate of energy release at Izu is about 100 times rate in Long Valley.

What Happpened?
We infer that the swarm was caused by a blade-like injection of molten rock into the earth’s crust over an event of 10 miles long and 10 miles deep. Fortunately, the magma did not reach the earth's surface, but the blade (or 'dike') was forced open by the magma pressure a total of about 65 feet. The magma got close enough to the surface under Miyake volcanic island to trigger several steam and debris eruptions, which caused the island to be evacuated.

Hazard Implications
During a volcanic crisis, if we have continuous GPS data and a good understanding of the geometry of the intruding magma, we can forecast the rate and likely distribution of damaging (M>6) earthquakes. Such a continuous network of GPS receivers now exists in several volcanic sites. (The ‘Plate Boundary Observatory proposal by the NSF, NASA, and the USGS to Congress would place them across the entire western U.S.)

Where Do Our Results Apply?
Volcanic areas, such as Hawaii, the Pacific Northwest, Alaska, Yellowstone, and parts of northern and southern California.

Science Implications
Most earthquakes are followed by aftershocks that are smaller than the mainshock and become less frequent with time. But some earthquakes occur in swarms--a sustained high rate of seismicity that eventually stops or peters out. Swarms are common in volcanic areas, but sometimes occur on tectonic faults such as the San Andreas. Why? We offer a new explanation for the occurrence of swarms, attributing them to a sustained increase in the rate at which the crust is stressed. We attribute mainshock-aftershock sequences to a sudden but permanent increase in stress, rather than a change ins stressing rate.

How Do We Use the Izu Swarm To Make This Case?
During the swarm, the calculated rate at which the surrounding crust was stressed increased by a factor of a thousand, triggering earthquakes more than 25 miles from the dike. The swarm was well recorded by Japanese instruments. We use this extraordinary occurrence to test a key prediction of the theory of 'rate/state friction,' (by Jim Dieterich of the USGS), which if validated can be used to forecast seismicity and earthquake hazards in all settings and countries. This theory predicts that the rate of earthquakes increases in direct proportion to the rate at which the crust is stressed, just as we observed. The theory also predicts that the duration of aftershocks of the M=6 mainshocks during the swarm will be shortened as the stressing rate increases. We found that aftershocks M=6 earthquakes at Izu normally last for a year; during the swarm, they lasted a day, in accord with the theory.

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