Thrust Faults: Forecasting megaquakes

Can slow creep along thrust faults help forecast  megaquakes?

Summary:

In Japan and areas like the Pacific Northwest where megathrust earthquakes are common, scientists may be able to better forecast large quakes based on periodic increases and decreases in the rate of slow, quiet slipping along the fault.

Diagram showing the edge of the continental plate on which the island of Japan sits (green). At the Japan Trench (right edge), the Pacific Plate (orange) pushes under the wedge-shaped Japanese plate, driving west and downward in quiet slips at a rate of about 8 cm/year. Some places (locked areas) get stuck, however, with small locked areas generating repeated small quakes and larger locked areas generating larger quakes. The new study found that an increase in the frequency of these small off-shore earthquakes presaged larger quakes, including the 2011 Tohoku-oki quake.Credit: Image courtesy of University of California, Berkeley

This hope comes from a new study by Japanese and UC Berkeley seismologists, looking at the more than 1,000-kilimeter-long fault off northeast Japan where the devastating 2011 Tohoku-oki earthquake originated, generating a tsunami that killed thousands. There, the Pacific Plate is trundling under the Japan plate, not only causing megaquakes like the magnitude 9 in 2011, but giving rise to a chain of Japanese volcanoes.

The scientists studied 28 years of earthquake measurements, looking at quakes of magnitude 2.5 or greater between 1984 and 2011. They discovered 1,515 locations off the coast of Japan where small repeating earthquakes happen -- 6,126 quakes in all.

According to co-author Robert Nadeau, a UC Berkeley seismologist and a fellow with the Berkeley Institute for Data Science (BIDS), an analysis of these quakes found that larger, more destructive earthquakes -- those of magnitude 5 or greater -- occurred much more frequently when the periodic slow-slip was fastest. This included the great Tohoku-oki earthquake, which also devastated a nuclear power plant and led to widespread radioactive contamination.

"The persistence of the periodic pattern over time may help us refine earthquake probabilities in the future by taking into account the times of expected slow-slip pulses," he said. "Right now, seismologists gives forecasts on a 30-year time frame and assume nothing is changing on a shorter time scale. Our study points out that things are changing, and in a periodic way. So it may be possible for scientists to give shorter time ranges of greater and lower probability for larger events to happen."
The research was led by Naoki Uchida, a seismologist at Tohoku University, and included UC Berkeley seismologist Roland Burgmann, professor of earth and planetary science. They published their findings in the Jan. 29 issue of Science.

Story Source:

The above post is reprinted from materials provided by University of California, Berkeley. The original item was written by Robert Sanders. Note: Materials may be edited for content and length.

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