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Earthquake forecast shows greater chance of magnitude-8 temblors

Study also finds decreased chances of strong Northridge-sized quakes

A new California earthquake forecast lowers the chances of an earthquake around magnitude-6.7 in the next 30 years, but nearly doubles the chances of a catastrophic magnitude-8 quake.

The study was a collaboration by the U.S. Geological Survey, Southern California Earthquake Center, the California Geological Survey and the California Earthquake Authority.

Called the Third California Earthquake Rupture Forecast, or UCERF3, the study found the chances of earthquakes around magnitude 6.7 — the size of the destructive 1994 Northridge earthquake — has gone down by about 30 percent, when compared with a 2008 assessment. The expected frequency of such events statewide has dropped from an average of one per 4.8 years to about one per 6.3 years.

But the estimate for the likelihood that California will experience a magnitude 8 or larger earthquake in the next 30 years has increased from about 4.7 percent in the 2008 assessment to about 7 percent in the new study.

"The new likelihoods are due to the inclusion of possible multi-fault ruptures, where earthquakes are no longer confined to separate, individual faults, but can occasionally rupture multiple faults simultaneously," lead author and USGS scientist Ned Field said. "This is a significant advancement in terms of representing a broader range of earthquakes throughout California's complex fault system."

Two kinds of scientific models are used to inform decisions of how to safeguard against earthquake losses: an Earthquake Rupture Forecast, which indicates where and when the Earth might slip along the state's many faults, and a Ground Motion Prediction model, which estimates the ground shaking given when one of the fault ruptures.

The study is latest earthquake-rupture forecast for California. It improves upon previous models by incorporating the latest data on the state's complex system of active geological faults and new methods for translating the data into earthquake likelihoods, the USGS said. It was developed and reviewed by dozens of scientific experts from the fields of seismology, geology, geodesy, paleoseismology, earthquake physics and earthquake engineering, according to the USGS.

"We are fortunate that seismic activity in California has been relatively low over the past century," said Tom Jordan, director of the Southern California Earthquake Center and a co-author of the study. "But we know that tectonic forces are continually tightening the springs of the San Andreas fault system, making big quakes inevitable."

"The UCERF3 model provides our leaders and the public with improved information about what to expect, so that we can better prepare."

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