Risk of 8.0 earthquake in California rises, USGS says

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By Ron-Gong Lin II & Rosanna Xia | The Los Angeles Times | March 10, 2015

Estimates of the chance of a magnitude 8.0 or greater earthquake hitting California in the next three decades have been raised from about 4.7% to 7%, the U.S. Geological Survey said Tuesday.

Scientists said the reason for the increased estimate was because of the growing understanding that earthquakes aren’t limited to separate faults, but can start on one fault and jump to others. The result could be multiple faults rupturing in a simultaneous mega-quake.

Stated another way, the chance of an 8.0 or greater quake in California can be expected once every 494 years. The old forecast calculated a rate of one 8.0 or greater earthquake every 617 years.

“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,” said USGS seismologist Ned Field, the lead author of the report.

“This is a significant advancement in terms of representing a broader range of earthquakes throughout California’s complex fault system.”

The report says that past models generally assumed that earthquakes were confined to separate faults, or that long faults like the San Andreas ruptured in separate segments.

But recent large California earthquakes showed how earthquakes can rupture across multiple faults simultaneously. Many are in the Los Angeles area.

The Whittier Narrows earthquake, a magnitude 5.9, struck on the Puente Hills thrust fault system on Oct. 1, 1987. Three days later, a magnitude 5.6 aftershock hit on a different fault. That aftershock killed one person, twisted several chimneys and broke windows. Damage was reported in Whittier, Pico Rivera, Los Angeles and Alhambra.

Bay Area Earthquake

Photo Credit Eric Risberg / Associated Press

Much larger quakes also showed how this could occur, including two that hit the Mojave Desert in the 1990s: the 1992 magnitude 7.3 Landers earthquake and the 1999 magnitude 7.2 Hector Mine earthquake.

It also happened in the 7.2 earthquake that hit along the California-Mexico border on Easter Sunday in 2010. Scientists said the border quake directed tectonic stress toward Southern California, putting the region at a higher risk for a future quake.

Data showed the April 4, 2010, quake and its aftershocks triggered movement on at least six faults, including the Elsinore and San Jacinto faults. Those faults run close to heavily populated areas in eastern Los Angeles County and the Inland Empire.

At the time, scientists said the imagery gave proof that earthquakes zipping along a fault can jump over gaps as long as seven miles. Previously, only jumps of three miles had been observed. There was also proof that earthquakes can reverse directions, an observation that had never been seen before.

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