New Official Federal Quake Model Raises Likelihood of the Big One

Wednesday, March 11, 2015

The odds used to be, if you lived in California for 617 years you would experience a monstrous 8.0 earthquake sometime in your life. Now scientists are saying 494-year-olds can expect to get the shock of a lifetime.

Of course, no one is planning to live that long. So a more meaningful expression of new calculations from the U.S. Geological Survey (USGS) is that the odds of an 8.0 quake in the next 30 years have risen from 4.7% to 7%. While that is only 2.3 percentage points higher, it can also be read as a much scarier 48.9% increase.

The new report abandons past models that assumed an earthquake happened along a single fault, and factors in multiple faults rupturing simultaneously. “This is a significant advancement in terms of representing a broader range of earthquakes throughout California’s complex fault system,” USGS seismologist Ned Field, the lead author of the report, told the Los Angeles Times.

The report is quick to point out that much is unknown about earthquakes and faults, and its projections are “approximations.” For instance, “it does not model the earthquake-triggering process that produces aftershocks.”

The new model takes into account a growing body of work on the complexity of California earthquake fault structure. “It has become difficult to identify where some faults end and others begin, implying many more opportunities for multifault ruptures,” the report says.

Southern California, including Los Angeles, earns a full 7% chance of an 8.0 shaker, compared to Northern California (5%) and the San Francisco region (4%). The Bay Area fares better than the south partly because the 1906 quake on the San Andreas fault that leveled San Francisco relieved some of the pressure there.

While 8.0 quakes are guaranteed to wreak havoc, residents who experienced the 1994 Northridge quake in Southern California can attest to the damage and fear a 6.7 shaker delivers. The new report says there is better than a 99% chance the state gets one of those in the next 30 years.

However, USGS’s new model, the Uniform California Earthquake Rupture Forecast (UCERF3), predicts a decrease in the average time between 6.7 quakes from 6.3 years to 4.8 years compared to its predecessor, UCERF2. That is a 30% decrease, and is the opposite of 8.0 quakes, whose frequency increased 20%. 

Not all faults are created alike. The new model predicts a three-fold increase in the likelihood of a 6.7 quake on the Calaveras fault and a three-fold decrease on the San Jacinto fault.

The new numbers from the USGS are important for more than just their ability to scare Californians witless. State and local governments use them to set building standards and insurance companies determine seismic risk and rates with them. The report emphasizes that the data has to be applied on a case-by-case basis, rather than across the board.

“Conclusions will vary depending on whether you are designing a single family dwelling in Sacramento, retrofitting the San Francisco–Oakland Bay Bridge, considering the location of a nuclear power plant, laying pipeline across the San Andreas Fault, or considering aggregate losses over a large insurance portfolio.

The new multifault model, which increases the likelihood of bigger quakes and consequently reduces the risk of smaller ones, poses a larger threat to big buildings and a lesser one to residential homes, the report said.

–Ken Broder


To Learn More:

Big Bay Area Quake: 72 Percent Chance in Next 30 Years, Scientists Say (by Lisa M. Krieger, San Jose Mercury News)

USGS Ups Odds of 8.0 Earthquake (by Gary Robbins, U-T San Diego)

Risk of 8.0 Earthquake in California Rises, USGS Says (by Rong-Gong Lin II and Rosanna Xia, Los Angeles Times)

Report Says Bay Area Earthquake Faults Are “Locked and Loaded” (by Ken Broder, AllGov California)

UCERF3: A New Earthquake Forecast for California’s Complex Fault System (U.S. Geological Survey) (pdf)

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