Report Says Bay Area Earthquake Faults Are “Locked and Loaded”

Tuesday, October 21, 2014

Four faults running beneath the Bay Area are “locked and loaded” and poised to devastate population centers and water sources, according to a new study published in the Bulletin of the Seismological Society of America.

James Lienkaemper, the study’s lead author and a research geophysicist at the U.S. Geological Survey, said a new analysis of data predicts a 70% likelihood that one of the four faults, segments of the 800-mile San Andreas Fault, will rupture within the next 30 years.

And by “the next 30 years,” they mean it could happen tomorrow.

All four have reached or are near reaching their average recurrence time. The study estimates the size of each quake: 7.1 for the Rodgers Creek and Green Valley faults, and 6.8 for the Hayward and Northern Calaveras faults. Those are about the size of the Loma Prieta quake 25 years ago that killed 63 people.

The Hayward fault is the most problematic. It is the one most likely to rupture and the one likely to do the most damage. The fault runs beneath Oakland, Berkeley and other East Bay locales where 5 million people live. The Hayward averages a major incident every 160 years and it’s been 146 years since the last one.

The Green Valley Fault, between Napa and Fairfield, is near dams, canals and aqueducts that supply a lot of the state’s water. It hasn’t ruptured since the 1600s, when none of that infrastructure existed or was needed.

Scientists believe that there is almost certainly going to be a big quake in California within the next 30 years, but have usually considered the state’s southern portion as being most at risk. The new study bears down on the northern half.

The surveys relied on in the study measured fault creep, the minute shifts in the Earth that can be used to calculate how much pressure is building along a fault. Twenty-nine sections of faults are measured at 80 monitoring sites.

The Pacific Plate and the North American Plate are always on the move along the San Andreas, quietly crunching past each other without causing any problems. But when something impedes their movement, they become locked and pressure builds.

“The Pacific Plate is having its way with the North American Plate and each fault has an assumed slip rate. But the locked patches are resisting,” Lienkaemper told NBC News. And in the case of earthquake faults, resistance really is futile.

–Ken Broder


To Learn More:

Four California Faults Are Ready to Rupture (by Becky Oskin, Discovery)

Locked Faults Could Pop Big Earthquake in Bay Area: Study (by Linda Carroll, NBC News)

4 Northern California Faults Primed for Big Quakes (by Ellen Knickmeyer, Associated Press)

4 NorCal Faults in Heavily Populated Areas Primed for Earthquake of 6.8 or Greater: Study (by Tracy Bloom and Wendy Burch, KTLA)

Huge Earthquake Could Hit San Francisco Any Day: Report (by Lydia O'Connor, Huffington Post)

Using Surface Creep Rate to Infer Fraction Locked for Sections of the San Andreas Fault System in Northern California from Alignment Array and GPS Data (by James J. Lienkaemper, Forrest S. McFarland, Robert W. Simpson and S. John Caskey, Bulletin of the Seismological Society of America)

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