What scientists learned about a Bay Area fault that could unleash a magnitude 6.9 earthquake

A new study shines a light on a system of earthquake faults in the San Francisco Bay Area that most residents don't even know exists. The Foothill Thrust Belt faults are deep under Silicon Valley, and researchers at Stanford found they're capable of generating a magnitude 6.9 earthquake every 250 to 300 years.

To put a magnitude 6.9 earthquake in perspective, the 1989 Loma Prieta earthquake measured this magnitude. It killed 63 people and injured 3,757 and was the largest tremor to occur on the San Andreas Fault since the 1906 earthquake, according to the U.S. Geological Survey.

"It's a reminder that we think a lot about San Andreas and Hayward, and those are faults that are very active, but we know there are other faults in the Bay Area that are less active but can still have earthquakes," said Stephen DeLong, a supervisory research geologist with the USGS who peer-reviewed the study.

Earthquake danger in the Bay Area continues to be dominated by faults such as the much longer San Andreas and Hayward, DeLong explained, but the Foothill Thrust could also produce a devastating shake that shouldn't be ignored.

Flanked by the Santa Cruz Mountains and the Santa Clara Valley, the Foothill Thrust Belt runs through the foothills of Silicon Valley and comprises two main so-called thrust faults: the Shannon-Monte Vista Fault and Berrocal-Sargent Fault. These two faults lie east of the San Andreas Fault, extending from south of Gilroy through Silicon Valley past Palo Alto.

The study, which has been peer-reviewed and accepted for publishing, is not forecasting when a magnitude 6.9 earthquake will happen on the Foothill Thrust faults, nor is it saying for certain that a big earthquake is going to happen on them every several centuries. Rather, it reveals the shortest recurrence time for an earthquake of that size (according to scientists, that time is 250 to 300 years), assuming "no other processes of seismic energy release occur along the faults such as creep," said Felipe Aron, the study's lead author and a Stanford postdoctoral research fellow. Faults can do what seismologists call "creep," moving slowly and continuously, never releasing an earthquake. At the other end of the spectrum, they can store energy and then explode with a massive earthquake. The team also found that a magnitude 6.5 quake could occur every 100 to 150 years.

Thrust, or reverse, faults are features in the Earth's crust that occur when a massive slab of rock moves up and over a lower block of rock, which moves deeper into the Earth, Aron explained. These are the types of faults that can form mountain ranges.

"An earthquake is produced when the crustal blocks slide past one another along the fault in a sudden rapid motion, liberating most of the energy accumulated when the fault was silent," wrote Aron, who works as a research fellow at the Research Center for Integrated Disaster Risk Management and is an adjunct assistant professor at Pontificia Universidad Católica de Chile. "Thrust faults are difficult to study because an earthquake doesn't usually rupture the ground, and that makes it hard to understand the fault motion or the fault's earthquake history."

In an effort to find a new way to look at these tricky-to-study faults, the researchers created a computer simulation that took into account the landscape around the fault, such as the river drainages within the surrounding mountains. They also looked at how fast other faults are moving.

"I think the main novelty of our study is that we were able to formally link two, apparently independent, natural processes, the topographic structure of the river drainage network of mountain ranges uplifted by faults, with the rate at which slip and earthquake magnitude potential accumulates on those faults over time, something that hasn't been done before," Aron said.

DeLong called the study method clever and elegant, saying researchers usually depend on direct evidence of a fault's motion, whereas this study used indirect data. George Hilley, a geological sciences professor at Stanford and co-author of the study, said the study's findings align with what seismologists already know about the fault. The more exciting finding, he says, is that the method of research used was successful.

"This is a new way of characterizing the rates of motion on these faults," Hilley told SFGATE. "Seismologists haven't done this before where they use the landscape itself as well as other geographic information to determine how faults might slip."

While this study looked at faults in a region where the USGS already has deep knowledge about earthquake hazards and knows about the potential for a large earthquake on the Foothill Thrust Belt, the method used is transferable to other geographic locations where information is lacking.

For everyday Bay Area residents, the study is an important reminder to be prepared for the next big one.

"The message isn't that we need to be more concerned," DeLong said. "These faults are in our USGS models. The main thing the public needs to keep in mind is to have a plan in mind for when strong shaking can happen. Being prepared for that is the most important message."