Potential for 6.9 quake along fault in Silicon Valley

A lesser-studied fault system along the western side of Silicon Valley could generate a magnitude 6.9 earthquake — the same size as 1989’s infamous Loma Prieta — every 250 to 300 years, a new Stanford study found.

The study adds to the understanding of how much risk the densely populated Silicon Valley region may face from the faults running underneath, which are particularly difficult to study using traditional geologic methods.

“The important message here is that we talk a lot about the San Andreas Fault and the Hayward Fault as being potentially hazardous, but we do know that there are many other faults underneath the San Francisco Bay Area that are capable of generating earthquakes,” said Stephen DeLong, a U.S. Geological Survey scientist who specializes in understanding earthquake hazards in Northern Cali

fornia. DeLong peer-reviewed the Stanford study.

“The possibility of these moderate earthquakes every few hundred years is consistent with what we’ve previously thought about these faults,” DeLong said.

The faults are the ShannonMonte Vista Fault and BerrocalSargent Fault; they are collectively known as the Foothill Thrust Belt faults. They lie east of the San Andreas Fault and span the inner edge of the Sierra Azul mountain range, extending from south of Gilroy through Silicon Valley past Palo Alto.

As always, “can” is different from “will”: The study isn’t predicting that a magnitude 6.9 earthquake will happen in the Silicon Valley area every 250 to 300 years, said Felipe Aron, who led the study as a Stanford postdoctoral research fellow and in his current job at the the Research Center for Integrated Disaster Risk Management at the Pontifical Catholic University of Chile. After all, it’s impossible with current science to predict when, where and what magnitude the next major earthquake will be.

Instead, Aron and the other researchers calculated the rate at which energy builds up along the Foothill Thrust Belt. Knowing how much energy is released in a magnitude 6.9 earthquake, they were able to calculate that the Foothill Thrust Belt could accumulate enough energy to

generate one every 250 to 300 years.

It’s unknown the last time the Foothill Thrust Belt released most of its stress in a major earthquake — and from when the countdown would begin, Aron said.

“Whenever we are able to learn when the clock really started ticking, we could use this type of metric (from the study) to interpret what would be more or less the timing for the next earthquake to happen. But we’re not quite there yet,” Aron said.

The Foothill Thrust Belt faults didn’t cause Loma Prieta, the last major earthquake to strike the Bay Area, which killed 63 people, injured thousands and inflicted as much as $10 billion in damages and business interruptions. (The question of which fault caused Loma Prieta is itself a subject of debate among earthquake geologists, Aron said.) Historical records indicate a major earthquake last occurred in the Silicon Valley area in 1865, but there isn’t enough evidence to link it to the Foothill Thrust Belt, Aron said.

The conclusion of a “magnitude 6.9 earthquake every 250 or 300 years” also assumes no other stress has been released from when the clock started ticking, Aron said. In reality, marginal amounts of stress are released all the time as faults move, sometimes in small earthquakes, so the metric serves more as a maximum bound, Aron said.

Part of the problem in understanding earthquake behavior,

especially in the area below Silicon Valley, is how difficult it is to gather direct evidence from faults, DeLong said. The USGS creates hazard models by calculating fault slip rates, which is how fast the fault moves over thousands of years, and by using the fault’s history of earthquakes, DeLong said.

Neither is well-known about the Foothill Thrust Belt, so the USGS uses what is known about the faults and others in the area to model hazard levels, Delong said. “What (the Stanford study has) done is integrate a lot of indirect evidence to support that (hazard) conclusion in a really elegant way,” DeLong said.

The Foothill Thrust Belt was formed by an approximately 11degree bend in the San Andreas Fault, said Stanford professor George Hilley, Aron’s research supervisor. The San Andreas Fault — which runs largely parallel to the Foothill Thrust Belt faults between Gilroy and Stanford until intersecting them near Stanford — is a strike-slip fault, which means the two blocks of the earth’s crust that make up the fault slide past one another in opposite directions.

But at the 11-degree bend — which, interestingly, occurs near the peak named Loma Prieta in the Santa Cruz Mountains — the San Andreas fault blocks no longer slide relatively easily past each other, causing rock to push up against each other and form the Sierra Azul, Hilley said. The Foothill Thrust Belt contributes to the uplifting of the earth’s surface to create the mountain range, Hilley said.

The Foothill Thrust Belt faults are thrust faults, meaning the upper block moves up and over the lower block that moves deeper into the earth. 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, Hilley said.

“The evidence for those old slips along that fault are constantly being buried because the mountains are being uplifted over,” Hilley said.

The method pioneered by Aron, Hilley and their colleagues’ research uses topographic information about the shape of mountain rivers that have been uplifted by earthquakes. They then used this information to infer the rate at which energy builds up along the Foothill Thrust Belt.

The basis of the new method “points to the interconnections between the natural systems,” Aron said. He’s hopeful this new method can be used to better understand earthquake hazard in other areas where such data is hard to collect, such as Silicon Valley.

“This is providing a new methodology that hasn’t been used before and can potentially be used in other places with less information to help communities understand the seismic hazards that they are living with,” Aron said.