An unknown fault system in Silicon Valley could cause a Loma Prieta earthquake. Here’s how often

A new study at Stanford has found that a less-studied fault system along the western side of Silicon Valley can generate a 6.9-magnitude earthquake — about the same size as the infamous Loma Prieta earthquake in 1989 — every 250 to 300 years.

The study adds to an understanding of how much the densely populated Silicon Valley region may face from faults taking place underneath, which is particularly difficult to study using traditional geological methods.

“The important message here is that we talk a lot about the San Andreas Fault and the Hayward Fault as a potential hazard, but we know that there are many other faults below the San Francisco Bay Area that can generate earthquakes,” said Stephen DeLong, a US Geological Survey scientist who specializes in understanding earthquake hazards. in Northern California. DeLong peer reviewed the Stanford study.

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

The defects are Shannon-Monte Vista and Berrocal-Sargent Fault; They are known collectively as Foothill Thrust Belt Faults. It lies west of the San Andreas Fault and extends at the inner edge of the Sierra Azul mountain range, extending from south Gilroy through Silicon Valley past Palo Alto.

The upper deck of Highway 880 in Auckland is seen after the 1989 Loma Prieta earthquake.

Michael McCorm/The Auckland Tribune

As always, “can” is different from “will”: The study does not expect a 6.9-magnitude earthquake to occur in the Silicon Valley region every 250 to 300 years, said Philip Aaron, a postdoctoral research fellow at Stanford who led the study. After all, it is impossible with current science to predict when, where and how much the next big earthquake will be.

Instead, Aaron and the other researchers calculated the rate at which energy accumulates along the ridgeside intrusion belt. Knowing how much energy is released in a 6.9-magnitude earthquake, they were able to calculate that the hillside storm belt could accumulate enough energy to generate one every 250 to 300 years.

Aaron said it’s unknown the last time the Foothill Thrust Belt released most of its stress in a major earthquake — and from when the countdown began.

“When we are able to tell when the clock has actually started ticking, we can use this kind of scale (from the study) to explain what will be more or less the timing of the next earthquake. But we’re not quite there yet,” said Aaron.

The Foothill Thrust Belt faults did not cause Loma Prieta, the last major earthquake to hit the Bay Area, killing 63 people, injuring thousands, causing up to $10 billion in damage and halting business. (The question of what fault caused the Loma Prieta is itself a topic of debate among seismic geologists, Aaron said.) Historical records indicate that a major earthquake last occurred in the Silicon Valley region in 1865, but there is not enough evidence to link it to it. Aaron said: The hillside storming belt.

Aaron said the “6.9-magnitude earthquake every 250 or 300 years” also assumes that no further pressure has been released since the clock started ticking. In fact, marginal amounts of stress are released all the time as the faults move, sometimes in small earthquakes, so the scale runs more at maximum, Aaron said.

Part of the problem in understanding earthquake behavior, especially in the region below Silicon Valley, DeLong said, is how difficult it is to gather direct evidence from faults. DeLong said the USGS creates hazard models by calculating fault slip rates, the speed at which the fault moves over thousands of years, and by using earthquake history.

Both are not well known about the Foothill Thrust Belt, DeLong said, so the USGS is using what is known about faults and others in the area to model hazard levels. “What it[the Stanford study]did was combine a lot of indirect evidence to support this (risk) conclusion in a really elegant way,” DeLong said.

Stanford University professor George Healy, Aaron’s research supervisor, said the Foothill Thrust Belt was shaped by a roughly 11-degree curve at San Andreas Faul t. The San Andreas fault – which largely parallels the Foothill Thrust Belt between Gilroy and Stanford until their intersection near Stanford – is a slip fault, meaning that two masses of Earth’s crust form the fault in front of each other. in opposite directions.

The severely damaged homes in the Marina district were demolished a few days after the 1989 Loma Prieta earthquake.

Fredrik Larson, Staff / San Francisco Chronicle

But at an 11-degree bend — which happens intriguingly near the summit called Loma Prieta in the Santa Cruz Mountains — the San Andreas fault blocks are no longer “basically parallel” to each other, causing rocks to push against each other and form the Sierra said, Healy said. Azul. Healey said the Foothill Thrust Belt contributes to raising the ground level to create the mountain range.

Foothill Thrust Belt faults are thrust faults, which means that the upper block is moving up and over the lower block moving deeper into the ground. Healey said it is difficult to study thrust faults because an earthquake does not usually rip the ground apart, making it difficult to understand the fault’s motion or the history of the fault earthquake.

“Evidence for ancient slips along this fault is constantly being buried because mountains are being lifted,” Healy said.

The method devised by Aaron, Healy and their colleagues’ research uses topographic information about the shape of mountain rivers lifted by earthquakes. They then used this information to infer the rate at which energy accumulates along the hillside intrusion belt.

The basis of the new method, Aaron said, “indicates the interconnectedness of natural systems.” He hopes that this new method can be used to better understand earthquake risk in other regions where such data is difficult to collect, such as Silicon Valley.

“This provides a new methodology that hasn’t been used before and can be used elsewhere with less information to help communities understand the seismic risks they live with,” Aron said.

Claire Howe is a writer for the San Francisco Chronicle. Email: [email protected], Twitter: clairehao_

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