Little Known SoCal Fault Could Trigger The Next Big One

LOS ANGELES, CA — When it comes to the Big One, all eyes turn to California’s San Andreas and Hayward Faults, but a trio of lesser-known faults have the potential to devastate both the Bay Area and Southern California.

Two studies out of Harvard and Stanford universities this week shed light on the secondary fault networks that could trigger quakes strong enough to topple buildings in the Golden State’s major metropolitan areas. Fractured and much more difficult to study, these faults are no less capable of calamity.

Findings in a study out of Harvard University paint a grimmer picture for Southern California. The Palos Verdes fault zone, a system of interconnected faults stretching the length of Los Angeles and Orange Counties, is capable of generating a magnitude 7.8 earthquake, the study concluded. What’s worse is that the fault is slipping at a much faster rate than previously understood.

The study published in the Bulletin of the Seismological Society of America, changes scientists’ understanding of the fault, which was previously thought to be a series of smaller faults. Connected over a larger surface area, it’s capable of a much larger rupture than previously understood.

“Historically, this fault has been seen as a segmented fault — lots of little pieces,” John H. Shaw, a professor of structural and economic geology at Harvard University and one of the study’s authors told the Los Angeles Times. “This seemed like a structure that wasn’t going to rupture in one big earthquake.”

However, using data from oil company drilling and ground sensors, the Harvard scientists were able to map the largely underwater fault along nearly 70 miles of Southern California’s coast.
According to the Harvard scientists, a single rupture along the fault system could create a quake as large as magnitude 7.4, and multiple ruptures could create quakes as large as magnitude 7.8.

The study concluded that quakes between magnitude 7.1 and 7.4 along the fault line could occur every 580 to 610 years while the magnitude 7.8 quake would be more likely to occur every 760 to 1170 years.

A quake of that magnitude beneath a population of nearly 16 million people would be catastrophic. It would be nearly 60 times more powerful and longer than the magnitude 6.7 Northridge quake that killed 57 people, injured thousands and caused tens of billions of dollars in damage in 1994.

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A good comparison would be the magnitude 7.1 Ridgecrest quake of 2019. It struck in a sparsely populated area, limiting its damage. However, it struck along a series of interconnected fault lines similar to the Palos Verdes Fault, and it triggered thousands of additional smaller quakes over the course of the ensuing months.

According to the U.S. Geological Survey, a quake of about a magnitude 8.0 in Southern California could kill more than 1,800 people, injure 50,000 more and cause hundreds of billions in damage. It would be felt across California, according to the Southern California Earthquake Center.

Historically, large earthquakes of that size have caused soil liquefaction in coastal areas and the Los Angeles basin where trees can actually sink into the soil.

The good news is that scientists don't see a major quake along the Palos Verdes Fault as very likely in our lifetime because it's not very active.

“California is already a center for earthquakes, and the danger for a large earthquake exists constantly,” Shaw told the Times. “This helps us ... understand this particular fault system location, where large earthquakes might occur. It helps us understand the hazards these earthquakes might create so we can prepare for them better."

The study out of Stanford University focuses on a system of faults that run the length of Silicon Valley. Scientists studied the Shannon-Monte Vista and Berrocal-Sargent faults, which are part of a system known as the Foothill Thrust Belt faults. They run east of the San Andreas from just north of Palo Alto through the Silicon Valley and south past Gilroy.

“We discovered that a quake of similar size to the 1989 Loma Prieta event, the last devastating earthquake affecting this populous economic hub, could occur every 250-300 years,” Stanford University scientists concluded.

The Foothill Thrust Belt is a fractured system of faults that have traditionally been difficult to study, in part, because they are thrust faults that don’t typically rupture the ground the way slip faults such as the San Andreas and Hayward faults do. They also have a lower slip rate, giving scientists less data about their energy buildup. However, scientists at Stanford used topographical data and models to help assess the buildup of energy in these unseen faults.

The study found that a system of faults could produce 6.9 temblor. Such a quake would be felt across the Bay Area.

A quake of that magnitude would be almost 10 times as powerful as the 2014 Napa Quake, which killed one person, injured hundreds of others and caused hundreds of millions of dollars worth of damage. That quake also damaged thousands of homes, buildings and businesses.

The last time a 6.9 magnitude earthquake struck the Bay Area was the Loma Prieta earthquake in 1989. It was epicentered under Santa Cruz County and caused 63 deaths, nearly 4,000 injuries and the collapse of a stretch of the San Francisco–Oakland Bay Bridge.

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According to Stephen DeLong, a U.S. Geological Survey scientist and expert on earthquake hazards in Northern California who peer-reviewed the Stanford study, the topographical model is an elegant approach that could help scientists better understand secondary faults known to loom beneath densely populated areas.

“It's a good reminder of these secondary faults,” said DeLong. “They pose a hazard to people who live in the Silicon Valley, and folks should have a plan in place for an earthquake.”

Residents can ready themselves for the next large quake by downloading the Shake Alert app to get advance warning as a major quake hits, maintaining an earthquake kit, and having a plan, he said.

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