A new study finds that the San Andreas and Cascadia faults could combine to cause a massive double earthquake.

It's like a plot from a Hollywood movie. A massive earthquake on a West Coast fault triggers other earthquakes farther away, causing widespread devastation over hundreds of miles.

A new study released Tuesday revealed that the scenario may not be as far-fetched as previously thought.

Scientists from Oregon State University studying sedimentary strata in the Pacific Ocean have discovered that two of the most famous faults on the West Coast — the San Andreas fault in northern California and the Cascadia subduction zone off Oregon and Washington — may be synchronous, with earthquakes on one fault having the potential to set off the other fault.

The study, published in the journal Geosphere, concluded that there have been at least three cases in the past 1,500 years, including the most recent case from 1700, when researchers believe massive ruptures occurred on each fault just minutes to hours apart. Scientists said it appears that a large earthquake in the Cascadia subduction zone triggered the San Andreas Fault in these cases, causing another large earthquake in Northern California.

“We're used to hearing that 'the big thing' — Cascadia — is this huge, catastrophic thing,” said Chris Goldfinger, a marine geologist at Oregon State University and lead author of the study. “It turns out that's not the worst case scenario.”

Large earthquakes rarely occur on either fault. But each has the potential to kill thousands of people and cause billions of dollars in damage, previous studies have shown.

The potential connection between the two faults means disaster planners must take into account the possibility of successive earthquakes, researchers said Tuesday.

“We can expect that an earthquake on one fault alone would exhaust the entire country's resources to respond,” Goldfinger said. “And if they both exploded together, San Francisco, Portland, Seattle and Vancouver would all likely be in a state of emergency in a compressed time frame.”

To be sure, strong earthquakes that occur on the two faults are very rare. The San Andreas Fault experiences a major earthquake about every 225 years on average. The Cascadia Subduction Zone, an offshore rift that extends from Cape Mendocino, in far northern California, to Vancouver Island in British Columbia, produces a massive earthquake every 500 years on average, according to geological records.

In other words, a Bay Area resident could have been born in 1907, lived to age 81, and not missed the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake, the two most devastating Northern California earthquakes of the 20th century on the San Andreas Fault.

“I don't think you should walk around all day worrying about the Cascadia and San Andreas faults coming up and giving us a double whammy,” said David Schwartz, a geologist and scientist emeritus with the USGS at Moffett Field. “If it happens, it will happen. But I'm more concerned about the Hayward Fault or the Calaveras Fault or Rogers Creek here in the Bay Area. You can't walk around worrying about those things. But you can prepare.”

The last major damaging earthquake in the Bay Area was the 6.0 magnitude South Napa earthquake on August 24, 2014, which killed one person, injured 300, and caused $1 billion in damage in Napa and Vallejo. This earthquake occurred on the West Napa Fault, which is part of the San Andreas Fault Zone.

Before that, it was the 6.9-magnitude Loma Prieta earthquake on October 17, 1989, that disrupted the World Series, destroying the Bay Bridge, the Oakland Cypress Expressway, downtown Santa Cruz, and parts of San Francisco, killing 63 people and causing $6 billion in damage.

Millions of new people have moved to Northern California and been born since 1989.

Schwartz said it's important for everyone to prepare for earthquakes by taking steps like mounting bookshelves to the wall, stocking up on backup water supplies, purchasing a battery-operated radio, and making a plan for their families in the event of multi-day power outages and other disruptions.

“It's a West Coast kind of life,” he said. “If you're prepared — you've got your house ready, your family's ready, you know what you're going to do — then whatever goes wrong you're prepared. I wouldn't worry about one mistake causing an earthquake in the next. If that happened, it would certainly be bad for the area. But all you can do is prepare yourself for any emergency that might arise.”

Although Californians are relatively familiar with the San Andreas Fault, the Cascadia Subduction Zone is more dangerous in many ways. This fault, where the Juan de Fuca plate subducts beneath the North American plate, is capable of producing earthquakes up to magnitude 9.0.

Studies have shown that an earthquake of this massive size can cause shaking for up to 5 minutes, with most of the damage occurring in Oregon and Washington, and trigger a tsunami wave up to 100 feet high. The last major earthquake in the Cascadia Fault System was on January 26, 1700, with an estimated magnitude of 9.0.

There is a 10-15% chance of a 9.0 magnitude event occurring in the Cascadia subduction zone within the next 50 years, according to the USGS.

The USGS concluded that there was a 72% chance of one or more earthquakes of magnitude 6.7 or greater occurring on any fault in the Bay Area, and a 60% chance in Los Angeles.

Tuesday's study was based on scientists sampling sediment on the ocean floor off northern California, Oregon and Washington. They analyzed layers known as “turbidites” where different sizes of sand and gravel are deposited by underwater landslides often caused by earthquakes, and used radiocarbon dating to study 3,100 years of geological history.

Seismic researchers have theorized about possible connections between the two massive faults for years, and the new paper is likely to expand the debate, Schwartz said.

“They put some specific things on the table to take a closer look at,” he added.

Originally Posted: October 7, 2025 at 3:13pm PDT

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