The San Andreas and Cascadia earthquake faults may be related

October 10, 2025

4 minutes read

New evidence suggests that the San Andreas and Cascadia faults may produce simultaneous earthquakes

Seafloor samples reveal evidence of several earthquakes along the West Coast's two major fault zones occurring in rapid succession over the past 3,000 years.

Written by Jack Tamesiya, edited by Andrea Thompson

An aerial view of the San Andreas Fault crossing the Carrizo Plain in California.

Cavan Images/Peter Essick/Getty Images

The west coast of North America is a geologically turbulent area where tectonic plates collide, sink under, and scratch each other. Over the centuries, this activity has regularly caused major earthquakes. New research reveals that some of these seismic events may have occurred simultaneously along two major faults on the coast: the San Andreas Fault and the Cascadia Subduction Zone.

A team of researchers analyzed a collection of seafloor sediments from the area where faults meet off the coast of Northern California. The researchers' findings, recently published in the journal Geosphere, reveal that fault systems have produced numerous simultaneous earthquakes over the past 3,000 years.

Chris Goldfinger, a marine geologist at Oregon State University and lead author of the new paper, compares the process to tuning an analog radio, where the device's oscillators are synchronized to transform incoming signals. “When you tune an old radio, you basically cause one oscillator to vibrate at the same frequency as the other oscillator,” he says. “When these faults coincide, one fault can set the other on hold and cause earthquakes in pairs.”

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The Cascadia subduction zone, where the Juan de Fuca and Jorda plates slide beneath the North American plate, extends all the way from Vancouver Island to northern California to meet the San Andreas Fault. This fault extends south for 750 miles along the boundary where the North American and Pacific plates slide past each other.

Chris Goldfinger, a marine geologist at Oregon State University, has samples of sediment on the seafloor.

Since 1999, Goldfinger and his team have been drilling into the seafloor at a tectonic crossroads, known as the Mendocino Triple Junction, to pull up cores that show a cross-section of the sediments that accumulated there. In the new study, the researchers examined more than 130 sediment cores that record nearly 3,000 years of geologic history. Many cores contain layered sediments known as turbidites, which are created by marine landslides that move large amounts of material around the ocean floor. Many of these landslides are caused by earthquakes, making turbidite layers a useful way to determine past seismic events.

Most turbidites have coarse sediment layers at the bottom and fine, silt-like sediments at the top, similar to what you get when you roll up a bucket of sand at the beach. But the turbidites in samples taken from Mendocino Triple Junction “appear to be upside down with all the sand on top,” Goldfinger says. “As far as we know, gravity hasn't changed.”

As they examined the puzzling features, Goldfinger realized that the cores contained two turbidites stacked on top of each other. This provides evidence that two separate earthquakes occurred in quick succession – the first settled a layer of silt on top of the ocean floor, and the second shock sent another avalanche of sand over the summit.

Some turbidity layers are so far apart that these events can occur anywhere within minutes to decades of each other. Analysis of the ages of shells in sediments indicates that there have been at least eight large earthquakes along the San Andreas Fault over the past 3,000 years that occurred during decades of large earthquakes along the Cascadia subduction zone.

The idea that fault systems near each other could coincide has been floating around for years, and has been seen at smaller fault boundaries over short periods, says Meng Wei, a marine geologist and geophysicist at the University of Rhode Island. But he says the new research is impressive because it shows that this phenomenon is possible with larger fault systems over thousands of years.

Although the Cascadia and San Andreas systems have apparently been linked for thousands of years, there appears to be some discrepancy when it comes to the timing between successive earthquakes. Wei, who was not involved in the new study, says it's possible that the two quakes could trigger tremors within a few years of each other at some point in the future, but more research is needed to measure how one earthquake triggers another. He adds: “Even if these two faults were synchronized, the time interval between earthquakes could reach decades.”

The two systems are also not completely in sync. The team discovered that some earthquakes, including the 1906 earthquake that devastated San Francisco, were one-time events caused exclusively by movements along the northern San Andreas Fault.

CT scan images of turbidites in deep-sea sediment cores. On the left, a thin layer of turbidites from the 1906 earthquake. On the right, from an earthquake about 1,500 years ago, typical “inverted double layers” — double or triple the thickness of the turbidite. The thick sand at the top is the San Andreas Bed, with the Cascadia Bed below.

But if a fault system produces major earthquakes in rapid succession, it could cause major disasters along the west coast of North America. An incipient earthquake along the Cascadia subduction zone would draw recovery resources into the Pacific Northwest, making response to the subsequent San Andreas earthquake difficult.

Goldfinger hopes the new work will help influence seismic risk planning for communities near the two fault systems. “In the paper we stuck to the geology rather than delving into the potential doom and gloom,” he says. “But it's very clear that if something like this happens — and we think the evidence for it is strong — we have to be prepared.”

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