For decades, the specter of the "Big One"—a catastrophic earthquake along California's San Andreas Fault—has haunted the minds of West Coast residents and emergency planners. But new research, published in late September 2025 and drawing on decades of geological detective work, suggests that the true worst-case scenario could be even more dire: two of North America's most dangerous fault lines, the San Andreas Fault and the Cascadia subduction zone, may be capable of rupturing in tandem, unleashing devastation across a vast swath of the continent.
According to a study led by Oregon State University geologist Chris Goldfinger and published in the journal Geosphere, these two faults have been moving in a kind of geological "dance" for thousands of years. By examining deep-sea sediment cores from the Cascadia megathrust, Goldfinger's team found evidence that major quakes on one fault may have helped trigger ruptures on the other. This pattern of near-simultaneous earthquakes has persisted through at least ten earthquake cycles over the past 3,100 years, with the most striking example occurring in 1700, when a magnitude 9.0 Cascadia quake was closely followed by a magnitude 7.9 event on the San Andreas.
The implications are staggering. As Goldfinger told Newsweek, "We’re used to hearing the ‘Big One’ – Cascadia – being this catastrophic huge thing. It turns out it’s not the worst-case scenario." Instead, a scenario in which both faults rupture within hours—or even minutes—of each other could present a seismic threat that dwarfs anything previously considered. Such a double disaster would simultaneously impact millions across California, Oregon, Washington state, and Western Canada, including major cities like San Francisco, Portland, Seattle, and Vancouver.
The research hinges on a detailed analysis of earthquake-triggered deposits known as turbidites—layers of sediment that form underwater landslides during intense shaking. By comparing turbidite layers from both fault systems, the team discovered matching pairs, or "doublets," in canyons off Northern California near the Mendocino triple junction, where the San Andreas, Cascadia, and Gorda faults converge. Each doublet represents a pair of earthquakes: one on Cascadia, followed closely by another on San Andreas.
“Northern San Andreas fault events have triggered turbidity currents in the southernmost Cascadia subduction zone, and vice versa,” the study authors wrote. This is evidence, they say, of "partial synchronization" between the two faults—a phenomenon in which stress released by one rupture triggers shaking on the other fault within minutes to hours. In three cases within the past 1,500 years, the researchers believe that fault ruptures at Cascadia and San Andreas occurred just minutes to hours apart.
Goldfinger, a professor emeritus at Oregon State University who studies marine geology, geophysics, paleoseismology, and subduction earthquakes, explained the mechanics to OregonLive: “When a fault ruptures, it’s relieving stress locally, but it’s transferring stress to areas nearby. So when Cascadia ruptures, it transfers stress to Northern California.”
This discovery, as Goldfinger noted, was made possible by a navigational error during research in 1999, which led scientists to collect sediment cores from both fault systems—an unexpected stroke of luck that has since yielded groundbreaking insights. The findings were further validated by examining historical and geological records, including accounts from coastal Japan, where a tsunami struck shortly after the 1700 Cascadia quake. While there is no colonial record of the San Andreas event due to the lack of European settlers in California at the time, the geological evidence is compelling: the sediment created by Cascadia had not yet settled when the San Andreas quake occurred, indicating a near-instantaneous succession.
What does this mean for the present day? As Goldfinger pointed out to Newsweek, “If Cascadia were to have an earthquake, it may serve as a natural warning system for the northern San Andreas. The majority of Cascadia events were followed closely by San Andreas, so I would rank the probability of this happening as high.” However, he also cautioned that while the 1700 earthquakes happened within minutes to hours of each other, the interval could be much longer in other cases—ranging from weeks to decades or even 50 years. The precise timing remains uncertain.
The potential consequences of a double rupture are difficult to overstate. “We could expect that an earthquake on one of the faults alone would draw down the resources of the whole country to respond to it,” Goldfinger said. “And if they both went off together, then you’ve got potentially San Francisco, Portland, Seattle, and Vancouver all in an emergency situation in a compressed timeframe.” The resources required to respond regionally would be roughly double that required for either fault alone. As Goldfinger noted, “The previous worst-case scenario that has been planned for is likely no longer the worst case.”
Despite the mounting evidence, experts warn that the West Coast's current level of preparedness for such a catastrophic event is "poor." Emergency managers at the state and national level need to take note of the possible implications of back-to-back—or even simultaneous—major earthquakes. “Regionally, having two disasters in close timing proximity would be a huge thing for the country to try to respond to,” Goldfinger told OregonLive.
The next step for researchers is to gather more evidence from San Andreas earthquake sites, including a promising location at Lake Merced that covers the last 200 years right in San Francisco. Goldfinger hopes that others will pursue additional records to independently verify—or challenge—the team's findings. He also mentioned that at least one researcher is investigating whether Indigenous oral histories in Northern California might contain accounts of the 1700 San Andreas quake, which could provide further corroboration.
While the study does not predict when the next rupture will occur, it fundamentally shifts how scientists and emergency planners evaluate West Coast earthquake hazards. The Cascadia subduction zone is capable of producing magnitude 9 earthquakes, like the one in 1700, which caused a tsunami that crossed the Pacific. The northern San Andreas, meanwhile, last unleashed major events in 1906 and 1989, both of which devastated San Francisco. Separate research published earlier this year in Proceedings of the National Academy of Sciences showed that a future Cascadia megathrust quake could instantly sink parts of the coastline by several feet and expand floodplains by more than 100 miles.
Ultimately, as Goldfinger and his colleagues have shown, even very dissimilar faults can end up synchronizing over time. Their findings add a sobering new layer of complexity to the challenge of preparing for earthquakes along North America's restless western edge. For millions living in the shadow of these faults, the message is clear: the true "Big One" may not come alone.
