As if one West Coast megaquake were not enough, new research suggests the region’s feared “Big One” might show up with a plus-one. A team led by Oregon State University geologist Chris Goldfinger argues that a massive Cascadia megathrust earthquake could, at least sometimes, trigger a major rupture on the northern San Andreas fault shortly afterward, turning a regional disaster into near-simultaneous crises from San Francisco through Portland and up to Vancouver.
The work appears in Geosphere, where Goldfinger and co-authors report combing through roughly 3,100 years of deep-sea sediment pulled from channels near the Cape Mendocino triple junction. In a university release, Goldfinger said the team kept finding paired, “upside-down” layers of sediment, with coarse sand sitting on top of finer silt. That pattern is hard to explain as ordinary aftershocks, and is best explained by a big Cascadia rupture followed minutes to hours later by motion on the nearby San Andreas fault, according to Oregon State University.
What the sediment record shows
Seafloor cores record underwater avalanches of mud and sand, known as turbidity currents, as stacked layers called turbidites. In this new study, the researchers say many sites contain distinctive “doublet” layers that invert the normal grain-size order, with the coarser material on top rather than at the bottom.
As summarized by ScienceDaily, the team identified about 18 likely Cascadia turbidites and 19 from the northern San Andreas system over the last three millennia, and ten of those appear closely linked in time. LiveScience reports that radiocarbon dating and sediment layering suggest at least three events, including the major Cascadia megathrust quake in 1700, where the two fault systems produced deposits within hours or days of one another.
How rare, and how bad
The authors stress that only a handful of these near-simultaneous cases occur in the late Holocene, suggesting this is not an every-century event. Still, they argue that the number of close matches implies partial synchronization is a real, if infrequent, hazard rather than a quirky footnote.
Modeling and scenario work already show that a full Cascadia magnitude 9 event would bring catastrophic shaking to the Pacific Northwest, along with an immediate tsunami racing toward the coast. A near-concurrent northern San Andreas rupture on top of that could lengthen the duration of strong shaking and pile on inland damage across parts of California, Oregon and Washington, according to the paper and coverage in The Guardian. The historical 1700 quake, the most recent candidate for a paired sequence, remains central to how the authors interpret those odd “doublet” turbidites.
What emergency planners are already doing
Emergency managers on the West Coast have long practiced for worst-case scenarios, even before this double-quake idea gained traction. Washington state and federal partners have run the Cascadia Rising exercises and Hazus modeling for a magnitude 9 Cascadia event, while California’s Great ShakeOut drills revolve around a magnitude 7.8 San Andreas scenario. Both efforts are meant to stress-test communications, evacuations and supply chains, according to the Washington Military Department and Great ShakeOut.
Those exercises show that tsunami waves can begin hitting some shorelines within 10 to 30 minutes of the initial rupture, leaving coastal communities with very little time to reach high ground. Layer in a big inland rupture at the same time and you are looking at overlapping demands on already thin emergency resources.
The study does not claim that a Cascadia and San Andreas double strike is imminent, and scientists emphasize uncertainties in radiocarbon dating and how some sediment layers are interpreted. The practical takeaway is more familiar: retrofit and strengthen older buildings where possible, secure heavy items inside homes and workplaces, stock up on emergency supplies and learn your local tsunami and earthquake evacuation plans. For authoritative preparedness guidance, residents can turn to the U.S. Geological Survey and to local emergency management offices.
