Cascadia and the San Andreas may have ruptured back to back, a sediment record suggests

Layers of seafloor mud off northern California come in the wrong order. Where a deposit from an undersea landslide should grade from coarse at the bottom to fine on top, some are stacked with fine material beneath and coarse above, and Oregon State University marine geologist Chris Goldfinger reads that inversion as the signature of two earthquakes arriving close together. His group’s reading, drawn from a roughly 3,100-year sediment record, is that the Cascadia subduction zone and the northern San Andreas fault may sometimes break in quick succession.

The idea is an inference from sediment, not an observed event, and other seismologists have not accepted it as settled. The record does not include a single instrumented example of the two faults rupturing together.

Inverted doublets near Cape Mendocino

The sediment puzzle is older than the interpretation. The Geological Society of America’s release says the breakthrough traces to a navigational error on a 1999 research cruise, when a graduate student entered incorrect coordinates and the drill ship ended up 90 kilometers south of the intended coring sites. The cores that mistake produced became part of the dataset behind the paired-layer interpretation.

The cores come from the area around the Mendocino Triple Junction, where the San Andreas meets the Cascadia margin. According to the Oregon State summary carried by ScienceDaily, the team examined turbidites, the sediment layers that shaking sends sliding down submarine slopes, in cores taken near Cape Mendocino and dated them by radiocarbon. The summary reports three instances in the past 1,500 years when both fault systems appeared to rupture close together, including one around 1700.

The paper itself appeared in the journal Geosphere in 2025, and the Geological Society of America’s release lists Goldfinger as lead author with co-authors including J. Beeson, B. Black and C.H. Nelson. It describes paired turbidite deposits in cores from both north and south of Cape Mendocino that were laid down at nearly the same time, with the second layer in some pairs deposited within minutes or hours of the first.

Minutes to decades between the paired quakes

Pair timing is where accounts diverge, and the spread matters for the phrase “back to back.” Scientific American counted more than 130 cores spanning 3,000 years and reported at least eight major San Andreas earthquakes coinciding within decades of big Cascadia events. Goldfinger put the logic this way: “When these faults synchronize, one fault could tune up the other and cause earthquakes in pairs.”

Eos described a tighter tally from the same work: 18 turbidite sequences from southern Cascadia, 10 of which showed clear temporal association with San Andreas events, with a median age difference between paired quakes of about 60 years, inside the dating uncertainty. Goldfinger is quoted there saying the best fit to the data, by far, is to have Cascadia go first. Geophysicist Kathryn Materna called the correlation pretty striking given that the two faults have different recurrence intervals.

The 1906 San Francisco earthquake, the same reporting notes, was an independent event, so the faults are not locked into a fixed sequence.

Goldfinger has been blunt about the stakes if the pairing holds. In the Geological Society of America’s release he said it is hard to exaggerate what a magnitude 9 earthquake would be like in the Pacific Northwest, and that a San Andreas rupture following it would be “movie territory.” That is a statement about consequences, not about frequency, and the sediment record can say only how often the layers pair up, not when the next pair might form.

Melgar, Beroza and the dating objection

Skepticism is specific. National Geographic quoted geophysicist Diego Melgar saying that what the study presents as synchronicity is “in no way a slam dunk,” and seismologist Greg Beroza urging caution in embracing the interpretation. Their objections center on dating: paired deposits that look simultaneous could be hundreds of years apart once radiocarbon error is counted, and similar-looking layers in cores separated by long distances are easy to match wrongly.

Another alternative is that some turbidites come from storms rather than earthquakes. Meng Wei of the University of Rhode Island, who was not involved, acknowledged to Scientific American that the work shows synchronization across large fault systems over long periods but cautioned that the interval between paired earthquakes can still be decades.

Oregon State’s account carries the practical stake. If both systems rupture within minutes or hours of each other, San Francisco, Portland, Seattle and Vancouver could need emergency help at once, stretching national response resources. Goldfinger, in the same summary, said the Big One is usually pictured as Cascadia alone and that “it turns out it’s not the worst-case scenario.” Whether the 3,100-year doublets mean hours or decades is the number on which that claim depends.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview