Two major fault systems along North America’s West Coast, the Cascadia subduction zone and the San Andreas fault, may be more closely connected than previously believed. A new study suggests that activity on one fault could trigger earthquakes on the other, raising the possibility of closely timed seismic events.
“We’re used to hearing the ‘Big One’ — Cascadia — being this catastrophic huge thing,” said Chris Goldfinger, a marine geologist at Oregon State University and lead author of the study. “It turns out it’s not the worst case scenario.”
Deep-Sea Evidence Reveals a Hidden Pattern
To investigate this possibility, Goldfinger and his colleagues examined sediment cores taken from the ocean floor. These cores preserve about 3,100 years of geological history. The team focused on turbidites, which are layers of sediment left behind by underwater landslides that are often triggered by earthquakes.
By comparing turbidite layers from areas influenced by both fault systems, the researchers identified similarities in their structure and timing. These patterns point to a potential synchronization between Cascadia and the northern San Andreas fault.
Pinpointing the exact timing between earthquakes on the two faults is challenging. However, Goldfinger noted three cases within the past 1,500 years, including the most recent event in 1700, where the data suggests the earthquakes occurred within minutes to hours of each other.
A Larger Disaster Scenario
This possible connection has major implications for earthquake preparedness.
“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.”
Scientists have long considered the idea that faults might interact in this way, but real-world evidence has been scarce. The only documented example occurred in Sumatra, where two large earthquakes struck three months apart in 2004 and 2005.
A Chance Discovery Leads to a Breakthrough
Goldfinger’s interest in this question goes back decades, including a key moment during a 1999 research cruise. While collecting sediment cores from the Cascadia subduction zone off Oregon and northern California, the team accidentally drifted off course. They ended up about 55 miles south of Cape Mendocino in California, within the San Andreas fault zone.
Instead of abandoning the location, the researchers decided to collect a core there as well. What they found turned out to be highly unusual.
“Doublets” Point to Back-to-Back Earthquakes
Under normal conditions, turbidites show a consistent pattern, with coarse material settling at the bottom and finer sediment layering on top. In this unexpected core, the pattern was reversed. Coarse, sandy material sat above finer, silty sediment.
This unusual structure suggested a two-step process. The lower, finer layer likely formed first during a major Cascadia earthquake. The coarser material on top appeared to result from a subsequent event along the nearby San Andreas fault.
To confirm this idea, the team used radiocarbon dating on this core and others collected near Cape Mendocino, where the two fault systems meet. The results supported the idea that these reversed layers, which the researchers call “doublets,” were created by earthquakes occurring close together in time, rather than aftershocks or unrelated events.
Researchers and Collaboration
The study also included contributions from Ann Morey, Christopher Romsos and Bran Black of Oregon State’s College of Earth, Ocean, and Atmospheric Sciences; Jeff Beeson of the National Oceanic and Atmospheric Administration Oregon State; Maureen Walzcak, University of Washington; Alexis Vizcaino, Springer Nature Group in Germany; Jason Patton, California Department of Conservation; and C. Hans Nelson and Julia Gutiérrez-Pastor, Instituto Andaluz de Ciencias de la Tierra in Spain.
