Two major fault systems along the west coast of North America, the Cascadia subduction zone and the San Andreas fault, may be more closely related than previously thought. A new study suggests that activity on one fault can trigger earthquakes on the other, resulting in closely timed seismic events.
“We’re used to the story of the ‘Big One’ — Cascadia — being this catastrophic behemoth,” said Chris Goldfinger, a marine geologist at Oregon State University and the study’s lead author. “It turns out that’s not the worst-case scenario.”
Deep-sea evidence reveals hidden patterns
To investigate this possibility, Goldfinger and his colleagues examined sediment cores taken from the ocean floor. These cores preserve approximately 3,100 years of geological history. The research team focused on turbidites, which are layers of sediment left behind by submarine landslides often triggered by earthquakes.
The researchers identified similarities in structure and timing by comparing turbidite layers in areas affected by both fault systems. These patterns indicate potential synchrony between the Cascadia and northern San Andreas faults.
The exact timing of earthquakes on the two faults is difficult to determine. But Goldfinger points out that there have been three instances in the past 1,500 years, including the most recent earthquake in 1700, where data suggests the quakes occurred within minutes to hours.
Larger disaster scenarios
This potential link has significant implications for earthquake preparedness.
“Even an earthquake on one of the faults would be expected to deplete the nation’s resources to respond,” Goldfinger said. “And if both go away together, San Francisco, Portland, Seattle and Vancouver could all be in a state of emergency in the short term.”
Scientists have long considered the idea that faults could interact in this way, but there was little actual evidence. The only recorded example occurred on the island of Sumatra, where two major earthquakes occurred three months apart in 2004 and 2005.
Accidental discoveries lead to breakthroughs
Goldfinger’s interest in this issue goes back decades, including a pivotal moment during a 1999 research cruise. While collecting sediment cores from the Cascadia subduction zone off the coast of Oregon and northern California, the team accidentally went off course. They ended up within the San Andreas Fault Zone, about 55 miles south of Cape Mendocino, California.
Instead of abandoning the site, the researchers decided to take cores there as well. What they discovered turned out to be extremely rare.
“Doublet” refers to consecutive earthquakes
Under normal conditions, turbidites exhibit a consistent pattern, with coarse material settling to the bottom and finer sediment layering on top. In this unexpected core, the pattern was reversed. Coarse sandy material was overlain by finer silty deposits.
This unusual structure suggests a two-step process. The lower, finer layers may have first formed during the massive Cascadia earthquake. The coarser material at the top appears to have been produced by later events along the nearby San Andreas Fault.
To confirm this idea, the researchers radiocarbon-dated this core and others taken near Cape Mendocino, where the two fault systems meet. The findings supported the idea that these inverted layers, which the researchers called “doublets,” were formed by earthquakes that occurred at the same time in close time, rather than by aftershocks or unrelated events.
Collaborate with researchers
The study also included contributions from Ann Morley, Christopher Lomsos, and Bran Black of Oregon State College of Earth, Ocean, and Atmospheric Sciences. Jeff Beeson of the National Oceanic and Atmospheric Administration in Oregon. Maureen Walzcak, University of Washington; Alexis Vizcaino, Springer Nature Group, Germany; Jason Patton, California Department of Conservation. C. Hans Nelson and Julia Gutiérrez Pastor of the Andalusian Institute of Geosciences, Spain;
