Scientists have uncovered evidence indicating that a magnitude 9 earthquake on the Cascadia subduction zone could trigger a subsequent major earthquake on California's San Andreas Fault. This finding, published in the journal Geosphere, suggests the possibility of a catastrophic chain reaction along the West Coast.
The research is based on the analysis of thousands of years of underwater sediment. Scientists discovered patterns in sediment cores that appear to show both fault systems rupturing in close succession in the past. These paired deposits, found in both Cascadia and areas influenced by the San Andreas Fault, often formed at nearly the same time according to radiocarbon dating.
"It’s kind of hard to exaggerate what a M9 earthquake would be like in the Pacific Northwest," said Dr. Chris Goldfinger, a paleoseismologist at Oregon State University, in a statement to the Geological Society of America. "And so the possibility that a San Andreas earthquake would follow, it’s movie territory."
This discovery was serendipitous. During a 1999 expedition aimed at reconstructing Cascadia's earthquake history, a research vessel veered off course and collected sediment samples from an area influenced by the San Andreas Fault off the coast of northern California. These samples revealed unusual sediment layers, described as "big, thick, sandy doublet events," which differed from the single layers typically found when reconstructing earthquake histories.
After ruling out other explanations, the research team concluded that the paired deposits likely represented two distinct, major seismic events: one from a Cascadia megathrust earthquake and another from movement on the San Andreas Fault. The findings suggest that the time between these events could have been as short as minutes or hours.
A magnitude 9 earthquake on the Cascadia fault would already be a catastrophic event, generating widespread violent shaking, massive tsunamis, and landslides across the Pacific Northwest. The new research indicates that California could face a follow-up disaster with little time for recovery.
Dr. Goldfinger stated that the findings have influenced his personal emergency preparedness. "I'm from the Bay Area originally," he said. "If I were in my hometown of Palo Alto, and Cascadia went off, I think I would drive east. There looks to me like a very high risk the San Andreas would go off next."
If this scenario were to recur, emergency response efforts could be strained by simultaneous or sequential disasters affecting vast areas from the Pacific Northwest down into California. The implications of such back-to-back seismic events could present unprecedented challenges for emergency responders and infrastructure across the region.