Two major fault systems along North America's West Coast - the Cascadia subduction zone and the San Andreas fault - appear to be on friendlier terms than geologists ever realized. A new study suggests that activity on one fault could trigger earthquakes on the other, raising the delightful prospect of closely timed seismic double features.
"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." Because, you know, why have one apocalyptic earthquake when you could have two?
To uncover this hidden relationship, Goldfinger and his colleagues examined sediment cores from the ocean floor, preserving about 3,100 years of geological history. They focused on turbidites - layers of sediment left by underwater landslides typically triggered by earthquakes. By comparing turbidite layers from both fault systems, the team found similarities in structure and timing, suggesting a potential synchronization between Cascadia and the northern San Andreas fault.
Pinpointing the exact timing is tricky, but 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. That's the geological equivalent of "we need to talk about your plans."
This connection has major implications for disaster 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 suspected faults might interact, but real-world evidence has been scarce - the only documented example happened in Sumatra, where two large earthquakes struck three months apart in 2004 and 2005. Goldfinger's interest dates back to a 1999 research cruise when his team accidentally drifted 55 miles south of Cape Mendocino in California, into the San Andreas fault zone. Instead of scrapping the trip, they collected a sediment core there - and found something weird.
Normally, turbidites show coarse material at the bottom and finer sediment on top. This core had the pattern reversed: coarse, sandy material above finer, silty sediment. The team concluded the lower layer formed during a major Cascadia earthquake, and the coarser material above came from a subsequent event along the nearby San Andreas fault. Radiocarbon dating confirmed these "doublets" were created by closely timed earthquakes, not aftershocks or unrelated events.
So, West Coast residents, your emergency kit should now include a sense of irony. The study included contributions from researchers at Oregon State, NOAA, University of Washington, Springer Nature Group in Germany, California Department of Conservation, and Instituto Andaluz de Ciencias de la Tierra in Spain.