US west coast faults could trigger catastrophic back-to-back earthquakes, study finds

"Warnings about the looming threat of the big one a catastrophic earthquake that could devastate cities have stoked fears across the US west coast for decades. But according to a new study, a high-magnitude earthquake in the Pacific north-west could set off a secondary one on California's San Andreas fault, causing an unrivaled catastrophe. The bigger one would have the potential to wreak havoc up and down the coast at once, researchers say."

"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, said lead author Dr Chris Goldfinger, a marine geologist and geophysicist. 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."

"Goldfinger said the findings came after decades of head-scratching and a lot of data that was difficult to explain. Drilling into deep-sea sediment cores that contain thousands of years of geologic history, the researchers combed through the layers left from ancient earthquakes. They thought the submarine landslide deposits were from San Andreas quakes but they were upside down from what we typically see, Goldfinger said. We realized that it wasn't a single deposit, he added, it was two."

A high-magnitude Cascadia subduction-zone earthquake can trigger secondary ruptures on the San Andreas fault, causing simultaneous major earthquakes along the Pacific coast. Submarine sediment cores and turbidite layers show paired deposits indicating near-synchronous ruptures separated by minutes to hours. Deep-sea landslide deposits previously misattributed to single events reveal dual deposits stacked in reverse order. Simultaneous Cascadia–San Andreas ruptures would strain national emergency resources and could place San Francisco, Portland, Seattle and Vancouver in concurrent crises, amplifying damage and complicating response and recovery across the West Coast.

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