They are two of the West Coast’s most destructive generators of huge earthquakes: the San Andreas fault in California and the Cascadia subduction zone offshore of California’s North Coast, Oregon, Washington and British Columbia.

The public has often thought of these danger zones as separate entities. But what if they were capable of back-to-back disasters?

That’s the unsettling possibility described in a groundbreaking new study published recently in the journal Geosphere and reported in the Los Angeles Times.

The authors suggest that, for thousands of years, large earthquakes on the Cascadia subduction zone were quickly followed by large earthquakes on the northern San Andreas fault.

In 1700, a Cascadia subduction zone earthquake is believed to have measured around a magnitude 9. Based on archaeological evidence, villages sank and had to be abandoned, according to the U.S. Geological Survey. That earthquake was so powerful, entire sections of the Pacific coastline dropped by as much as 5 feet. In the Pacific Northwest, Native American stories told of “how the prairie became ocean” and canoes were flung into trees.

The study suggests the Cascadia earthquake was followed by a northern San Andreas fault earthquake from Cape Mendocino toward San Francisco, with a magnitude of around 7.9.

“What that suggests is that the San Andreas earthquake happened very closely in time after the Cascadia earthquake,” said Jason R. Patton, engineering geologist with the California Geological Survey and a co-author of the study.

The evidence suggests the San Andreas fault in 1700 ruptured in an earthquake within hours to days of the Cascadia earthquake. “It could have even been minutes, but we can’t nail it down,” said the study’s lead author, Chris Goldfinger, a paleoseismologist with Oregon State University and a professor emeritus of marine geology.

Story Date: November 3, 2025