One-two punch of massive quakes

Study suggests one fault often triggered another in California and could do so again.

- BY RONG-GONG LIN II

THE SAN ANDREAS fault at California's Carrizo Plain National Monument. Researchers say a quake in another zone could trigger a rupture in the fault.

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 study published recently in the journal Geosphere.

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 coauthor of the study.