The Cascadia Subduction Zone

(Graphic from oregon.gov)

The San Andreas fault, which is responsible for the great San Francisco earthquake of 1906, is less destructive than the Cascadia Subduction zone. The Cascadia has triggered most of the San Andreas' earthquakes in the past 3,000 years (but not in 1906), which will make a future Cascadia/San Andreas combined quake the most destructive natural disaster in American history by far.

A megaquake in the Pacific Northwest could trigger a large earthquake along California’s San Andreas Fault, creating an unprecedented catastrophe up and down the Pacific Coast, a new study has found.

The study suggests that the fearsome Cascadia subduction zone, a fault line running offshore from Northern California to British Columbia that is capable of producing earthquakes of magnitude 9 or higher, has triggered large quakes in San Francisco and elsewhere along the northern part of the San Andreas Fault. In some cases, it’s possible that quakes on the San Andreas followed the first quake within minutes or hours, according to the study, which was published Sept. 29.

For example, researchers found evidence that the last magnitude 9 earthquake in the Cascadia subduction zone — in 1700 and so large that it caused a tsunami in Japan — also produced a major earthquake on the northern San Andreas Fault. That part of the fault extends from the Mendocino Junction offshore of Humboldt County, where it meets the Cascadia, to Hollister (San Benito County). That earthquake could have been as large as the magnitude 7.9 earthquake in 1906 San Francisco, said Jason Patton, a co-author of the study and engineering geologist at the California Department of Conservation.

According to lead author Chris Goldfinger of Oregon State University

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.”

Mr. Goldfinger said that if he heard that the Cascadia Subduction zone went off, he would immediately leave his home in Palo Alto and head east. This is one expert's advice that we intend to follow.