Bay Area earthquakes unlikely to trigger deadly tsunamis, experts say

Onlookers watch for large tidal waves at Poplar Beach in Half Moon Bay Friday morning. (Photo: Solly Mirell)

Californians may wonder if an earthquake hits Northern California, would it trigger a tsunami of a similar magnitude to the one seen in Japan?

The answer, according to Paul Segall,  professor of geophysics at Stanford University, is probably not in the Bay Area.

The 810-mile San Andreas fault, which runs through several coastal counties including San Mateo, Santa Clara, and San Francisco, would not cause large vertical motions in the event of an earthquake. Vertical motions are necessary to displace large amounts of water, Segall explained.

In contrast, the Cascadia Fault, which is north of the Bay Area and runs from Mendocino up to Vancouver Island, is a thrust fault with lots of vertical motion. It can cause a large tsunami, Segall said.

Scientists can precisely predict when a tsunami will arrive by calculating the speed of the waves based on the depth of the water and other factors. But, Segall said California was lucky to have had so much time to prepare for today’s wave surges and set up voluntary evacuation shelters.

Had an earthquake of similar magnitude struck Oregon, a location much closer to home, for example, Segall predicted that officials would have had 15 minutes or less to issue a tsunami warning.

For those who live along the northern California coast, Segall says it is not always possible to receive an official alert or warning in time.

“If you feel an earthquake, just go,” he said, advising residents not to wait until it may be too late.

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