San Francisco

Bay Area Tsunami Risk Very Low, Scientists Say

It's unlikely that a major earthquake will send a tsunami to devastate the shores of the Bay Area, scientists told NBC Bay Area.

A local earthquake, along the San Andreas fault for example, would not even cause a tsunami because the plates on that fault line move side to side and, thus, do not disrupt water.

An earthquake along a subduction zone, a type of fault line where one plate essentially gets pushed under the other, could trigger a tsunami that reaches the shoes of the Bay Area, but citizens would have hours to evacuate and prepare.

This type of seismic event is most likely to occur in far away locations, in places like Alaska, Chile or Japan, says Dr. Steve Ward, a research geophysicist at UC Santa Cruz.

Bathymetry, or the shape of the earth's features underwater, plays a big role in helping waves from those locations reach the Bay Area, says Brian Garcia, a meteorologist with the National Weather Service.

“It can actually steer the tsunami waves into different regions,” he said. “So, we can get impacts from those areas.”

Ward says another way to think about this is to imagine the beam from a flashlight. Much of the energy from an earthquake is directional. It goes forward in the triangular shape of a flashlight beam, he says. Tsunamis follow the path of that energy.

“So if you’re on the flashlight beam, you’re going to get wet,” he said. “If you’re not on the flashlight beam, you’ll probably be safe.”

The Bay Area is on the flashlight beam for a seismic event in Alaska near the Aleutian Islands. If the fault line there ruptures, water displaced at the source would rush down the Canadian coast near Vancouver and saturate the Pacific Northwest coastline before settling in the Bay Area. The water might even pierce the San Francisco Bay by the Golden Gate Bridge, but it shouldn’t do much damage from there.

“It will get into the Bay to a degree, but it’s not going to have any sort of impact especially in the North or South Bay,” Garcia said. “Directly across from the entrance, though, under the Golden Gate, they could have some impacts.”

After the 2011 earthquake in Japan, a small tsunami wave did make its way into the Bay. Pictures of the wave reaching Emeryville can be found online. That wave took several hours to reach the Bay Area, and by that time was less than a foot high.

What’s the tsunami risk where you live? Visit Know Your Zone on the California Governor’s Office of Emergency Services’ website and type in your address to find out.

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