As part of our series Bay Curious, we’re answering questions from KQED listeners and readers. This question comes from Steven Horowitz, who wanted to know:
If a tsunami were to hit the Golden Gate, what would be its real effect on communities facing the San Francisco Bay?
Steven’s question came from watching the summer’s action flick, “San Andreas.”
“I was sitting there watching the giant tsunami course through the Golden Gate and into the bay,” he says. “I looked at that and thought: Wouldn’t there be some kind of dissipation coming through the Golden Gate?”
It’s All About Our Faults
Despite the terrifying image of a 500-foot wave about to wash over the Golden Gate Bridge, tsunamis do not actually pose a considerable threat to the Bay Area.
It all has to do with the kinds of geologic faults that we have (and don’t have).
Tsunamis are caused when a tectonic plate under the ocean smashes into and slides underneath a continent. That process, known as subduction, never happens smoothly or quietly. It shakes up the seabed, displacing a huge volume of ocean water that races across the ocean, and eventually floods the shore.
But the San Andreas Fault is different. It’s called a slip-strike fault because the two plates slide past each other horizontally. Of course, any time plates move, the ground shakes. But here, there is no subduction and no displaced ocean.
Meaning no killer tsunamis. Even San Francisco’s infamous 1906 earthquake generated only a 4-inch wave at the Presidio gauge station.
Small Waves Still Pack a Punch
Although they aren’t generated here, tsunamis do occasionally hit our shores. Since 1850, more than 50 tsunamis have been recorded in San Francisco Bay. Most were generated by earthquakes in subduction zones near Russia, Japan or Alaska.
Eric Geist, a geophysicist at the U.S. Geological Survey in Menlo Park, says that size is the most important factor in evaluating risk.
“We can look at anything, from huge waves to micro-tsunamis, that you’d never see with your eyes but our instruments can detect,” he says.
The worst tsunami to hit the Bay Area was triggered in 1964 by a magnitude 9.4 earthquake in Alaska, Geist says. That wave rolled in at just under 4 feet and damaged marinas and private boats in Marin County.
The Cascadia subduction zone, which runs roughly from Mendocino County to Vancouver Island, could also produce a massive earthquake and tsunami. But Geist says it’s unclear how a tsunami from “The Really Big One” would affect the Bay Area.
“Oregon, Washington and California north of Eureka would really bear the brunt of that tsunami,” he explains.
But What If a Big One Arrived?
In Ward’s simulations, the incoming wave stands just over 16 feet tall. This is much larger than historical tsunamis, but Geist agrees that a wave this size is theoretically possible.
Approaching the Golden Gate at 55 mph, the wave would first hit the outlying areas of Point Reyes National Seashore and Montara. It would then start to flood low-lying areas like Half Moon Bay.
“It’s not like splash and dash,” explains Ward. “When the water comes in, it’s going to flood.”
It would feel like a 12-hour tidal cycle was packed into an hour.
“And it will do as much damage when it goes back out and drags along cars and debris,” he adds.
The original wave and splashbacks from shore would then start to pile up as they squeeze through the 1-mile-wide Golden Gate Strait. In Ward’s simulations, the wave reaches a maximum height of about 30 feet.
“That’s barely to the top the pylon,” says Ward, who is confident that the bridge would have no trouble withstanding the wave energy. “It probably wouldn’t even touch the steel.”
Finally, the wave would fan out into San Francisco Bay. Parts of Mission Bay and the Marina could see significant flooding, but by the time it reached Treasure Island or the East Bay, the wave would be less than 3 feet tall. It would probably not even make it to the South Bay.
Verdict: San Francisco Is Relatively Safe
Steven Horowitz, who asked Bay Curious the question, was glad to hear that the tsunami would be nothing like the movie.
“By the time it gets to Berkeley, which is where I’m sitting right now, I think I’m pretty safe,” he says. “Sounds like it’s not going to come rushing up University Avenue.”
Bay Area residents can also rest assured that there have been no recorded deaths from tsunami-related events in San Francisco. And even a worst-case-scenario Cascadia tsunami would take several hours to reach the city, providing ample time to mobilize a response.
And just in case, the City and County of San Francisco has a tsunami plan in place. It includes a strategy for evacuating people from vulnerable areas like Ocean Beach, coordinating basic services (like shelter, water, food, and medical attention) and performing search and rescue.
Still, “if you get a warning and are in a tsunami zone, follow the evacuation instructions,” says Ward. “What do you have to lose, a couple hours of your time?”