By Michael Burge
After the utter devastation that befell Indonesia, everyone should have known to stay away from the ocean during and after a tsunami, right?
When the Tohoku earthquake struck Japan on March 11, the Pacific Tsunami Warning Center advised people on the U.S. Pacific coast to stay clear of the beach.
A lot of San Diegans did the opposite, heading to the ocean to see the mammoth wave roll in. Some showed up with surfboards to spit in the tsunami’s eye.
“You just can’t underestimate people’s ability to do dumb things, but I’m not going to give up trying to educate (them),” says John Sandmeyer, a sergeant with San Diego’s Lifeguard Service who helped develop the city’s tsunami disaster plan.
Recent catastrophic tsunamis in the Pacific and Indian oceans have shown that these speeding walls of ocean water-which can cross the ocean at 600 mph-can be more destructive than the earthquakes that spawn them.
According to some estimates, the tsunami that hit Japan produced wave heights of up to 100 feet.
Rick Wilson, a senior engineering geologist with the California Geological Survey, says a local tsunami would bring a 10- to 15-foot wave to the San Diego coastline.
“People think a 10-foot wave wouldn’t do much, but if you think about it, the water column would be twice our height,” Wilson says. “You can’t swim a tsunami, because these things are going at about 30 to 40 miles an hour. It’s really just a hard wall that’s hitting you, and sometimes these tsunamis have cars and boats and buildings caught up in them.
“It only takes about a one- to two-foot wave to lift a car,” Wilson says. “Those cars now become projectiles.”
The Tsunami Research Center at the University of Southern California has published “Tsunami Inundation Maps” that show lowlying places like Coronado, Mission Beach and Ocean Beach would be highly vulnerable to flooding if a giant wave were to strike. Anyone living in these neighborhoods that feels a prolonged quake should flee two miles inland or 30 feet above sea level immediately.
“It’s really up to the individual to react,” Wilson says. “There will be no warning from the government, because you only have about 10 to 15 minutes before the tsunami could arrive.”
Nearly 60 percent of the time, the tide will recede an unusually long distance in the minutes before a tsunami strikes, offering an early warning for those paying attention, Wilson says.
The obvious question: Can it happen here? Seismologists and tsunami researchers say it’s unlikely, but not impossible.
Scripps Institution of Oceanography seismologist Debi Kilb says faults near San Diego can generate quakes of a magnitude 7.0 or higher, which is severe enough to cause injury or death, and crack water lines and freeway overpasses like bread sticks. However, they’re not likely to trigger a significant tsunami.
This region’s most active fault is the San Andreas, which is 100 miles inland and thus too far away to cause a tsunami. The Rose Canyon fault, which runs from Downtown north through La Jolla and into the Pacific Ocean, may be capable of triggering a devastating quake, but Kilb says the fault isn’t very active. Nor is it the type that would create a tsunami.
The quake that devastated Japan happened along what is known as a subduction zone-where one tectonic plate of the Earth’s crust is being forced beneath an adjacent plate. Such quakes lift and drop the ocean floor violently, thrusting massive volumes of water upward, causing a tsunami.
“It’s like somebody took the ocean and tilted it toward the land, and now you’ve got this flood of water coming on shore,” Wilson says.
The faults found off San Diego’s coast are known as slip-strike, move horizontally and are therefore less prone to create tsunamis.
What could lead to a local tsunami, Scripps seismologist Duncan Agnew says, is a quake that causes a landslide on the ocean floor. However, such events are extremely rare, he says.
The tsunami that struck Japan in March gave the world its biggest nuclear scare since the Chernobyl event in 1986.
The disaster at Japan’s Fukushima Daiichi nuclear power plant occurred when the tsunami overwhelmed protective barriers, inundated the plant and knocked out backup power sources designed to keep cooling systems operating.
Southern California Edison, which operates the San Onofre Nuclear Generating Station in northern San Diego County, recently completed a statemandated study of tsunami and seismic risks, concluding that the plant can withstand the largest possible quake and tsunami that local faults can throw at it. The maximum tsunami would hit the plant at a height of 23 feet, the study says. San Onofre’s seawall is 28-30 feet above sea level.
While the region’s topography and manmade protections reduce the threat of tsunamis overwhelming San Diego, maximizing safety also requires common sense. The next time the ground shakes, the best bet is to grab the loved ones, not the surfboard.
-Pat Sherman contributed to this story