Reassessing L.A.’s tsunami danger
Revised maps expand the f lood evacuation zones in a worst-case scenario.
When state geologists went looking for the hypothetical origin of the worst tsunami that could strike Southern California in 1,000 years, they found it in the Aleutian Trench off the Alaskan coastline.
A magnitude 9.3 underwater earthquake there could generate a wave that would hit Southern California several hours later and inundate portions of Malibu, Santa Monica, Venice, Marina del Rey, Long Beach and the two busiest ports in the nation.
That was the worst-case scenario, but there were plenty of other possibilities for catastrophe. Earthquakes along undersea faults near Catalina and Anacapa islands — as well as submarine landslides off the Palos Verdes Peninsula — could generate tsunamis capable of flooding those same areas in just minutes.
To help local emergency officials prepare, the California Geological Survey has released new maps that show the extent of flooding the worst tsunamis could produce in Los Angeles County.
The maps were released during Tsunami Preparedness Week, when coastal residents are reminded to get ready for disasters that may or may not occur in their lifetimes.
In some ways, that 1,000-year quake in Alaska would be the
easy one to respond to: There would be hours to evacuate after an official warning was issued.
But there would be little to no time between an official warning and massive flooding in the smaller offshore quakes, said engineering geologist Nick Graehl, who helps local agencies plan evacuation strategies.
“You’re going to feel that strong ground shaking,” Graehl said of the closer quakes. “You don’t wait for that official warning. You feel that earthquake, you go. And you go to a safe area, and you stay there until there’s an official all-clear.”
In the case of tsunamis, the recommended mode of evacuation is by foot, not automobile, according to senior engineering geologist Rick Wilson.
“We’ve done some studies with [the] U.S. Geological Survey and found if people get in their cars and try to drive out of areas like Marina del Rey and Port of Los Angeles, Port of Long Beach, they’ll be stuck in traffic and they won’t get out in time,” Wilson said. “So the recommendation is for people to evacuate on foot. If they do that, almost everybody can get out of the zone in this critical worst-case scenario that we’re looking at.”
The best preparation would be for people to check the maps, find an evacuation route and practice it, Graehl said.
“We’re encouraging people to maybe take a tsunami selfie at higher ground,” Graehl said.
The new maps are revisions of earlier work that predicted the f looding from a so-called 500-year flood.
The Tohoku earthquake that caused a devastating tsunami in Japan a decade ago provided the basis for the update, said state geologist Steve Boylen.
“Japan had sea walls that they had built for a 500-year event,” Boylen said. “The 1,000-year event topped over that.”
In Southern California, the two standards did not produce dramatically different results.
The new maps, built on a probability analysis, “reinforce where we believe the line was back in 2009, but we have added an additional buffer to be extra cautious with the errors and uncertainties in the modeling,” Wilson said.
A Times analysis of the changes since 2009 shows that Long Beach has the largest additions. Much of the Belmont Shore neighborhood and locations north of Colorado Lagoon and Los Cerritos Channel, as well as west of the Los Angeles River, are included in the 2021 evacuation zone areas.
Also, roughly two dozen blocks along Beach Drive, an upscale area in Hermosa Beach, have been added.
The updated plans include a handful of upscale townhome complexes in Santa Monica as well as a few blocks in Marina del Rey, which include single-family homes as well as a number of apartments and hotels.
While not extending too far inland in Malibu, the new maps include a number of retail locations and restaurants, as well as a museum.
The maps are designed to support city and county evacuation plans, which can vary widely due to local conditions.
In the Naples area of Long Beach, for example, there is an elderly population that requires extra attention from the city’s emergency management teams, Graehl said.
With its pier and beachfront, Santa Monica has a large daytime population of tourists and beachgoers who would need to be notified.
Santa Monica visitors as well as residents can sign up for emergency alerts on the city’s website or by text, said Chief Resilience Officer Lindsay B. Call.
“These maps are incredibly important tools to enable emergency managers to provide the best education to our local residents,” Call said.
The new maps incorporate a philosophical shift in how the agency hopes to support local emergency planning. Some of the squiggly topography lines on previous maps have been pushed out to the nearest major street, so that evacuation zones could be more easily described to the public.
The Los Angeles County maps are part of a phased release of statewide maps that is expected to be completed by early 2022.
So far, in addition to L.A. County, updated maps have also been posted for Alameda, Del Norte, Humboldt, Mendocino, Monterey and San Mateo counties.