An earthquake just off California's coast poses dire tsunami risk for many communities

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California does not often experience dangerous tsunamis, but there is one type of ocean surge some experts are increasingly concerned about.
Near-shore tsunamis — those triggered by earthquakes just offshore — could pose a particularly dire risk for California's heavily populated coastal communities, according to experts, disaster modeling and local hazard plans.
Depending on the strength and location of the quake, life-threatening waves could approach the coastline in as few as 10 minutes, perhaps not even enough time for an emergency alert to be issued, much less arrive on your cellphone.
And it's a potential threat all along California's 840-mile coast, from the dense cities in the south to more remote reaches in the north.
Read more: These California coastal cities face heightened flood danger from tsunami, data show
Data published in both the city and county of Los Angeles' hazard mitigation plans show how locally generated tsunamis could reach heights of up to 5 feet above the mean sea level in Marina del Rey; 7 feet in Manhattan Beach and San Pedro; 8 feet in Long Beach and the Santa Monica Pier; 9 feet in Malibu; and 30 feet on Catalina Island — depending on the fault that ruptures or where an underwater landslide is triggered.
In Orange County, a magnitude 6.8 quake just offshore could bring a local-source tsunami to the coast in as little as 15 minutes, county documents say. A quake-triggered submarine landslide could inundate areas in Newport Beach — including Balboa Island, Balboa Peninsula and Lido Isle — and potentially surge so far inland up San Diego Creek that water approaches the 405 Freeway in Irvine.
And in the San Francisco Bay Area, near-shore tsunami could cause waves to surge 4 feet above mean sea level at San Francisco's Aquatic Park, Alcatraz Island, Redwood City and Richmond; 5 feet in Alameda; 6 feet at San Francisco's Ocean Beach; 7 feet in Sausalito; 8 feet in Pacifica in San Mateo County; and 9 feet in Bolinas in Marin County, according to the California Geological Survey.
Reginald Harrison, director of disaster preparedness and emergency communications for the city of Long Beach, said 'tsunamis are a rare but real threat to our community."
"Unlike earthquakes, however, tsunamis provide natural warnings before they arrive," Harrison said in a statement released ahead of Tsunami Preparedness Week. "If you are near the beach when this occurs, you should move to higher ground as quickly as possible."
Read more: The California tsunami danger is real. The 7.0 earthquake is wake-up call to prepare
California is home to a number of communities where evacuating quickly could be immensely challenging.
A study published in 2020 identified nine densely populated and particularly problematic places. They include Naples island in Long Beach; Oxnard Shores and southern Channel Islands Beach in Ventura County; the islands in Huntington Harbour; Balboa Island; Balboa Peninsula/Lido Isle; San Diego's Mission Beach peninsula; the Coronado peninsula, and Bay Farm Island in Alameda.
Alameda is particularly vulnerable to tsunami. More than half its about 75,000 residents live in a designated hazard zone, and there are few exits from the community — which is split between Bay Farm Island (actually a peninsula) and the main Alameda island.
However, officials there contend "there is minimal risk of any significant tsunami occurring" from a local fault. Instead, they say, the greatest risk is from distant areas, which could produce tsunami of as much as 18 feet above sea level in Alameda.
In other places, though, a near-shore tsunami could produce the highest waves. In Redondo Beach, for instance, a near-shore tsunami could bring waves of up to 11 feet above mean sea level, compared to 9 feet possible coming from a distant earthquake.
Katie Eing, the emergency services coordinator for Newport Beach, estimated 45,000 people live in the potential local inundation zone, about half of the entire city's population. However, she noted many more visitors and workers could also be present, especially during tourist season.
Eing said a near-shore tsunami could be devastating. The city's hazard mitigation plan cites a study from USC that estimates "a potential submarine landslide anywhere along the steep Southern California offshore escarpment could generate a tsunami" that reaches an elevation of 30 feet to 33 feet above sea level.
"The concern with these local tsunami sources is that travel time between the local source of an earthquake and the arrival of the first waves along the coastline is estimated at 10 to 20 minutes, which does not allow much time for broadcasting of warnings and evacuation,' the report said. "Several wave crests are likely, with the second and third waves likely to be higher than the first."
Read more: Are you at risk of tsunami flooding in Southern California? Check these maps
Evacuating all areas of the inundation zone, by comparison, would take hours, Eing estimated. That timeline is backed up by a 2015 study, which concluded a full evacuation of Newport Beach's tsunami-vulnerable areas could take almost 5 hours.
Depending on the scenario, Eing said officials may recommend some people shelter-in-place rather than chance getting stuck in evacuation traffic at or below sea level.
'If it was a local-sourced earthquake… it's probably safer for [residents] to go to the third story,' she said. "It's important to know where you live, if you're in the inundation zone.'
The potential for chaos and confusion in an evacuation became all-too-apparent last December, when a magnitude 7 earthquake struck about 30 miles off the Humboldt County coast.
An expansive tsunami warning arrived minutes later — sending alarms blaring on the cellphones of Californians from the North Coast through the San Francisco Bay Area, including Oakland, Berkeley and Alameda.
Read more: Where in Northern California is most at risk for tsunami flooding? Check these maps
Fortunately, only a small tsunami ended up materializing, just 2 inches high in Mendocino County. But the response and reaction shows plenty of room for improvement.
Some of those who chose to evacuate didn't do so promptly, and there was widespread confusion whether evacuation was necessary at all.
"Many people questioned how a tsunami could enter the Bay and stated it must only be people on the coast who need to evacuate," one person told the city of Berkeley in a survey. "There was just tons of confusion all around, lots of complacency and not taking it seriously."
Another person complained that the only warning they got was the broad "emergency alert" sent to all cellphones, as they weren't signed up for Berkeley's alert systems. As a result, "we had no information at all about who should evacuate and who should not, access to a map, how much time we had," that person wrote.
Hesitation can be deadly. The 2011 magnitude-9.1 earthquake that struck near Japan spawned a tsunami that rose as high as 45 feet and reached the coast within just 30 minutes.
More than 18,000 people died in the disaster, a toll exacerbated by an initial warning that underestimated the size of the tsunami before communications systems cut out entirely. Another issue, as documented by seismologist Lucy Jones in her book "The Big Ones," was the reluctance of some people to evacuate.
Read more: Monster earthquake could sink swath of California, dramatically heightening flood risk, study says
That reluctance, as the Berkeley survey found, could be fueled by a lack of timely information. California's tsunami hazard map website didn't work during the alert period, for instance.
In response, San Mateo County, just south of San Francisco, built its own online tsunami dashboard that lets residents and visitors plug in their location to see if they're in a hazard zone, as well as search for nearby safe places.
Last December, Highway 1 was clogged with motorists trying to flee Pacifica. What many didn't realize is that in some vulnerable areas, heading just a few blocks inland was enough to safely be out of a tsunami's reach.
Some evacuees also went in the wrong direction, driving from higher ground into lower-lying areas in an attempt to flee.
Shruti Dhapodkar, director of emergency management for San Mateo County, heard from someone who lives in the coastal community of Montara — which is high enough in elevation that no tsunami would be expected to ever hit it. Yet that resident fled on Highway 1, directly into a low-lying area.
"I want to make this point very, very clear ... avoiding low-lying areas and moving to higher ground" is the objective, "not necessarily getting in your car and coming over the hill," Dhapodkar said.
Read more: The tsunami that battered Santa Cruz highlights the threat facing California's coast
Understanding what areas of a city are threatened by tsunami are crucial — as is knowing where the safe areas are. If you're at the Santa Monica Pier, just head up above the bluffs, which are at about 100 feet above sea level. You don't need to run to Mount Baldy.
In many parts of San Francisco, heading a few blocks away from the coast is enough to get to safety.
The general advice is to evacuate by foot or on bicycle if possible to avoid traffic jams. But some officials acknowledge that could be a tough ask in car-crazy California.
For more distant tsunamis, where hours may be available for an evacuation, scenarios can vary.
"For most of L.A., a huge chunk of our evacuation zone really is the beach and a couple of blocks inland," said Jennifer Lazo, division chief of the innovation and technology division at the city's Emergency Management Department.
But there are a number of areas in which people may need to drive out. In distant-source tsunami scenarios, firefighters and police officers would be dispatched to direct people out, Lazo said.
Read more: Why a massive California tsunami alert was issued — then soon canceled
Additional, crucial planning remains in the works. San Francisco says it needs to establish an evacuation strategy for people with disabilities, which it plans to complete by 2027.
"In areas where high ground is not immediately available, vertically evacuating and seeking refuge in tall buildings might be the best option for life safety, especially for people with disabilities or access and functional needs," the city's hazards plan says.
San Francisco also needs to develop and install signs on sidewalks and streets in low-lying waterfront areas to help people know where to go in case of tsunami and other coastal flood hazards, such as king tides, the city says.
Some coastal areas have installed tsunami warning signs, but they're not ubiquitous.
"There is a lack of signage just throughout the entire state. Marin County — I'll speak to us — we definitely have limited signage throughout the county identifying tsunami inundation zones," said Steven Torrence, Marin County's director of emergency management. The county Board of Supervisors doesn't have jurisdiction everywhere, he noted, but "we need to make sure that the jurisdictions are clearly identifying these inundation zones."
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This story originally appeared in Los Angeles Times.