Tsunamis have wreaked havoc along the world’s coastlines since before recorded history. Neglecting the possibility of one reaching West Marin’s shores, county and state officials say, would be unwise.
Such was the focus of a tsunami readiness workshop held Tuesday morning in Stinson Beach, where local law enforcement, fire protection and utilities officials joined a couple dozen residents to hear from state geological and emergency services experts who emphasized the need to have their own evacuation plans locked-and-loaded.
“If you’re strolling on the beach and San Andreas decides to start rumbling, get the heck out of there!” said Ursula Hanks, the coordinator for Marin County Emergency Services. “That’s step one: get to higher ground.”
A submarine earthquake striking near West Marin shores would leave coastal residents and visitors mere minutes to act quickly, experts said. In particular, where the San Andreas Fault links up with the San Gregorio Fault off Bolinas’s shore, a large earthquake could hurl 30-foot waves onto Stinson, Bolinas and Muir Beaches. Geologists predict a 7.0-quake has a 50-percent chance of occurring in the Bay Area by 2045.
More likely, Marin’s coast might endure 10-foot waves sent from a quake near Alaska’s Aleutian Islands, far enough away to give West Marin residents at least four-hours notice from local warning systems and media outlets.
What are those warning systems? Having received a heads-up from the National Tsunami Warning Center in Alaska, the county’s Office of Emergency Services would broadcast messages to local disaster groups able to warn neighbors about an approaching tsunami through calls, texts and radio transmissions to launch a localized evacuation effort.
Residents can sign up to receive real-time alerts in the event of a tsunami or other disaster through the county’s AlertMarin Emergency Notification System at alertmarin.org. Residents can also see if they live in a tsunami zone by entering their addresses into the search engine at myhazards.caloes.ca.gov.
In both Muir Beach and Stinson Beach, fire stations have sirens that would blare out the alarm. Depending on weather conditions, the Marin County Sheriff’s Office would send out a small airplane equipped with a loudspeaker issuing evacuation instructions. Following tsunami evacuation signs posted throughout town and not hesitating to act could prevent dangerous traffic congestion—especially on busy, tourist-heavy days—and save lives, experts said.
“We can’t make people evacuate,” said Stinson Beach Fire Chief Kenny Stevens. “But we can prepare ourselves to be safe.”
As a general rule of thumb, experts recommend moving a mile inland and 50 feet high to keep a safe distance from a tsunami’s sweep.
“I would not feel safe during a modern-day tsunami in a two-to-three story wood-frame house, even if it’s elevated as much as 10 feet,” said Rick Wilson, a senior engineering geologist with the California Geological Survey. He added that were landslides to block the few roads leading into or out of West Marin towns, residents should be prepared to be isolated from the outside world for at least a week.
Eight tsunamis have reached California shores over the past 70 years. In 1964, a 9.2-magnitude quake from Alaska sent a tsunami that killed one person in Bolinas 13 hours after the first wave arrived.
Most recently, in 2011 California coastlines—including in Bolinas—felt ripples from a massive tsunami that ravaged Japan’s coastal towns in the wake of an 8.9-magnitude earthquake that brought waves in excess of 100 feet in certain spots. Stressing the need to move fast during a tsunami, Mr. Wilson recalled the tragedy of a Japanese elementary school where more than 70 students and teachers died because they argued among each other and could not decided quickly enough which evacuation route to use.
“The key is to have a plan to get to high ground and to follow that plan immediately,” Mr. Wilson said. “Don’t wait. Don’t argue.”
That tsunami—which experts classified as a “distant-source” event—caused minimal property damage to ports along the California coast, due to the lucky fact that it arrived during low tide. A much greater threat to local property and lives would be a “near-source” event, such as what might result from a rift along one of the Bay Area’s many fault lines.
One quake zone—the Point Reyes thrust fault—lies offshore close to West Marin, ripe for tsunami conditions should the fault ever thrust. But geological evidence taken from pre-historic soils indicate little chance of a large quake hitting nearby, and there is no evidence that a tsunami has ever originated from within the Bay Area.
But if a massive quake were to happen among any of the Bay Area faults, tsunami wave lengths could stretch hundreds of miles long, sending out multiple columns of fast-moving waves lasting 20 minutes or more for as long as an entire day. Once they crash through a developed area, the huge waves transform into deadly mudflows, wiping out everything in their path and stopped only by elevated structures such as cliffs or tall walls. Along the way, the tsunami would inundate buildings, sling around dangerous debris, create massive erosion and landslides and liquefy subterranean soils that would undermine road foundations.
The chance of any quake occurring in the Bay Area above a magnitude 8.0, however, is four percent, according to Mr. Wilson. The last near-source tsunami to strike California’s coastline struck in 1700, when a 9.0-quake erupted along the Cascadia Subduction Zone in the Pacific Northwest. The likelihood of a large quake in that zone in the next 50 years is 40 percent.