New study finds these California coastal cities face heightened tsunami danger
James WardThe California Geological Survey recently released new maps showing that potential tsunamis increase the threat of flooding in Alameda, Monterey, and San Mateo counties, along with other coastal areas designated as threatened by earlier studies.
The updated maps — released to coincide with California's Tsunami Preparedness Week that starts March 22 — use new data and improved computer modeling to show how far inland a surge of seawater might go in a worst-case scenario, according to the California Department of Conservation.
The agency started updating California's tsunami danger maps after 2011's Tohoku-Oki earthquake and tsunami in Japan that killed approximately 15,000 people and caused massive damage, including to the Fukushima Nuclear Power Plant.
“Japan utilized data from several hundred years of tsunami records in its planning, which seemed perfectly reasonable, then was hit by a once-a-millennium tsunami," said Steve Bohlen, Acting State Geologist of California and head of CGS. "So, we’re using a thousand-year scenario as the baseline for our new maps, hoping to avoid the tragic loss of life experienced there."
Here's what to know about the recently released California tsunami reports.
What new areas were included in the California Tsunami Hazard Maps?
For most of California, the biggest tsunami threat would be another massive earthquake in the Aleutian Islands in the northern Pacific Ocean, the California Geological Survey found.
"These maps are based on the best data we have, but there's a margin of error when you’re trying to consider a thousand-year event, so we err on the side of caution,” said Rick Wilson, head of the CGS Tsunami Program. “The buffer zones on the maps account for that uncertainty. We worked closely with local emergency planners to identify places where mapped tsunami hazards have changed so that they can update their evacuation plans.”
The survey modeled what may happen if a major earthquake hits the eastern Aleutian Islands and caused a tsunami:
- The City of Monterey and Moss Landing saw slight increases in previously included Tsunami Hazard Areas due to higher projected surges. High dunes near the Salinas River State Beach and Monterey Dunes Colony Association were removed from the hazard area.
- The new maps account for both local- and distant-source tsunamis. The San Gregorio Fault runs through the offshore Monterey Canyon, and a large enough earthquake could generate a landslide capable of producing a surge that would hit the coast in minutes.
- In a worst-case tsunami, sizeable areas of Alameda, Oakland, and Berkeley could be flooded up to an 18-foot elevation.
- The Tsunami Hazard Area is extended inland by 4 or 5 blocks in parts of southwestern Albany and northwestern Berkeley — as far as 6th Street in places.
- There are minor adjustments to the 2009 flood lines in Emeryville, West Oakland, the City of Alameda, San Leandro, and San Lorenzo.
- On the Bay side, there are minor changes in South San Francisco, San Mateo, Foster City, East Palo Alto, and Menlo Park. In Burlingame, the hazard area has been extended inland to Highway 101. In Redwood City, the hazard area covers the Pacific Shores Center Business park and the low-lying regions off of Seaport Boulevard.
- On the Pacific side, the most significant change is in Half Moon Bay, where seawater could cross Highway 1 through much of the community north of Arroyo Leon. Parts of western Half Moon Bay, both north and south of Kelly Avenue, have a slightly greater risk.
- The Tsunami Hazard Area is expanded in Princeton and Half Moon Bay Airport, El Granada, and Miramar. The hazard area is slightly larger in Pacifica, Pescadero Creek Valley, and reaches as far as Highway 1 in parts of Moss Beach.
What other parts of California could be hit by a tsunami?
In areas previously identified as in danger of a tsunami, flooding could reach up to 30 feet above high tide along the outer coast of Humboldt Bay in the Eureka area, and up to 50 feet in Crescent City and Cayucos.
In Southern California, a large tsunami could flood parts of Marina del Rey, Long Beach, Malibu, and Huntington Beach up to 15 feet above sea level.
You can visit the California Geological Survey's website for the extensive California tsunami hazard map.
How long would Californians have to evacuate if a tsunami happened?
According to CGS's large Aleutian Island scenario, the first tsunami surges would reach the Bay Area and Monterey in about five hours.
"That may seem like a lot of time, but it will take an hour or so for the National Tsunami Warning Center to issue a warning to California and then additional time for local authorities to determine whether an evacuation is necessary," Wilson said. "The bottom line is, if you’re near the coast and feel strong shaking from a local earthquake or get an official notification to evacuate, move inland as soon as possible.”
California's Tsunami Preparedness Week's webpage has multiple resources on how to prepare for a potential tsunami.
How many tsunamis have hit California?
More than 150 tsunamis have hit California’s shore since 1800, according to the California Geological Survey. Many were barely noticeable, but a few have caused fatalities or significant damage — most recently, the 2011 tsunami that not only devastated Japan but caused $100 million of damage to California ports and harbors.
Moss Landing Harbor in Monterey County sustained severe damage during the 2011 tsunami in Japan. Almost all dock piles were damaged and had to be replaced. Damages totaled more than $2 million with 2020 inflation.
The most destructive tsunami to hit California occurred on March 28, 1964, in Crescent City in northern California. Several surges reaching 21 feet high swept into the city near the Oregon-California border four hours after a magnitude 9.2 earthquake in Alaska. The tsunami killed 11 people and destroyed or damaged 289 buildings, causing an estimated $15 million in damage in 1964 money.