March 23, 2021
SACRAMENTO - As California recognizes Tsunami Preparedness Week
, the California Geological Survey
is releasing interactive maps covering Alameda, Monterey, and San Mateo counties. These maps allow users to enter an address to determine whether the property is within an area that may be subject to tsunami flooding.
These Tsunami Hazard Area
maps serve two purposes. First, using new data and improved computer modeling, they update 2009 tsunami inundation maps showing how far inland a surge of seawater might go in a worst-case scenario. Second, they tie a small buffer beyond the modeled inundation area to roads or landmarks to help local officials easily communicate evacuation plans. As local tsunami evacuation material is developed, it will be added to CGS’ online map interface.
"After 10 years of research following the Tohoku-Oki earthquake and tsunami
in Japan, we're releasing maps with many improvements to keep Californians safe,” said Dr. Steve Bohlen, Acting State Geologist of California
and head of CGS. “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. 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. While damaging tsunamis are infrequent in California, if you’re on the coast, you need to be aware of this potential hazard.”
More than 150 tsunamis have hit California’s shore since 1800. 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.
The most destructive tsunami to hit California occurred March 28, 1964. Several surges reaching 21 feet high swept into Crescent City four hours after a magnitude 9.2 earthquake in Alaska, killing 12 and leveling much of the town's business district. For most of California, the biggest tsunami threat would result from another huge earthquake in the Aleutian Islands.
Changes to the existing tsunami hazard maps are show larger areas of potential inundation along the outer coast of San Mateo County but relatively modest changes in the other areas assessed, with the inundation potential upgraded in some areas and downgraded in others, noted Rick Wilson, head of the CGS Tsunami Program.
"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,” Wilson said. “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.”
A closer look at the counties covered by these maps, which utilize a magnitude 9.3 earthquake in the eastern Aleutian Islands as the source of the tsunami:
- The City of Monterey and Moss Landing saw slight increases in 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.
- Moss Landing harbor in Monterey County sustained severe damage during the 2011 tsunami from Japan. Almost all dock piles were damaged and had to be replaced. Damages totaled more than $2 million with 2020 inflation.
San Mateo County
- 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 inundation 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 low-lying areas off of Seaport Boulevard.
- On the Pacific side, the biggest 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.
According to the large Aleutian Island scenario used by CGS, the first 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 cautioned. "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.”
CGS, part of the California Department of Conservation, has issued Tsunami Hazard Area Maps in seven counties and hopes to update the maps for the rest of California’s coast within the next two years.