California Geological Survey Issues New Tsunami Hazard Maps for San Francisco, Contra Costa, and Santa Clara Counties

​​​​​July 8, 2021

SACRAMENTO -- The California Geological Survey (CGS) has released new interactive Tsunami Hazard Area maps for San Francisco, Santa Clara, and Contra Costa​​ counties​, useful for evacuation planning in an extraordinary event. These maps allow users to type in an address to determine whether the property is within a tsunami hazard area. Local tsunami evacuation material will be added to the online map interface as it is developed. 

Working with the California Governor's Office of Emergency Services (Cal OES), t​hese Tsunami Hazard Area maps include new data and improved computer modeling results, and replace maps published by CGS in 2009. The new maps are not only based on how far inland a surge of seawater might go in a worst-case scenario but also include minor inland "buffers" to roads and landmarks to clarify where people must evacuate to be safe. These buffer areas also account for potential errors and uncertainties in the modeling.  

The maps incorporate hard lessons learned by the nation of Japan a decade ago during the Tohoku-Oki earthquake and tsunami, noted Dr. Steve Bohlen, the 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,” he said. “Then it was impacted by a once-a-millennium tsunami. So, we’re taking a very conservative approach and using a thousand-year scenario as the baseline for our new maps, hoping to avoid the tragic loss of life experienced in Japan. 

“While damaging tsunamis are infrequent in California, they have and do happen. If you live on o​​​r visit the coast, you need to be aware of this potential hazard.”

While the new maps for the three Bay Area counties take a variety of potential tsunami sources into account, the worst-case scenario would result from a magnitude 9.3 earthquake in the eastern Aleutian Islands. Examples of new information provided by these maps:

  • The most significant increases in the mapped tsunami hazard area are in and around North Beach and downtown around Market Street. These are both areas with very low land elevations that could be impacted by a small increase in waterfront flooding. The new Tsunami Hazard Area maps around Market Street look similar to predicted flooding from long-term sea-level rise, so any waterfront flood prevention measures in that area may help reduce the risk from both tsunamis and climate change.
  • On the west side of the city, the tsunami hazard for the Ocean Beach has only a few minor increases. The tsunami hazard has increased in the area of the zoo, with potential flooding of the grounds during extreme tsunami events.

  • ​Larger areas around the Port of Richmond, which includes many residential and commercial business areas, are now included in the tsunami hazard area. The 2009 map did not show much flooding beyond the port itself. 
  • Tsunami flow-depths are expected to be relatively shallow in low-lying areas of both San Francisco and Richmond, but the flooding of buildings and transportation and shipping facilities could be costly and cause significant delays in recovery.
  • The tsunami hazard for the western and northern parts of the county has stayed essentially the same as was depicted in 2009. Although CGS doesn’t expect to see significant flooding in these areas to the north and into the delta, harbors may see strong and potentially damaging currents during extreme tsunami events.
  • ​The tsunami hazard is slightly higher in the south bay. However, the residential and commercial areas near the bay are protected by the levees and ponds in the area.

In the large Aleutian Island scenario used by CGS, the first surges would reach the Bay Area in about five hours.

“That time will go by fast, as scientists and emergency planners at the federal, state, and local levels will all need to assess the threat level before issuing an evacuation warning,” said Rick Wilson, head of the CGS Ts​unami Program​.

"Bottom line: If you’re near the coast and feel strong shaking from a local earthquake or get an official notification to evacuate, move inland or to a higher elevation as soon as possible. A large tsunami surge might be fascinating to watch but you don’t want to be anywhere near it.” 

These maps inform the State’s Hazard Mitigation Plan and emergency planning efforts so that state agencies, local governments, tribal governments and others can effectively mitigate and respond to emergencies that might occur in their areas of responsibility. 

“We appreciate the collaboration with the Department of Conservation’s California Geological Survey to provide Californians who live, work or visit in a Tsunami Hazard Area the tools they need to develop evacuation plans for themselves, their families and their businesses,” said Lori Nezhura, Deputy Director of Planning, Preparedness and Prevention, Cal OES. ​​

More than 150 tsunamis have hit California’s shore since 1800. Most were barely noticeable, but a few have caused fatalities or significant damage. The 2011 tsunami that devastated Japan also 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.

CGS, part of the California Department of Conservation, has issued Tsunami Hazard Area Maps in 13 counties and hopes to update the maps for the remaining seven coastal counties within the next year.
“Hazards management – for geologic events such as tsunamis as well as mining and petroleum production – is one of our department’s pillars, or cornerstones,” Bohlen said.​


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