The California Geological Survey has issued new tsunami hazard maps for San Francisco, Santa Clara and Contra Costa counties, now updated to account for a “worst-case scenario.”

The 2021 maps, crafted by the CGS in partnership with the California Governor’s Office of Emergency Services, replace 2009 versions with new estimates covering larger areas that may need to be evacuated in the event of a tsunami.

According to Steve Bohlen, acting state geologist of California and head of the CGS, the heightened level of precaution was inspired by the 2011 Tohoku-Oki earthquake and tsunami.

“Japan utilized data from several hundred years of tsunami records in its planning, which seemed perfectly reasonable,” Bohlen said in a statement. “Then it was impacted by a once-a-millennium tsunami.”

California has released updated Tsunami Hazard Area Maps for the Bay Area. The maps take into account a “worst-case scenario” that could affect the region during a major earthquake out at sea. (Image courtesy of California Geological Survey)

The CGS’s “worst-case scenario” for the Bay Area would be a 9.3-magnitude earthquake in the Aleutian Islands off the west coast of Alaska. A tsunami caused by such an earthquake, the CGS said, would reach the Bay Area within five hours, leaving little time for an evacuation warning to be issued and residents to safely evacuate.

In 1964, a 9.2 earthquake in Alaska caused a tsunami to reach Crescent City in four hours, killing 12 and destroying much of the town’s business district.

Using the Aleutian Islands scenario, along with other sources, the CGS developed new tsunami hazard maps to inform residents and emergency response planners what areas may need to be evacuated and how far inland they would have to move.

The most significant increase in the tsunami hazard area, according to the CGS, is low elevation land in San Francisco’s North Beach and downtown Market Street, as well as residential and commercial area around the Port of Richmond.

Footage shows the devastation that resulted to Crescent City following the Great Alaskan Earthquake of 1964. (Video courtesy of mcab1993/YouTube)

The new maps will help inform the State Hazard Mitigation Plan so state agencies, local and tribal governments can prepare to respond to potential tsunamis. The maps may also help inform climate change response, the CGS said, as the same low-level areas that are vulnerable to flooding during a tsunami are also vulnerable to flooding from rising sea levels.

Thirteen new maps total have been released by the CGS in 2021, and it hopes to release maps for the remaining seven California counties within the next year.

Residents can view the tsunami hazard area map for their county via the CGS website, as well as access tsunami safety materials.