October 7, 2022
The California Geological Survey (CGS) today completed its interactive
Tsunami Hazard Area maps for the entire California coast by releasing updated maps for Santa Cruz, Ventura, Marin, Napa, Solano, Sonoma, and San Diego counties. CGS, which is housed inside California's Department of Conservation, created the maps to help users determine whether they are in an at-risk area for tsunamis and plan accordingly. Tsunamis are uncommon but have hit California numerous times in the past.
Completed with support from the California Governor’s Office of Emergency Services (Cal OES), these Tsunami Hazard Area maps use new data and improved computer modeling to update an earlier series of maps published in 2009.
"Hazard management -- particularly natural hazards -- is one of our department's key priorities," Conservation Director David Shabazian said. "Our geologists utilized the best scientific information and techniques available to develop maps that will help communities identify and prepare for tsunami risk. Our geologists also work with communities to reduce the impact and cost if a tsunami occurs."
Evacuation material for each county will be added to the online map interface as it is developed, and Cal OES will ensure that local informational tsunami kiosks and signage are updated inform the public.
“We learned a great deal about the importance of preparedness and being conservative in our approach to the mapping from all the research conducted after the
2011 Tohoku-Oki earthquake and tsunami in Japan,” said Rick Wilson, who heads the CGS Tsunami Program. “These maps have several improvements to help keep Californians safe.
“Although Japan prudently considered data from several hundred years of tsunami records in its planning, it unfortunately was struck by a once-a-millennium event that also impacted our state. Our new maps consider the best information we have about California tsunamis going back more than a thousand years. Damaging tsunamis are infrequent in California, but if you’re on the coast, you need to be aware of this potential hazard. Our goal is to avoid the tragic loss of life suffered in Japan.”
A closer look at the counties covered by these maps, which utilize a magnitude 9.3 earthquake in the eastern Aleutian Islands as the worst-case distant source of the tsunami. Most of these counties also have potential local sources that CGS accounts for in the maps.
A closer look at the counties covered by these maps, which utilize a magnitude 9.3 earthquake in the eastern Aleutian Islands as the worst-case distant source of the tsunami. Most of these counties also have potential local sources that CGS accounts for in the maps.
- New modeling indicates that a tsunami generated from a subduction zone earthquake off the Aleutian Islands could produce a tsunami 18-25 feet above the Santa Cruz Boardwalk -- more than twice as high as envisioned in the 2009 map. A worst-case scenario tsunami would hit low-lying areas near Santa Cruz Harbor, Capitola Beach, Seacliff Beach, La Selva Beach, and Pajaro River Beach particularly hard.
- While a distant-source tsunami would take approximately 4½ to 5 hours to reach Santa Cruz County, an underwater landslide in the nearby Monterey Canyon could arrive at local beaches within minutes.
- New modeling indicates that either a distant- or local-source tsunami could cause flooding up to a 15- to 20-foot elevation in cities such as Ventura, Oxnard, and Port Hueneme.
- A magnitude 7 to 8 earthquake along the offshore portion of the Ventura-Pitas Point fault zone could send a tsunami to the county’s shores with only a few minutes of warning. An Aleutians-generated tsunami would take 5 hours to reach the county.
Marin County
- There are significant changes to the mapped Tsunami Hazard Area along the inner coast of Marin County on San Francisco Bay, including San Rafael, Larkspur, and Corte Madera. Tsunami flooding generated by an exceptionally large Alaskan quake could reach 10 feet or greater along portions of Marin’s inner coast.
- The Pacific coast of Marin has some modest increases to the hazard area with tsunami flood waters reaching 25- to 30-foot elevation in extreme cases.
- There are local tsunami sources related to earthquakes on the offshore Point Reyes thrust fault. Surges could arrive within minutes of the earthquake, produce 10-foot flooding, and put people on beaches at risk.
Sonoma County
- Updated modeling shows modest changes to the existing hazard areas on the Pacific coast, such as Bodega Bay. However, there are larger changes to the hazard area mapped on the inner coast, such as along the water in Petaluma. Tsunami flood levels in these areas during extreme events would still be relatively small (4- to 6-feet).
- Like Marin County, the local source tsunami threat is a large quake on the Point Reyes thrust fault.
Solano County
- Along the bay shoreline in western Solano County, maximum tsunami flooding could reach 4 feet but is not expected to flood Highway 37.
- In Benicia, worst-case tsunami flooding is 5 feet and might reach inland nearly to Interstate 780 along the Benicia State Recreation Area.
- In Vallejo, tsunami flooding could reach 5 feet around Mare Island, inundating low-lying undeveloped areas adjacent to the Napa River.
- Near the mouth of the Napa River, the maximum tsunami flood elevation is about 6 feet, impacting wetlands near the river, its tributaries, and along Sonoma Slough.
- Low-lying wetland areas of American Canyon could experience tsunami flooding of up to 5 feet.
- Further up the Napa River towards Edgerly Island, the maximum tsunami flood elevation is only 2 to 3 feet. Flooding is unlikely to overtop the riverbank levees.
“For distant-source tsunamis, local emergency officials will be able to give the public ample warning to move away from the danger,” said Wilson. “However, if there’s a nearby offshore earthquake or landslide, there won’t be time for an official warning. People will have to rely on natural tsunami warning signs – such as feeling strong shaking while at the beach or seeing the ocean recede suddenly – and get inland or to higher ground as soon as possible.”
CGS has updated Tsunami Hazard Area maps for the entire coast of the California mainland and Catalina Island. It will produce maps for the rest of the Channel Islands in the next few years.
California shores have been struck by more than 150 tsunamis since 1800. Although most were minor, some have caused fatalities and/or significant damage. One of the more recent damaging tsunamis was generated by the 2011 Japan earthquake; it caused roughly $100 million of damage to California ports and harbors. Last January, a volcanic eruption in Tonga caused roughly $10 million in damage, mainly to harbors in Santa Cruz and Ventura counties.
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.
