March 22, 2021
SACRAMENTO - As California begins Tsunami Preparedness Week, the California Geological Survey (CGS) has released a new interactive Tsunami Hazard Area map for Los Angeles County.The map allows users to type in an address to determine whether the property is within a zone.
The Tsunami Hazard Area map serves two purposes. First, using new data and improved computer modeling, it updates 2009 inundation maps showing how far inland a surge of seawater might go in a worst-case scenario. Second, it ties a small buffer beyond the modeled inundation area to roads or landmarks to help local officials easily communicate evacuation plans. Local tsunami evacuation material will be added to the online map interface as it is developed.
More than 150 tsunamis have hit California’s shore since 1800. Most 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.
While the new Los Angeles County map takes 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.
Changes to the existing Los Angeles County inundation map are modest, with the inundation potential upgraded in some areas and downgraded in others, noted Rick Wilson, head of the CGS Tsunami Program. CGS and the California Governor's Office of Emergency Services (Cal OES) have advised several communities with isolated pockets of population and few roads for evacuation -- such as in Alamitos Bay, Marina del Rey, and the ports of Long Beach and Los Angeles -- about their increased inundation areas.
“A large tsunami could flood sizeable areas of Marina Del Rey and Long Beach to an elevation of 15 feet,” Wilson said. “Flood levels for the Port of Los Angeles and the Port of Long Beach could reach elevations of 12 to 15 feet, which would inundate almost all of the land in the ports and some of the surrounding communities. Local officials have indicated that if both ports were shut down for one day, the economic loss to the county would be over $1 billion.”
Los Angeles County’s beaches can have over a million visitors per mile of beach during summer weekends and holidays, posing an evacuation challenge. The Tsunami Hazard Area maps will help communities understand their threat and plan appropriately.
"This map is 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 Nick Graehl, the CGS engineering geologist who developed the Los Angeles County map. “The buffer zones on the map accounts 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.”
According to the scenario used by CGS, the first surges of a worst-case tsunami would reach the Los Angeles coastline in about six 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 has issued seven Tsunami Hazard Area maps and hopes to update the maps for the rest of California’s coastal counties in the next two years.