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On the 10-year remembrance of a tsunami that devastated Japan and damaged many California ports and harbors, the California Geological Survey released two new maps created to ensure public safety on the North Coast.

These Tsunami Hazard Area​ maps, which cover Del Norte and Mendocino counties, serve two purposes. First, using new data and improved computer modeling, they update 2009 inundation maps showing how far inland a surge of seawater might go in a worst-case scenario. Second, the maps include a small buffer beyond the modeled inundation area that enhances the model results and can help local officials create or update evacuation plans.​

​"After 10 years of research following the Tōhoku-oki earthquake and tsunami in Japan, we're releasing maps with many improvements to keep Californians safe, including a once-a-millennium tsunami event as the minimum baseline for inundation," said Dr. Steve Bohlen, the acting state geologist of California and head of CGS, part of the California Department of Conservation.

"Although damaging tsunamis are not a frequent event, anyone who has lived on the North Coast for any length of time knows that they are not a hazard to ignore."

Changes to the existing inundation maps for Del Norte and Mendocino counties are relatively modest but nonetheless important, noted Rick Wilson, head of the CGS Tsunami program. In downtown Crescent City, 9th Street remains the evacuation boundary, but the updated map shows potential for tsunami flooding further up creek valleys and includes more areas to the east and north of the city. In Mendocino County, where most of the populated coastline is protected from tsunamis by tall cliffs, the new map depicts a higher tsunami hazard up many river valleys.

"There is some uncertainty when we draw the inundation lines," Wilson said. "The 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. The buffer zones in the new maps account for that uncertainty.

“CGS collaborated with the California Governor's Office of Emergency Services throughout this process and has worked closely with coastal communities to identify places where mapped tsunami hazards have changed so that they can update their evacuation plans appropriately.”

The Redwood Coast Tsunami Workgroup has developed evacuation brochures for Humboldt and Del Norte counties and plans to create them for Mendocino County soon.

More than 150 tsunamis have hit California’s coast since 1800. Many are barely noticeable but nearly a dozen tsunamis have caused fatalities or significant damage, most recently during the March 11, 2011, tsunami generated by a magnitude 9.1 earthquake in Japan. After traveling 10 hours across the Pacific Ocean, the Tōhoku-oki tsunami caused $28 million in damage in Del Norte County, $4 million in damage in Mendocino County, and $100 million statewide.

The most devastating 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. Ten people died, and half of the waterfront business district was destroyed.

South of Cape Mendocino, the biggest tsunami threat to most of California would result from another massive earthquake in the Alaska and Aleutian Islands regions. However, the biggest threat to the northernmost part of California is a major earthquake on the Cascadia subduction zone -- the 700-mile undersea boundary where tectonic plates are colliding. Scientists have evidence that the subduction zone generated a magnitude 9 earthquake and tsunami in 1700.

A similar event could send surges onshore up to 50 feet high toward Crescent City, which has the highest tsunami risk in California, and 30 feet high along the outer coast of Humboldt Bay and the Eureka area. A big quake on the Cascadia megathrust subduction fault might cause five to six minutes of shaking and give people along the coast a maximum of 10 minutes to get inland or to high ground.

CGS plans to issue Tsunami Hazard Area maps for some other parts of the state during California Tsunami Preparedness Week, March 22-26, and hopes to update existing maps for the entire California coast within two years.

While the Tsunami Hazard Area maps are informational, not regulatory, CGS also is working with an advisory panel to develop tsunami zone maps to guide local land-use planning under the Seismic Hazards Mapping Act. CGS currently creates regulatory zone maps for several earthquake-related hazards, helping to ensure public safety and minimize economic impacts.

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