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2021 Multijurisdictional Local Hazard Mitigation Plan  March 28, 1964 (Distant Tsunami)—An M-9.2 earthquake in Anchorage, Alaska, generated a tsunami that struck the Pacific Northwest and northern California. Twelve people were killed in California, and a surge approximately 20-feet high flooded 29 city blocks of Crescent City.  March 11, 2011 (Distant Tsunami)—An M-9.0 earthquake in Tohuku, Japan generated a moderate tsunami in California. While the tsunami did not cause significant flooding, it did lead to one death and more than $100 million in damages to 27 harbors statewide. The most significant damage occurred in Crescent City and Santa Cruz. 15.2.2 Location The California Department of Conservation maintains detailed tsunami inundation maps for San Mateo County and other parts of the State. These maps are generated through computer modeling of the areas most likely to be affected by a tsunami event and serve as an important preparedness tool. The tsunami hazard areas identified in the mapping are based on a suite of tsunami sources, both local and distant, and does not, therefore, represent risk from a single sources event. Tsunami risk areas are shown in Figure 15-3. 15.2.3 Frequency The frequency of tsunamis is related to the frequency of the events that cause them, so it is similar to the frequency of seismic or volcanic activities or landslides. Generally, four or five tsunamis occur every year in the Pacific Basin, and those that are most damaging are generated in the Pacific waters off South America rather than in the northern Pacific. Based on risk factors for the County and past occurrences, it is highly likely that tsunamis will continue to strike the coastline in San Mateo County. Tsunami probabilities are tied to earthquake and other geologic events; however, not all earthquakes or submarine landslides will trigger a tsunami. 15.2.4 Severity A tsunami’s size and speed, as well as the coastal area’s form and depth, affect the impact of the tsunami. At some locations, the advancing turbulent wave front will be the most destructive part of the tsunami wave. In other situations, the greatest damage will be caused by the outflow of water back to the sea between crests, sweeping away items on the surface and undermining roads, buildings, bulkheads, and other structures. This outflow action can carry enormous amounts of highly damaging debris, resulting in further destruction. Ships and boats, unless moved away from shore, may be forced against breakwaters, wharves, and other craft, or be washed ashore and left grounded after the withdrawal of the seawater (National Tsunami Hazard Mitigation Program, 2001). 15.2.5 Warning Time Warning System The tsunami warning system for the Pacific Ocean evolved from a program initiated in 1946. It is a cooperative effort involving 26 countries with numerous seismic stations, water level stations and information distribution centers. The National Weather Service operates two information distribution centers: The Pacific Tsunami Warning Center in Ewa Beach, Hawaii; and the National Tsunami Warning Center covering the California coast in Palmer, Alaska. The warning centers issue tsunami watches, warnings, and advisories. A watch is issued when a large earthquake has occurred far away from the region and the threat is still being determined. 15-4