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Tsunamis are extremely long ocean waves caused by undersea earthquakes, volcanic eruptions or massive landslides into adjacent water bodies. Tsunamis are generally small in height in deep water, but very long and can rise to 20 feet or more when they approach the coast, causing great destruction in shoreline locations. No tsunamis have been known to strike San Mateo County. However, the 1964 Alaska earthquake resulted in a tsunami striking Crescent City in Del Norte County just south of the Oregon border, killing eight people. The origin of the wave was assumed to be the Aleutian Trench off the Alaskan Coast. The U.S.G.S. has produced a map delineating areas subject to tsunami inundation, based on a 20-foot runup along coastal areas and also at the Golden Gate. Such a runup is estimated to occur an average of once every 200 years.This information has been incorporated into the Natural Hazards map. The areas of the County which would be most heavily damaged by a tsunami are those along the Pacific Coast: Point Ano Nuevo, the Pescadero Creek and San Gregorio Creek estuaries, Half Moon Bay, portions of San Pedro Valley, Rockaway Beach and Laguna Salada. The degree of damage experienced by these areas would depend on the local sea bottom and coastal topographic characteristics, as well as the incoming direction of the tsunami. The potential tsunami inundation areas are indicated on the Natural Hazards map. A 20-foot tsunami runup at the Golden Gate would be attenuated to a maximum of 8 feet at Sierra Point (Brisbane) and roughly 4 feet at Ravenswood Point (East Palo Alto), based on the U.S.G.S. analysis. Bayshore locations which are most subject to tsunami damage include Sierra, Oyster, Coyote and Ravenswood Points; the San Francisco Airport area; the Burlingame Marina; Brewer Island in Foster City; Belmont Slough; Greco Island and portions of Bair Island off Redwood City. Seiches are oscillating waves in an enclosed or partly-enclosed body of water, caused by earthquakes or landslides which displace part of the water body. Four water bodies in San Mateo County are believed to be large enough to pose significant seiche potential: Upper Crystal Springs Reservoir, Lower Crystal Springs Reservoir, San Andreas Lake and Pilarcitos Lake. If an earthquake similar in magnitude to the 1906 earthquake were to occur at a time when these water bodies were at the high water mark, seiches could overtop the spillways of these water bodies by several feet, causing large scale inundation downstream.Tsunami: Long, high-velocity sea waves resulting from seismic events which have relatively small wave height in deep water, but which rise significantly in shallow water.Seiche: Oscillating waves in an enclosed or partly enclosed body of water caused by seismic activity or sudden changes in atmospheric conditions. |