Within the State of California, 23 separate volcanic areas and more than 500 volcanic vents have been identified (Jennings, 1975). California volcanoes demonstrate great variety in their types and in their geologic settings; potential volcanic hazards within the State vary accordingly. The tectonic settings of volcanic centers range from subduction-related volcanism in the northern part of the State (Mount Shasta and Lassen Peak), to volcanism related to crustal stretching and thinning along the Sierra Nevada escarpment (Mono-Inyo volcanoes and Long Valley caldera), to volcanism in an area of active crustal spreading in the Salton trough (Salton Buttes rhyolite domes) (figure 1). Past eruptions within the State have run the gamut from small basaltic eruptions through catastrophic caldera-forming eruptions of rhyolite such as the one that formed the Bishop Tuff about 700,000 years ago; virtually every known type of eruptive activity has occurred within California.
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Volcanic activity within the State of California has occurred on the scale of "human time" as well as "geologic time" as evidenced by eruptions at Lassen Peak in 1914-1917 and recent unrest in the Long Valley-Mono Lake area of central-eastern California. Although relatively minor in scale compared to many prehistoric eruptions within the State, the Lassen Peak eruptions included at least two blasts that devastated areas to the east of the peak and produced mudflows that inundated the valley floors of Hat and Lost Creeks. Tephra from the most violent eruption, on May 22, 1915, was carried by prevailing winds as far as about 500 km to the east where it fell in Elko, Nevada.
Following four large (M>6) earthquakes that occurred in the Long Valley region during May 1980, numerous swarms of relatively shallow earthquakes occurred within the south moat of the caldera near the community of Mammoth Lakes. These earthquake swarms continued from 1980 until late 1983 and were accompanied by uplift and deformation within the caldera that continues at the time of this writing (fall 1987). These events have greatly increased concerns about the possibility of renewed eruptive activity in the Long Valley-Mammoth Lakes area and have resulted in increased monitoring efforts in the region and preparation of emergency response plans by local, State, and Federal officials.
Sooner or later, a volcano in California will erupt again, and the ever-expanding use of areas near volcanoes increases the potential impact of an eruption on the State's economy and on the health and safety of its citizens. The elements at risk from future eruptions in California include its population, power resources including nuclear reactors, water supplies, transportation, communications, agriculture, industry, and recreation. Alfors and others (1973) estimated very conservatively that losses in California due to volcanic eruptions could amount to 50 milion dollars between 1970 and the year 2000. The results of the 1980 Mount St. Helens eruptions, however, suggest that far greater losses are likely from even small future eruptions in California. Eruptions of Mount St. Helens in May and June 1980, that were small in volume relative to possible future events in California, resulted in estimated shortterm losses to the economy of Washington State of 970 million dollars (MacCready, 1982). Moreover, 60 lives were lost during the May 18 eruption, and additional economic losses are still accumulating at the time of this writing (fall 1987).
The purpose of this report is to describe potential hazards from future eruptions of volcanoes in California. This assessment is based on the locations, types, and scales of past eruptions and the nature, distribution, and hazardous effects of products from those eruptions. By anticipating the nature and extent of potential volcanic hazards and the nature and likelihood of possible warnings, planners and public officials such as the State Office of Emergency Services (OES) and local governments can:
thereby mitigating the effects of future eruptions.
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