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Figure 1: Areas subject to potential hazards from future eruptions in California. Areas numbered I to VI are shown in detail as separate illustrations on Plate 1 -- (Web note: not available).
The potentially more hazardous eruptions in the State are those that involve explosive eruption of large volumes of silicic magma. Such eruptions could occur at vents in as many as four areas in California. They could eject pumice high into the atmosphere above the volcano, produce destructive blasts, avalanches, or pyroclastic flows that reach distances of tens of kilometers from a vent, and produce mudflows and floods that reach to distances of hundreds of kilometers. Smaller eruptions produce similar, but less severe and less extensive, phenomena.
Hazards are greatest close to a volcanic vent; the slopes on or near a volcano, and valleys leading away from it, are affected most often and most severely by such eruptions. In general, risk from volcanic phenomena decreases with increasing distance from a vent and, for most flowage processes, with increasing height above valley floors or fan surfaces. Tephra (ash) from explosive eruptions can affect wide areas downwind from a vent. In California, prevailing winds cause the 180-degree sector east of the volcano to be affected most often and most severely. Risk to life from ashfall decreases rapidly with increasing distance from a vent, but thin deposits of ash could disrupt communication, transportation, and utility systems at great distances, and over wide regions, in eastern California and adjacent states.
Volcanic eruptions are certain to occur in California in the future and an be neither prevented nor stopped, but actions can be taken to limit damage from them. Reduction of risk to life and property can be effected by avoiding threatened areas and by taking protective measures to reduce the effects when and where vulnerable areas cannot be avoided. Monitoring of volcanic precursors generally can identify the locality of impending volcanic activity, even though it often does not pinpoint the nature or timing of an eruption, or even its certainty. Hazard-zonation maps can then be used to guide decisions regarding evacuation and other response activities. Thus, effective monitoring of volcanoes in the State, combined with preparation of contingency plans to deal with future eruptions, can help reduce risk to lives and property.
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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Figure 1:
Areas subject to potential hazards from future eruptions in California.
Areas numbered I to VI are shown in detail as separate illustrations on
Plate 1 -- (Web note: not available).
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 million 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.
Late Pleistocene and Holocene vents are further divided on the map into three general categories based on character of past eruptions and partly on compositions of erupted products as follows:
Classifications of volcanoes, based on composition of products erupted, style of eruption, or both, is useful for providing general information about the potential location, nature, and timing of future eruptions. large, central-vent, silicic volcanoes are likely to erupt more frequently and more explosively in the future than smaller, mafic volcanoes, most of which are widely scattered in volcanic fields. Most central-vent volcanoes and silicic volcanic centers are located above large, shallow masses of differentiated magma that sporadically erupt viscous gas-rich magma and, thus, have a greater tendency to erupt explosively and repeatedly.
In contrast, vents located in mafic fields erupt less viscous magma from which gas generally escapes nonexplosively. Mafic magma may come to the surface from great depths and commonly is not stored in large chambers in the crust; such eruptions are less explosive than those at more silicic centers and are less likely to occur repeatedly from the same vent. However, mafic magmas, like silicic magmas, may interact with ground water and cause phreatic explosions. Thus, large silicic central-vent volcanoes like Mount Shasta and Lassen Peak can be expected to erupt repeatedly in the future; however, eruptions in mafic volcanic fields probably will occur at new vents within the field rather than at previously active vents.
Close to an erupting vent, the main hazards to property posed by eruptions of tephra include high temperatures, burial, and impact of falling fragments; large falling blocks can kill or injure persons who cannot find shelter. Significant property damage can result from the weight of tephra, especially if ti is wet, and 20 centimeters or more of tephra may cause structures to collapse. Hot tephra falling near a volcano may set fire to forests and structures. Farther away, the chief danger to life is the effect of ash on the respiratory system. Even 5 centimeters of ash will stop the movement of most vehicles and disrupt transportation, communication, and utility systems. Machinery is especially susceptible to the abrasive and corrosive effects of ash. These effects, together with decreased visibility or darkness during an eruption, may further disrupt normal transportation, communication, and electrical services; they can also result in psychological stresses and panic among people whose lives may not be endangered.
A wide variety of compositions and volumes of tephra have been erupted during the last 10,000 years in California (Table 2). Silicic volcanic centers like those at the Medicine Lake and the Mono-Inyo volcanoes have produced many lobe-shaped tephra deposits, some of which extended hundreds of kilometers downwind. Such eruptions are likely to occur in the future at those and other silicic volcanoes like Mount Shasta and Lassen Peak.
Eruptions of relatively small volumes of basaltic tephra have occurred at many vents during Holocene time. Such eruptions have been far less explosive than more silicic eruptions and have produced cinder cones at the vents and tephra deposits within a few kilometers downwind. Similar small-volume eruptions of tephra can be expected to occur again at many vents within mafic volcanic fields in California.
The outher boundaries of hazard zones are generalized and enclose minimum areas that would be endangered by eruptions like those used as models. Within these zones, relative hazard is gradational and decreases with increasing distance form a vent and for most flowage phenomena with increasing height above valley floors or basins.
The various hazard zones depicted show differences in the possible kinds and extents of hazards. Hazard zones termed "locally precedented" at a given volcano are based on events that have occurred at that volcano during the last 10,000 years. A pyroclastic-flow-hazard zone termed "locally unprecedented" is shown at some silicic volcanic centers where large, shallow, differentiated magma chambers are thought to exist and where explosive eruptions larger than any identified at that volcano in the past are thought to be probable enough to consider in hazard planning. The eruption of Mount Mazama (at Crater Lake, Oregon) about 6,800 years ago serves as a model to define the extent of hazards from pyroclastic flows at these centers. Although hazard zones are based on specific, identified "model" events, the size, location, or nature of the next eruption in California are impossible to forecast.
An inverse relationship exists between the size of eruptions (volume of material ejected) and their frequency; like earthquakes, small eruptions occur more frequently than large ones. Studies of past eruptive activity at Mount St. Helens (Crandell and Mullineaux, 1978, p.17), for example, suggested that a tephra eruption of small volume can be expected there as often as once every 100 years. An eruption of moderate volume might occur once every 500-1,000 years, and a significantly larger eruption no more than about once every 2,000-3,000 years. A few even larger, cataclysmic eruptions have occurred in the western United States, during the last 2 million years, including one in Long Valley, California, about 700,000 years ago (Bailey and others, 1976). These very large eruptions deposited ash over much of the western United States. Such cataclysmic eruptions are very infrequent, however, and are difficult to plan for. Tephra-hazard zones for locally unprecedented eruptions hare briefly discussed in the section titled Tephra-Hazard Zones.
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People living near volcanoes may detect premonitory events before and eruption. Both the frequency of occurrence and intensity of felt earthquakes commonly increase before eruptions begin. Eruptions may also be preceded by noticeable steaming or fumarolic activity and perhaps by new or enlarged areas of hot ground. However, most precursory changes are subtle and the most effective means of monitoring are instrumental and include a variety of geophysical, geodetic, and geochemical techniques. Seismometers are used to detect and locate earthquakes associated with the rise of magma. Swelling of the ground surface can be detected by using precision instruments and techniques that measure minute changes in slope, distance, or elevation at the ground surface. Other techniques involve measurement of changes in heat flow at a volcano by repeated infrared surveys or by direct measurements of hot spring or fumarole temperatures. Changes in the composition or relative abundances of fumarolic gases may also precede eruptions and can be detected by frequent or continuous analysis of gases.
These and other types of monitoring may be useful in detecting warning signs of impending eruption (Unesco, 1972; Decker, 1973; Lipman and Mullineaux, 1981). However, the overall success of a monitoring system depends on detection an interpretation of precursory events in time to warn and evacuate people from threatened areas and to initiate other measures to mitigate the effects of the eruption. Although monitoring systems may be useful in indicating an increase in the probability of volcanic activity and its possible location, they typically do not indicate the kind or scale of an expected eruption, particularly the first magmatic event, or the surrounding areas that might be affected. Precursors to volcanic activity may continue for weeks, months, or even years before eruptive activity begins, or they can subside at any time and not be followed by an eruption. Thus, monitoring of volcanic precursors may provide a general warning that volcanic activity in a specific area is becoming more likely, but it often does not pinpoint the nature or timing of an eruption or even its certainty.
Once such preparations are completed, they should be modified as new information becomes available and land-use patterns change. Plans to deal with future eruptions should be developed on a cooperative basis by local, State, and Federal officials and agencies, and the duties and responsibilities of each group should be decided in advance of an emergency. Land-use and possible evacuation decisions during an emergency will be made by State and local officials, and persons or agencies responsible for making such critical decisions should be identified before an emergency begins. Once made, preparations like those described above may not be needed or utilized for years or even decades, but they could conceivable be needed in the near future. To be most effective, preparations must be made before the next eruption occurs.