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"spasmodic tremors," a seismic phenomena commonly associated with active volcanoes and considered to be the result of rock fracture caused by the pressure of intruding magma or of gas released from it. These revelations prompted considerable concern among Geological Survey volcanologists present, particularly those from the Cascades Volcano Observatory familiar with the activity at Mount St. Helens. That concern was dramatically heightened on May 7, the last day of the workshop, when departing scientists were awakened by a swarm of moderate earthquakes. A telephone call from the University of Nevada Seismological Laboratory indicated that the quakes were located at the persistently active epicentral area just east of Mammoth Lakes and were now at a depth of only 2 miles! A second swarm of earthquakes, including one of magnitude 4.2, later that evening precipitated a meeting of Geological Survey volcanologists and University of Nevada seismologists at the U.S. Forest Service's Visitor Center where careful reviews of seismograms and prolonged discussions led to concurrence that a small offshoot, or
tongue of magma, was intruding fractures beneath the southern edge of the caldera and that it could potentially reach the surface and produce an eruption within the foreseeable future. This conclusion was reinforced by the knowledge that the earthquakes in the Mammoth Lakes area tended to be seasonal; earthquakes are relatively infrequent during the winter months and increased to a maximum during the summers. That the May 7-8 swarm was the first of the 1982 summer season and one of the strongest in the previous 2-year interval lent some urgency to beginning additional monitoring activities, as well as to the need to provide some form of warning of the potential hazard to the local inhabitants.
During the following 2 weeks, a succession of meetings and review sessions were held at Geological Survey centers in Menlo Park, California, Vancouver, Washington, Denver, Colorado, and Reston, Virginia, during which time a formal notice of potential volcanic hazard, the lowest level of warning issued by the Geological Survey, was prepared and finally released on May 27, 1982.
Monitoring Activities Expanded
Even before release of the volcanic hazards notice, the Geological Survey began intensifying its monitoring activities in the Long Valley-Mammoth Lakes area. The seismic network deployed by the University of Nevada Seismological Laboratory, with funding from the Geological Survey, was reconfigured to better determine the location and depth of earthquakes in the Mammoth Lakes epicentral area, and, over the following several weeks, additional seismic stations were added to the network. Arrangements were also made at the Geological Survey's Menlo Park Center to automate seismic computations so that analysis of the data from all the stations could be done in real time (that is, at the actual instant the data were being measured).
Within days after the shallow May 7-8 earthquake swarm, a crew from the Cascades Volcano Observatory established four dry-tilt stations on the flanks of the Long Valley resurgent dome and a fifth station near the Mammoth Lakes epicentral area to supplement the leveling and geodimeter trilateration surveys that were to be undertaken in June. Later in July, four additional tilt stations were added in the vicinity of the epicentral area by the University of California, Santa Barbara. In addition,
hydrogen gas sensors were immediately installed, one on the resurgent dome and another on the southern edge of the caldera, to detect changes in hydrogen emissions that might foretell of further rise of the magma. Data from these continuously recording sensors are radiotelemetered to the Forest Service's Mammoth Lakes Visitor Center and thence relayed via GOES satellite to the National Oceanic and Atmospheric Administration National Environmental Satellite Service's Wallops Island, Virginia, receiving station, where it can be retrieved by telephone from any Geological Survey center in the country. In the near future, data from continuously recording borehole tiltmeters will be similarly transmitted via satellite to Geological Survey centers.
By early June, a surveying crew from the Geological Survey's National Mapping Division was in the field beginning the long painstaking task of remeasuring the 60 miles of level lines established in 1975 and adding 40 miles of new lines across the resurgent dome and elsewhere within the caldera to better define the pattern of uplift since 1980. Before the end of the summer, they were joined by a Los Angeles Department of Water and Power surveying crew, which greatly speeded the work. At the same time, the Geological Survey's Water Resources Division Subsidence Research Group began
reoccupying the 1975 trilateration (geodimeter) network previously occupied in 1978 and adding additional stations to provide better coverage of horizontal changes accompanying uplift of the resurgent dome. In late August, they returned to reoccupy the network a second time, aided by helicopter support provided by Los Angeles Department of Water and Power.
During the course of the summer, previously established Geological Survey gravity and magnetic surveys were expanded within the caldera and elsewhere in the region to help substantiate measured elevation changes and to detect other possible physical changes accompanying the rise of the magma.
In addition to these Geological Survey monitoring activities, many other organizations, including the California Division of Mines and Geology, and many university teams joined in the monitoring and research effort to better understand the causes and possible consequences of the rising and quaking landscape.
to emerge. By late August, it was clear that uplift of the resurgent dome was continuing but at a slower rate than in the previous 2 years. Lateral spreading of the dome, however, seemed to be going on at about the same or even somewhat faster rate. At the same time, the uplift, which initially extended well beyond the margins of the caldera, was now confined within the caldera. Most puzzling was a sudden decline in frequency of earthquakes at the Mammoth Lakes epicentral area and the complete cessation of spasmodic tremor. However, small earthquakes continue in a diffuse zone peripheral to the resurgent dome along what is most likely the caldera ring fracture.
Has the tongue of magma beneath the epicentral area ceased rising toward the surface, or is it only taking a "breather"? Is the decline in seismicity a comforting sign, or is it just "the calm before a storm"? Is the magma chamber responding to regional tectonic forces, or are independent chemical and physical processes going on within causing it to rise and generate earthquakes? All are critical questions that will be answered by continued monitoring. However, it is a very slow and painstaking process. The hope is that the results obtained will provide enough insight to give adequate warnings, if more serious activity should develop.'
Meanwhile, the citizens and public officials of Mammoth Lakes and neighboring communities are taking no chances. Spurred by the admonition to "Prepare for the worst, and hope for the best" voiced at a volcanic hazards workshop, which was held in Mammoth in late August and was jointly organized by the California Office of Emergency Services, the California Division of Mines and Geology, and the U.S. Geological Survey, they are going about business as usual in the midst of their winter ski season.