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Intense seismicity in the Long Valley caldera area of California last summer may have been hydrothermally driven and triggered by escalating magmatic unrest in the resurgent dome area, results from the MAMMOTH 97 Seismic Experiment suggest. The experiment involved enhanced seismic monitoring in an area that is already closely watched for earthquake activity. Sophisticated instruments were placed in the caldera's south moat section, Mammoth Mountain, and the Casa Diablo geothermal area (Figure 1). In early July, just as the enhanced network became fully operational, there was a major increase in seismic activity.

Preliminary analysis has yielded new insights into the structure and tectonics of the south moat and suggests the activity occurred above a vertical, east-west trending, dike-like structure beneath the moat. A series of vigorous swarms was accompanied by accelerated inflation of the resurgent dome (Figure 2). The permanent network in the area, using single-component analog instruments, recorded more than 1,600 earthquakes of magnitude up to 3.5 during July and August, but the MAMMOTH network, with three-component digital instruments, located about six times as many (Figure 3). About 63 “long period” (LP) earthquakes west of Mammoth Mountain were also recorded (Figures 1 and 4).