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Introduction
A moderate-magnitude earthquake [ML=5.8, California Institute of Technology (CIT), Seismological Laboratory] occurred at 0453 Pacific Day1ight Time (PDT) on 30 September 1981 approximately 10.5 km southeast of Mammoth Lakes, Mono County, California (figure 1). The earthquake epicenter is located by the California Division of Mines and Geology (CDMG) at 37.585°N and 118.886°W in the vicinity of Laurel Mountain at a focal depth of 6.2 km. Numerous aftershocks were generated after the main event. As of 1800 PDT on 1 October 1981, thirty-four ML=3.0-3.9 and five ML=4.0-4.9 aftershocks had occurred (Smith, 1981). The largest aftershock in this earthquake series occurred at 0606 PDT on 30 September and is located by CDMG to be at 37.642°N and 11 8.872°W (figure 1). Its magnitude is calculated by several agencies and varies from ML=4.5 (Office of Emergency Services- Sacramento) to ML=5.5 (CIT, National Earthquake Center- Golden). CDMG seismologists estimate the event at ML~5.2 based on magnitudes calculated by CIT, University of California, Berkeley, and University of Nevada, Reno.
The main shock of 30 September 1981 is in an area of the east-central Sierra Nevada that has experienced numerous moderate-magnitude events since occurrence of the 4 November 1978 Bishop earthquake (near Crowley Lake). This region was most active during the ML>6.0 (CIT) Mammoth Lakes earthquakes of 25 and 27 May 1980 (see McJunkin and Bedrossian, 1980; Sherburne, 1980; Turpen, 1980; Clark and Young, 1981; Cramer and Toppozada, 1981).
Damage from this recent episode of earthquake activity was minor. No structural damage was reported in Mammoth Lakes, the closest community to the main event. The most notable damage was non-structural and occurred in the Hot Creek area approximately 10 km east of Mammoth Lakes. In this location, much of the shelf stock at the Mammoth Lakes School District bus barn was toppled onto parked buses in the structure and the building floor. At the adjacent State Fish Hatchery, many fish died from suffocation when filters of the holding tanks were clogged by high concentrations of silt that was added to tributaries from discharges of local springs; additionally, concrete channels at this facility were cracked in several places from seismic shaking. As a precautionary measure, the U.S. Forest Services closed access to Hot Creek and Convict Canyon until earthquake activity subsided. The Hot Creek area was closed because of surges and unpredictable behavior in geothermal emissions; access into Convict Canyon was halted because of rock slide danger from steep canyon walls which are underlain by loose rock and slope debris. Numerous rock slides did in fact occur
