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by R.D. McJunkin and N.A. Kaliakin
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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 in mountainous regions
of the earthquake area during the first few days after the main event.
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