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Two moderate earthquakes occurred in the vicinity of Mammoth Lakes, California on the evening of 6 January 1983 (approximately 17:38 and 19:24 PST). These events, both of magnitude 5 – 5½ (USGS) occurred in the same general area as the series of magnitude 6 events in May, 1980 and the magnitude 5½+ event in September 1981. The maximum accelerations from the most recent earthquakes are less than those of the 1980 earthquakes. The 1980 strong-motion data from most of the same stations discussed in this report are given in Turpen (1980), and the 1981 strong-motion data are given in McJunkin and Kaliakin (1981). Discussions of the Mammoth Lakes area geology, recent volcanism and crustal movement are included in the special report published after the 1980 earthquakes (Sherburne, 1980).
At the time of this writing, the USGS estimate of the epicenter of the first earthquake is 37.63N, 118.93W (3 km depth), and 37.63N 118.94W (4.5 km depth) for the second earthquake (R. Cockerham, USGS, personal communication). Numerous smaller earthquakes occurred during the days following these earthquakes.
The two main earthquakes were felt as far away as Reno, Nevada (~160 km to north) and Stockton, California (~190 km to west). However, except for shelf stock in stores and power outages, the earthquakes did little local damage. The greatest reported damage (Sacramento Bee - January 9, 1983) was a hangar that collapsed on a private plane at Mammoth Lakes airport, approximately 2½ km northwest of the CDMG Convict Creek Station.
Fourteen CDMG strong-motion stations are located within 60 km of the epicenter area. These stations and their coordinates, site geology, housing structures, and instrument location(s) are listed in Table 1. Four of the stations are instrumented with multi-channel central-recording accelerograph systems. Data from three of these stations (Mammoth Lakes- High School Gym, Long Valley Dam, Bishop-Office Building) are presented in this report. Schematics indicating the locations of the accelerometers in these three structures are shown in Figures 2, 3, and 4 respectively. The fourth structure, Vermilion Dam, is inaccessible (as are the ground stations Vermilion Dam-Downstream and Mono Lake) until after the spring snowmelt.