abstract

Anomalous temporal variations in the southern part of the Sierra Nevada-Great Basin boundary zone (SNGBZ) from 1977 to 1980 followed a pattern that several authors have identified as precursory to strong earthquakes. From January 1977 to September 1978, a general decrease in seismicity was observed in the southern part of the zone, followed by a burst of moderate earthquakes from Doyle in the northern part of the SNGBZ to Bishop in the south. The 1977 to 1979 variations are particularly noteworthy because they occurred over the entire SNGBZ, indicating a regional, rather than local cause. In the southern part of the zone, a magnitude 5.7 shock 25 km northwest of Bishop on 4 October 1978 was followed by an earthquake swarm in the Mammoth Lakes area, with peaks in activity during late 1979 and the spring of 1980. The Mammoth Lakes activity spread gradually to the west during the first year of this sequence, apparently involving several northerly-trending fracture zones in an area of about 900 km2. Bursts of activity from December 1979 to April 1980 had increasingly dense spatial clustering. Temporal clustering showed monthly peaks in the spring of 1980, and intense swarming occurred in mid-May, about 10 days before the largest events in the sequence.

A search for changes in P-wave velocity changes within the SNGBZ, based on recordings of Nevada Test Site explosions at pairs of stations, has so far produced negative results.

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