Strain accumulation near the epicenters of the 1978 Bishop and 1980 Mammoth Lakes, California, earthquakes

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Abstract
The Bishop, California, earthquake (ML = 5.8) of 4 October 1978 occurred virtually beneath one of the stations in a trilateration network used by the U.S. Geological Survey to monitor strain accumulation in northern Owens Valley. Subsequently (late May 1980), three earthquakes (ML = 6.0, 6.3, and 6.0), the Mammoth Lakes sequence, occurred on the edge of the trilateration network 10 to 20 km from the Bishop earthquake epicenter. The deformation of the network observed in the epoch 1976 to mid-1979 was not significantly different from that observed in the epoch 1973 to 1976. Thus, the Bishop earthquake itself did not produce a measurable deformation, nor was anomalous deformation detected prior to the Mammoth Lakes earthquakes. The deformation of the trilateration network between July 1979 and September 1980 indicates significant extension (50 mm in 25 km) in the direction N65°E, essentially perpendicular to the alignment of the epicenters of the Mammoth Lakes earthquakes. This deformation suggests normal faulting, whereas the preliminary nodal-plane solutions seem to require left-lateral slip. Secular strain accumulation was detected in northern Owens Valley (latitude 37°15′ to 37°45′) in the 1972 to 1979 interval. The principal strain rates with standard deviations were 0.08 ± 0.02 μstrain/a N83°E ± 15° and 0.02 ± 0.02 μstrain/a N07°W ± 15°, extension reckoned positive. The observed strain accumulation is consistent with normal faulting along the Sierra escarpment in Owens Valley north of Bishop. This accumulation appears to contrast with southern Owens Valley where strain accumulation is primarily right-lateral shear acorss the axis of the valley.