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Bulletin of the Seismological Society of America; June 1979; v. 69; no. 3; p. 751-756
© 1979 Seismological Society of America
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Strain accumulation from 1964 to 1977 near the epicentral zone of the 1976-1977 earthquake swarm southeast of Palmdale, California

M. LISOWSKI and J. C. SAVAGE

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

Strain accumulation in a 5 by 7 km 10-station network that spans the San Andreas fault 20 km southeast of Palmdale, California has been calculated by comparing a 1964 triangulation survey with a 1977 trilateration survey. The network is located within the epicentral area of the anomalous 1976-1977 earthquake activity reported by McNally and Kanamori (1977). The comparison indicates shear strain accumulation of {gamma}1 = 10.8 ± 1.8 ppm and {gamma}2 = 5.1 ± 1.8 ppm for the 13-year period, where {gamma}1 = {varepsilon}11 - {varepsilon}22 and {gamma}2 = 2{varepsilon}12 and {varepsilon}ij are the tensor strain components in a coordinate system with the 1-axis directed to the east and the 2-axis to the north. A simple dislocation model of the 1971 San Fernando earthquake predicts coseismic shear strains {gamma}1 = 2.8 ppm and {gamma}2 = 1.7 ppm in the area of the network. The 1964-1977 shear strains corrected for the San Fernando earthquake and translated into yearly rates are Formula1 = 0.60 ± 0.14 ppm/yr and Formula2 = 0.26 ± 0.14 ppm/yr with the direction of maximum extension N75°E ± 6°. Shear strain rates Formula1 = 0.38 ± 0.06 ppm/yr and Formula2° = 0.20 ± 0.06 ppm/yr with the direction of maximum extension N80°E ± 7° in the period August 1971 to July 1977 have been reported for the Palmdale network located 30 km northeast along the San Andreas fault. Corrected for the effects of the 1971 San Fernando earthquake, the 1964-1977 shear strain rates for the network near the 1976-1977 earthquake swarm are not significantly higher than and agree in orientation with those for the Palmdale network in the 1971-1977 period. Thus no strain event of magnitude greater than a few parts per million has been associated with the anomalous earthquake activity.




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M. LISOWSKI and W. H. PRESCOTT
Short-range distance measurements along the San Andreas fault system in central California, 1975 to 1979
Bulletin of the Seismological Society of America, October 1, 1981; 71(5): 1607 - 1624.
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