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 γ1 = 10.8 ± 1.8 ppm and γ2 = 5.1 ± 1.8 ppm for the 13-year period, where γ1 = ε11 - ε22 and γ2 = 2ε12 and εij 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 γ1 = 2.8 ppm and γ2 = 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 γ˙1 = 0.60 ± 0.14 ppm/yr and γ˙2 = 0.26 ± 0.14 ppm/yr with the direction of maximum extension N75°E ± 6°. Shear strain rates γ˙1 = 0.38 ± 0.06 ppm/yr and γ˙2° = 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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