Abstract

Three series of aftershocks occurred in the region near Hollister, less than 15 km from the San Andreas Geophysical Observatory in 1968, 1970, and 1971 following shocks of magnitudes 2.3, 3.2, and 4.0, respectively. The high-gain system operating at SAO permitted a full study of these series for events as small as magnitude -0.7. The distribution of time intervals between consecutive shocks and magnitudes show certain definite differences in the time, space, and magnitude structure of the three sequences. The 1968 and 1970 series are highly concentrated in time and originate from quite small source volumes. Both correspond to the mainshock-after-shock type with high values of b (1.1 and 0.91). The 1971 series located in the Bear Valley area about 15 km south of the other two belongs to the swarm type with a low value of b (0.41). The situation suggests different physical conditions in two neighboring areas of the same fault with less homogeneous material and higher stresses acting at the area of the 1971 series. Variations in the b value with time for an area of 2500 km2 around SAO can be interpreted as due to migration of seismic sources from one area to another, or definite changes in rock properties with time.

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