A suite of 111 strong-motion accelerograms for 14 aftershocks of the Oroville, California, earthquake (ML = 5.7, 1 August 1975) that range in local magnitude (ML) from 2.8 to 5.2 has been analyzed to obtain estimates of seismic moment (Mo), source radius (ro), and stress drop (Δσ) in addition to the focal parameters of location, depth, and fault-plane solution. This data set, which is unusually complete for near-source (Δ ≲ 20 km) on-scale readings, allows for greater precision in the calculation of various measures of stress difference as represented by the Brune stress drop, the apparent stress, the arms stress drop, and the dynamic stress drop. In addition, the seismicity following each aftershock and state-of-stress seem to correlate with particular estimates of stress drop.
Seismic moments were calculated from the asymptotic long-period spectral levels which were corrected for the radiation pattern of a double-couple point source. They range from 1.4 × 1021 dyne-cm for a ML = 2.8 shock to 3.3 × 1023 dyne-cm for a ML = 5.1 event. A least-squares fit between ML and the logarithm Mo yields
These relationships are qualitatively in agreement with the response of the Wood-Anderson instrument to a Brune pulse. Stress drops from the Brune formulation range about 14 to 170 bars. Stress drop is correlated with depth in that the deepest events have the largest stress drops and no large stress drops occur at the shallow depths. Apparent stresses are smaller than the Brune stress drops and show a weaker depth dependence over the depth interval for which they are available. The stress drop calculated from the rms of acceleration (arms) was approximately constant at about 90 bars for 5 of the 7 larger events analyzed; the two high values of 160 and 190 bars were obtained only for the two events which had marked aftershock sequences of their own. These results may be interpreted in terms of the state-of-stress, simple fracture criteria, and mechanisms for the generation of aftershocks. The increase with depth of the envelope of the Brune stress drops may be caused by an increase in shear stress from overburden pressure. Smaller stress drop events can occur at any depth interval. The causal mechanism of aftershocks is not known, but probably includes a change in the frictional properties of the fault, suggesting that the arms stress drop is a measure of the frictional or dynamic stress release.