A suite of 111 strong-motion accelerograms for 14 aftershocks of the Oroville, California, earthquake (M (sub L) = 5.7m 1 August 1975) that range in local magnitude (M (sub L) ) from 2.8 to 5.2 has been analyzed to obtain estimates of seismic moment (M (sub O) ), source radius (r (sub O) ), and stress drop ( sigma ) in addition to the focal parameters of location, depth, and fault-plane solution. Various measures of stress difference as represented by the Brune stress drop, the apparent stress, the a (sub rms) stress drop, and the dynamic 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 1.4X10 (super 21) dyne-cm for a M (sub L) = 2.8 shock to 3.3X10 (super 23) dyne-cm for a M (sub L) +(16.8+ or -1.1) for M (sub L) 〉 or =4.3 and log M (sub O) = (1.1+ or -0.14)M (sub L) +(18+ or -0.51) for M (sub L) 〈 or =4.1. Stress drops from the Brune formulation range about 14 to 170 bars. Stress 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. The stress drop calculated from the rms of acceleration (a (sub rms) ) 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 probably includes a change in the frictional properties of the fault, suggesting that the a (sub rms) stress drop is a measure of the frictional or dynamic stress release.--Modified journal abstract.