Seismic estimates of stress release

Five seismic estimates of stress release are compared for eight multiply recorded aftershocks of the 1975 Oroville, California, earthquake. The dynamic stress drop, the a rms stress drop, and the apparent stress provide stable estimates which are strongly correlated across the data set. The uncertainties of these estimates (the standard error of the mean divided by the mean) are approximately 15%. Estimates of the Brune stress drop are uncorrelated with estimates of the average static stress drop calculated from the seismic moments and the rupture geometries of the aftershocks, determined from the signal durations of the recorded S waves. The Brune stress drops are most strongly correlated with the a rms stress drops, implying that the Brune stress drop provides an estimate of the average dynamic stress drop, as Brune originally proposed.