Stable Stress-Drop Measurements and their Variability: Implications for Ground-Motion Prediction

We estimate the a(rms)-stress drop Delta sigma-a(rms), (Hanks, 1979) using acceleration time records of 59 earthquakes from two earthquake sequences in eastern Honshu, Japan. These acceleration-based static stress drops compare well to stress drops calculated for the same events by Baltay et al. (2011) using an empirical Green's function (eGf) approach. This agreement supports the assumption that earthquake acceleration time histories in the bandwidth between the corner frequency and a maximum observed frequency can be considered white, Gaussian, noise. Although the Delta sigma-a(rms) is computationally simpler than the eGf-based f(c)-M-0-stress drop, and is used as the stress parameter to describe the earthquake source in ground-motion prediction equations, we find that it only compares well to the Delta sigma-eGf at source-station distances of similar to 20 km or less because there is no consideration of whole-path anelastic attenuation or scattering. In these circumstances, the correlation between the Delta sigma-eGf and Delta sigma-a(rms) is strong. Events with high and low stress drops obtained through the eGf method have similarly high and low Delta sigma-a(rms). We find that the inter-event standard deviation of stress drop, for the population of earthquakes considered, is similar for both methods, 0.40 for the Delta sigma-eGf method and 0.42 for the Delta sigma-a(rms), in log(10) units, provided we apply the similar to 20 km distance restriction to Delta sigma-a(rms). This indicates that the observed variability is inherent to the source, rather than attributable to uncertainties in stress-drop estimates.