Stable stress‐drop measurements and their variability: Implications for ground‐motion prediction

We estimate the arms‐stress drop, Graphic, (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 Graphic is computationally simpler than the eGf‐based Graphic‐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 Graphic at source‐station distances of ∼20  km or less because there is no consideration of whole‐path anelastic attenuation or scattering. In these circumstances, the correlation between the Graphic and Graphic is strong. Events with high and low stress drops obtained through the eGf method have similarly high and low Graphic. 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 Graphic method and 0.42 for the Graphic, in log10 units, provided we apply the ∼20  km distance restriction to Graphic. This indicates that the observed variability is inherent to the source, rather than attributable to uncertainties in stress‐drop estimates