5.3 Implications for stress drop scaling with depth and magnitude
We find that including reasonable corrections for depth dependent attenuation and rupture velocity can remove all need for any systematic dependence of stress drop on source depth in the upper 15 km at Parkfield. This is consistent with the previous results of AS2007. It is also in agreement with a recent meta-analysis (Abercrombie et al., 2020) that showed that previously reported increases in stress drop with depth could potentially be artefacts of inadequate correction for depth dependent path effects.
Less intuitively, improving corrections for depth variation in attenuation and velocity also decreased the resulting magnitude dependence of the stress drop results. The magnitude dependence is weak to the point of negligible above M1.75, but at lower magnitudes, some increase in stress drop with magnitude remains, regardless of correction strategy (see Figures 3, S6 and Table 1). We also observe increased variability at smaller magnitudes (Figure 8), probably largely reflecting increasing uncertainties. Based on previous analysis of the effects of frequency bandwidth limitations (e.g., Abercrombie, 2021) and our own analysis discussed above, we interpret this is a resolution effect, rather than a real physical effect.