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Earthquake stress drop is an important source parameter that directly links to strong ground motion and fundamental questions in earthquake physics. Stress drop estimations may contain significant uncertainties due to such factors as variations in material properties and data limitations, which limit the applications of stress drop interpretations. Using a high‐resolution borehole network, we estimate stress drop for 4551 (M0‐4) earthquakes on the San Andreas Fault at Parkfield, California, between 2001 and 2016 using spectral decomposition and an improved stacking method. To evaluate the influence of spatiotemporal variations of material properties on stress drop estimations, we apply different strategies to account for spatial variations of velocity and attenuation changes, and divide earthquakes into three separate time periods to correct temporal variations of attenuation. These results show that appropriate corrections can significantly reduce the scatter in stress drop estimates, and decrease apparent depth and magnitude dependence. We find that insufficient bandwidth can cause systematic underestimation of stress drop estimates and increased scatter. The stress drop measurements from the high‐frequency borehole recordings exhibit complex stable spatial patterns with no clear correlation with the nature of fault slip, or the slip distribution of the 2004 M6 earthquake. Temporal variations are significantly smaller, less well resolved and varying spatially. They do not affect the long‐term stress drop spatial variations, suggesting local material properties may control the spatial heterogeneity of stress drop.

We calculate rupture directivity and velocity for earthquakes in three well‐recorded repeating sequences (2001–2016) on the San Andreas Fault at Parkfield usingPwaves from borehole recordings and the empirical Green's function method. The individual events in each sequence all show the same directivity; the largest magnitude sequence (M ~ 2.7, 8 events) ruptures unilaterally NW (at ~0.8Vs), the second sequence (M ~ 2.3, 9 events) ruptures unilaterally SE, and the smallest magnitude sequence (M ~ 2, 11 events) is less well resolved. The highly repetitive rupture suggests that geometry or material properties might control nucleation of small locked patches. The source spectra of theM ~ 2.7 sequence exhibit no detectable temporal variation. The smallerMsequences both exhibit a decrease in high‐frequency energy following theM6 earthquake that recovers with time. This could indicate a decrease in stress drop, an increase in attenuation, or a combination of the two, followed by gradual healing.

We combine earthquake spectra from multiple studies to investigate whether the increase in stress drop with depth often observed in the crust is real, or an artifact of decreasing attenuation (increasingQ) with depth. In many studies, empirical path and attenuation corrections are assumed to be independent of the earthquake source depth. We test this assumption by investigating whether a realistic increase inQwith depth (as is widely observed) could remove some of the observed apparent increase in stress drop with depth. We combine event spectra, previously obtained using spectral decomposition methods, for over 50,000 earthquakes (M0 to M5) from 12 studies in California, Nevada, Kansas and Oklahoma. We find that the relative high‐frequency content of the spectra systematically increases with increasing earthquake depth, at all magnitudes. By analyzing spectral ratios between large and small events as a function of source depth, we explore the relative importance of source and attenuation contributions to this observed depth dependence. Without any correction for depth‐dependent attenuation, we find a systematic increase in stress drop, rupture velocity, or both, with depth, as previously observed. When we add an empirical, depth‐dependent attenuation correction, the depth dependence of stress drop systematically decreases, often becoming negligible. The largest corrections are observed in regions with the largest seismic velocity increase with depth. We conclude that source parameter analyses, whether in the frequency or time domains, should not assume path terms are independent of source depth, and should more explicitly consider the effects of depth‐dependent attenuation.