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 (increasing Q) 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 in Q with 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.

We calculate high-precision absolute and relative earthquake relocations to investigate the relationship between seismicity and major active faults, and to explore variation in seismogenic depths across the Northern Walker Lane. We first compute datum-adjusted and station-residual-corrected absolute relocations, before relocating events using waveform cross-correlation. Of 40,581 routinely located earthquakes between 2002 and 2018, we relocate 27,132 (66.9%) with resulting median horizontal and vertical location uncertainties less than ~100 m. We then compute 95thpercentile depths as a proxy for seismogenic depth and compare to published Moho depths. Microseismicity occurs in large highly clustered source areas, often consisting of many short, distinct fault structures. Activity concentrates near the ends of mapped Quaternary faults rather than along them. Microseismicity-defined structures in transition zones between major surface faults may identify active fault networks that link faults at the depth. Seismogenic depth shallows away from the Sierra Nevada to the east-northeast over approximately 80 km, from an approximate depth of 17 km to 13 km. This follows, to scale, the decrease in Moho depth across the same region from about 35 km to 30 km. We compare seismogenic and Moho depths to topographic relief and heat flow measurements to discuss controls on the depth of seismicity in the region. Heat flow increases smoothly over the same region of the decreasing seismogenic and Moho depth, increasing by as much as 20 mW/m2.