Abstract
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.