Does Earthquake Stress Drop Increase With Depth in the Crust?

Abstract

The stress release, or stress drop, during earthquake rupture has long been thought to be directly related to the magnitude of the ambient stress (e.g.,

Spatiotemporal Variability of Earthquake Source Parameters at Parkfield, California, and Their Relationship With the 2004 M6 Earthquake

“…This is consistent with the previous results of AS2007. It is also in agreement with a recent multi-region analysis (Abercrombie et al, 2021) that showed that previously reported increases in stress drop with depth could potentially be artifacts of inadequate correction for depth dependent path effects.…”

“…The Spectral Decomposition process assumes a simple attenuation structure in the study region in which PT(t) depends only on the travel time. Any spatial variation in attenuation, including a dependence on source depth is not included, and will be absorbed into the event terms and bias the resulting source spectra (e.g., Abercrombie et al, 2021;Shearer et al, 2019). In other words, ET(t) in Equation 1 will include the real source term and a function that includes both common effects at all events and all sites, and source region specific attenuation.…”

“…They also showed that the frequency range of the Southern California regional seismic network data is insufficient to resolve source scaling and absolute values of stress drop, and their variation, independently. This limitation is supported by the contrasting results of Trugman (2020), Bindi, Zaccarelli, and Kotha (2020), Bindi et al (2021), and Abercrombie et al (2021) for aftershocks of the 2019 Ridgecrest, California earthquake. Shearer and Abercrombie (2021) combined surface network with high-frequency deep borehole recordings to resolve the ambiguity near Cajon Pass.…”

“…However, independent studies of the same region have resolved different spatial and temporal distributions; for example, the spatial pattern in Japan from Oth (2013) appears different from that by Uchide et al (2014) and Yoshida et al (2017), who used more localized attenuation corrections. Some studies have also observed an increase in stress drop with depth that may reflect increasing stress on the faults, but such dependence may be, at least partially, an artifact of changes in attenuation and velocity with depth (Abercrombie et al, 2021;Allmann & Shearer, 2007;Sumy et al, 2017). Abercrombie (2014) analyzed three repeating clusters of small earthquakes on the San Andreas Fault at Parkfield, and found the temporal changes of only the best recorded, largest magnitude, cluster agreed with the results of Allmann and Shearer (2007) for the same earthquakes.…”

Resolving Differences in the Rupture Properties of M5 Earthquakes in California Using Bayesian Source Spectral Analysis

“…These two events occurred in the Salton Trough region in rapid succession on 26 August 2012, with the first one having a deeper hypocenter and higher stress drop than It is often difficult to disentangle the effects of source and path when considering depth trends in source properties. Abercrombie et al (2021) found that spectral decomposition approaches that do not explicitly account for hypocentral depth in their empirical path corrections can bias the observations toward an apparent depth trend in frequency content, since sources at greater depths will have seismic raypaths that preferentially sample deeper crustal rocks that are less attenuating. Our EGF selection process takes depth proximity into account and thus should mitigate potential artifacts of this form, lending some confidence that the observed depth trend is a real one.…”