Stress Drops Without Fitting Corner Frequencies: A Graph Inversion of Spectral Ratio Asymptotes for the Ridgecrest Earthquake Sequence

Trey C. Knudson, Bruce E. Shaw, William L. Ellsworth, & Gregory C. Beroza

In Preparation October 28, 2026, SCEC Contribution #15698

We analyze spectral stress drops for 2192 M2.5–4.0 earthquakes of the Ridgecrest, California sequence using the low- and high-frequency asymptotes of spectral ratios, which measure the relative moment and, for a Brune source, the relative slip of a pair. Because no corner frequency is fit (corners enter only to place the asymptote bands and set the absolute scale), pairs need no magnitude separation, P and S phases enter as independent measurements of the same asymptotes, and errors do not depend on the cube of corner frequency. We link spectral ratios across a graph of thousands of events, which stabilizes the inversion and allows the empirical Green’s function (EGF) assumption to be tested against pair separation. Relative stress drops scatter by 0.32 log10 units while measurement uncertainty and finite-bandwidth bias are small corrections to this. Stress-drop scaling with moment (+0.060 per log10(M0)) is one third of the community validation study’s median over the same magnitudes and is entirely explained by measurement in finite frequency bands. Stress-drop dependence on depth (+0.036 log10 units/km), 70% of the community median, depends on pair separation: near zero for pairs closer than ~500 m and plateauing beyond ~800 m. We interpret the depth trend in sparse regions as a path artifact from failure of the EGF assumption, and the densely sampled low-stress-drop northwest segment as the only large-scale feature attributable to source. Absolute calibration is a constant shift that leaves the population’s shape unchanged, and with independent moments, the median stress drop is 5.7 MPa.

Citation
Knudson, T. C., Shaw, B. E., Ellsworth, W. L., & Beroza, G. C. (2026). Stress Drops Without Fitting Corner Frequencies: A Graph Inversion of Spectral Ratio Asymptotes for the Ridgecrest Earthquake Sequence. Journal of Geophysical Research: Solid Earth, (in preparation).