Magnitude, Depth, and Methodological Variations of Spectral Stress Drop Within the SCEC/USGS Community Stress Drop Validation Study Using the 2019 Ridgecrest Earthquake Sequence

ABSTRACT We present the first ensemble analysis of the 56 different sets of results submitted to the ongoing Community Stress Drop Validation Study using the 2019 Ridgecrest, California, earthquake sequence. Different assumptions and methods result in different estimation of the source contribution to recorded seismograms, and hence to the source parameters (principally corner frequency, fc, spectral stress drop, Δσ, and seismic moment, M0) obtained from modeling calculated source spectra. For earthquakes smaller than magnitude (M) 2.5 there is negligible correlation between the fc values obtained by different studies, implying that no present method is reliable using available data. For larger magnitude events, correlation between fc measurements of different studies, within even a small M range is always higher than spectral Δσ, because the fc measurements simply reflect the underlying physical decrease in fc with increasing M. We model the observed trends of submitted fc with both magnitude and depth. Most methods report an increase in spectral Δσ with M, although a magnitude-invariant spectral Δσ is within the confidence limits. The depth dependence is smaller and depends on whether a study allows attenuation to vary with source depth; a combination of depth-dependent attenuation correction, and depth-dependent shear-wave velocity can compensate for reported depth trends. We model the submitted values to remove differing M and depth variation to investigate the relative interevent variability. We find consistent relative variation between individual events, and also lower relative spectral Δσ in the northwest of the aftershock sequence, and higher on the cross fault and in the region of main fault intersection. This large-scale comparison implies that absolute spectral Δσ estimates are dependent on the methods used; studies of different regions or using different methods should not be directly compared and improved constraints on path and site corrections are needed to resolve these absolute spectral Δσ differences.