From active subduction zone to reactivated near surface faults: using stress drop estimates to infer the relative influence of geological conditions and observational constraints

Earthquake stress drop estimates commonly vary by several orders of magnitude, particularly for small earthquakes (~M < 3). Stress drop values have been found to correlate with faulting style, faulting type (intraplate, interplate), depth, and to exhibit differences between natural and induced earthquakes. Nevertheless, consistent trends across data sets are difficult to establish, partly due to the variation in methodological approaches. However, while data coverage and observational limitations hinder obtaining the robust source spectral estimates necessary for calculating stress drop, some of the differences in stress drop values almost certainly reflect varying fault strength and geological conditions. We present our stress drop estimates from data sets representing a wide range of fault loading conditions and geological environments, from interplate and intraslab events in a subduction zone, to volcanic, intraplate, and human induced events. Stress-drop values range primarily between 1 100 MPa for events that meet the criteria for spectral-ratio analysis. We present correlations of low relative stress drop values in areas of high seismic attenuation indicative of lower rock strength, suggesting stress drop estimates may provide clues about faulting conditions during rupture. We also show one notable subset of induced events near active injection wells that exhibit stress drop values of ~0.1 MPa and have distinctive low-frequency content. Their spatial distribution, waveform, and source spectral characteristics suggest either slower rupture, lower stress drop values, or a combination of both, and may represent part of the transition between aseismic and seismic slip. We show using a Large-n array that while stress drop values are roughly constant (within 2 orders of magnitude), estimates can vary by roughly 25% when station coverage is limited to 15 stations or less with a maximum azimuthal gap of 90°. Our findings highlight the importance of the community-led experiment now being organized for the Ridgecrest sequence to explore relative influence of fault strength and methodological approaches in stress drop variation. As part of this community-led effort, we will present preliminary source parameter estimates of the sequence using spectral analysis and stopping phases.