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Abstract We estimate stress drops for 148 shallow (<6 km) earthquakes in the complex 2008 Mogul, Nevada, swarm using empirical Green's function‐derived spectral ratios. Near‐source, temporary broadband seismometers deployed before the M w 4.9 main shock provide high‐quality records of many foreshocks and aftershocks, and an ideal opportunity to investigate uncertainties in corner frequency measurement as well as stress drop (Δ σ ) variation related to space, time, depth, mechanism, and magnitude. We explore uncertainties related to source model, measurement approach, cross‐correlation limit, and frequency bandwidth. P ( S ) wave Δ σ results range from 0.2 ± 0.15 (0.3 ± 0.15) to 36±20 (58±7) MPa, a variation greater than the error range of each individual estimate. Although this variation is not explained simply by any one parameter, spatiotemporal variation along the main shock fault plane is distinct: coherent clusters of high and low Δ σ earthquakes are seen, and high‐Δ σ foreshocks correlate with an area of reduced aftershock productivity. These observations are best explained by a difference in rheology along the fault plane. Average Δ σ s of 3.9±1.1 (4.0±1.1) MPa using P ( S ) are similar to those found for earthquakes in a variety of settings, implying that these shallow, potentially fluid‐driven earthquakes do not have systematically lower Δ σ than average tectonic earthquakes (~4 MPa) and, therefore, have similar (or higher, due to proximity to the surface) expected ground motions compared to typical earthquakes. The unprecedented detail achieved for these shallow, small‐magnitude earthquakes confirms that Δ σ , when measured precisely, is a valuable observation of physically meaningful fault zone properties and earthquake behavior.
Ruhl et al. (Sun,) studied this question.