Abstract

The behavior of earthquake source scaling has been the topic of significant debate in the earthquake source community over the past two decades, and validating recent results has been difficult. In this study, we focus on high quality records from the 21 February 2008 Mw 5.9 Wells, Nevada, earthquake, and its aftershocks, which provide an unprecedented opportunity to take an in‐depth look at the source scaling and ground motions. For this earthquake sequence, conflicting scaling relations were reported in two previous studies (Baltay et al., 2010; Mayeda and Malagnini, 2010). In addition, recent comparisons of the ground‐motion prediction equations (GMPEs) with this data set have shown significant overestimation of ground motions for the aftershocks, while being in rough agreement with the mainshock ground motions (Petersen et al., 2011). In order to evaluate the reported scaling relationships and better understand the observed discrepancy in the GMPE’s, we chose to revisit the Wells, Nevada, earthquake sequence. We investigate the source parameters of the earthquakes using the S‐wave and coda spectral ratio methods (Mayeda et al., 2007) and find that the stress drops of the aftershocks are 2–5 times lower than that of the mainshock. We compute pseudospectral acceleration (PSA) ratios using direct S waves from broadband records and compare with theoretical source ratios assuming the self‐similar and non‐self‐similar source scaling assumptions as well as ratios derived from state‐of‐the‐art GMPE estimates. We find that we can only simultaneously match source ratios between the mainshock and selected aftershocks if we use non‐self‐similar scaling. Accounting for the significant differences in the stress drop between small and large earthquakes will help to enhance the prediction capability of ground motions for this region. By validating the source scaling with PSA ratios, these results can be used as constraints in stress parameterization used in the GMPE’s.

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