Project Abstract
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We continue our efforts to understand the earthquake source spectra and subsequently improve our ability to predict ground motion for future earthquakes. First, when computing broadband ground motion using the UCSB method, we explicitly specify a source spectrum for its moment rate function. During our recent modification, we assume the target source spectra is JA19_2S, inferred from our study of the NGA West-2 dataset (Ji and Archuleta, 2020). After testing it with BBP events, we identify two modifications for the scaling relations. Second, the “stress parameter” (hereinafter referred to it as ∆σ) (Boore, 1983) is used during stochastic ground motion simulations but the physical meaning of ∆σ has not been clearly defined. We conducted a review of a collection of theoretical source spectral models (Ji et al., 2022). We find despite the well-known variation in predicting static stress drop {\Delta\sigma}_s\ from the seismic moment and corner frequency, all models, especially three conventional models, suggest that earthquakes radiate about half of the available strain energy into the surrounding medium. This suggests not only a robust method to estimate stress drop using source spectra but also a physical interpretation to ∆σ. We infer that the constancy of the “stress parameter” (∆σ) found in engineering seismology (Boore, 1983; Atkinson and Beresnev, 1998) is similar to having constant apparent stress, \sigma_a (e.g., Ide and Beroza, 2001). |
