SCEC Award Number 21115 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Name Organization
Ralph Archuleta University of California, Santa Barbara Chen Ji University of California, Santa Barbara
Other Participants
SCEC Priorities 4b, 4c, 4a SCEC Groups GM, Seismology, EEII
Report Due Date 03/15/2022 Date Report Submitted 12/13/2022
Project Abstract
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).
Intellectual Merit The earthquake source spectra count important information about earthquake rupture. Most published stress estimates, especially for moderate and small earthquakes, are inferred from the corner frequency of source spectra. However, the stress estimates are model dependent. The intellectual merits of our works include c) two new modifications when we apply the DCF source spectral models to constrain the realistic rupture realization. b) a robust method for stress drop estimation; c) a clear physical mean of “stress parameter” (∆σ).
Broader Impacts A critical need for earthquake engineering is knowledge of near-source ground motion from damaging crustal earthquakes. While the data are becoming more plentiful (e.g., Ancheta et al., 2014), there is a notable lack of data within 20 km of the causative fault for earthquakes with M>6. Physics-based kinematic earthquake scenarios can provide computed broadband accelerograms for a wide range of magnitudes and distances.
Exemplary Figure Figure 3