SCEC Award Number 20114 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 4c, 4b, 4a SCEC Groups GM, EEII, Seismology
Report Due Date 03/15/2021 Date Report Submitted 05/12/2021
Project Abstract
In the UCSB method for computing broadband ground motion we explicitly specify a source spectrum. In the past, we have used a single corner Aki(1967)/Brune(1970) spectrum to constrain the source realizations. However, using data from the NGA West-2 (Ancheta et al., 2014), we have determined that the spectrum should have two corners: one related to the overall duration and one related to the spectral level of the high frequency radiation (Ji and Archuleta, 2020). In essence, the classical Aki (1967)/Brune (1970) single-corner source spectrum should be modified to include a second corner, i.e., a double-corner frequency (DCF) source spectrum (e.g., Gusev, 1983; Luco, 1985; Atkinson, 1993; Atkinson and Silva, 1997).
Intellectual Merit The classical Aki (1967)/Brune (1970) source spectrum has a single corner that can be related to stress drop (Brune, 1970, 1971). The single corner frequency is related to the overall duration of the rupture process. There is a stress parameter that controls the high frequencies which are directly related to root-mean-square acceleration (Hanks, 1979; McGuire and Hanks, 1980). There is clearly a difference in estimating the stress parameter from acceleration and stress drop estimated from a spectrum, assumed to have a single corner (Cotton et al., 2013). Using data from crustal earthquakes (Ancheta et al, 2014), we have determined scaling relations for the two corners of a double-corner source spectrum (Ji and Archuleta, 2020; Archuleta and Ji, 2016). This spectrum resolves the quandary discussed by Cotton et al. (2013) as to what is the stress drop of an earthquake.
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 1. Comparison of synthetic acceleration Fourier amplitude spectra of M 5 and 7 earthquakes predicted using SCF spectral models AS09 and BH14 as well as DCF spectral model JA19_2S. Note that for AS09, we predicted the S wave corner frequency using β=3500 m/s and adopt its falloff rate of 1.6 (Allmann and Shearer, 2009). The corner frequency estimated using BH14 is 1.76 times larger than AS09 (Ji and Archuleta, 2020).
This Figure was included in
Ji, C., and R. J. Archuleta (2021). A source physics interpretation of non-self-similar double-corner frequency source spectral model JA19_2S, submitted to Seismological Research Letters