Two empirical double-corner frequency source spectra and their physical implications
Abstract
The best-known part of Brune's (1970) spectral model is the single corner source spectrum. However, Brune noted that a realistic spectral model of the general high-frequency and low-frequency asymptotes needs an intermediate f-1 branch. This "f0-f-1-f-2" shape source spectrum features two corner frequencies: fc1 and fc2>fc1. Brune (1970) associated the emergence of the fc2 with the partial stress drop over a fault considering a rupture with heterogeneous stress release. Here we introduce two double-corner source spectral models JA19 and JA19_2S for 3.3≤ M ≤7.3, constrained by stochastic modeling the mean PGA and mean PGV of the NGA West-2 database. JA19 is self-similar. Its two corner frequencies fc1 and fc2 scale with moment magnitude (M) as (1) log(fc1)=1.754-0.5M and (2) log(fc2)=3.250-0.5M. We find that relation (1) is consistent with the known self-similar scaling relations of the rupture duration (Td), where Td=1/πfc1. Relation (2) may reflect scaling relation of the average rise time (TR), where TR~0.8/fc2. Stochastic simulations using JA19 cannot reproduce the sharp change in magnitude dependence of PGA and PGV at M 5.3, suggesting a breakdown of self-similarity. To model this change, JA19_2S is found by perturbing the scaling relationship in JA19. For JA19_2S: log(fc1)=1.474-0.415M for M<5.3; log(fc1)=2.375-0.585M for M>5.3. In both models the relation fc1<<fc2 applies. Seismic radiated energy scales with M02fc12fc2. The fc1/fc2 ratio scales not only with the ratio of effective stress drop and static stress drop as Brune (1970) pointed out but also with the fault aspect ratio. In short, the source spectral shape "f0-f-1-f-2", proposed originally by Brune (1970), provides a bridge to reconcile the known scaling relationships in source duration, static stress drop, seismic radiated energy, fault aspect ratio, and ground motion parameters within acceptable uncertainties. It also explains why the stress parameter would generally be larger than static stress drop which is related to the lower corner frequency.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMS054.0017J
- Keywords:
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- 7203 Body waves;
- SEISMOLOGY;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7215 Earthquake source observations;
- SEISMOLOGY