Validation Exercise for Two Southern California Earthquakes

We are in the process of a comprehensive validation exercise constraining parameters describing Q(f) and statistical models of small-scale heterogeneities for finite-difference earthquake simulations in Los Angeles basin. The parameters for our model of shallow crustal heterogeneities are constrained by inverting 38 deep borehole logs in Los Angeles basin (Hurst number 0.0-0.1, correlation length 50-150 m, ~5% σ), which are superimposed onto the SCEC CVM-SI 4.26. We simulate viscoelastic waves for the 2008 Mw 5.4 Chino Hills Event (0-2.5 Hz, modified finite fault source from Shao et al., 2012) and the 2014 Mw 5.1 La Habra event (0-1 Hz, point source), and compare our simulations to strong-motion records. The optimal linear Qs-Vs relation derived from our results is Qs=0.1Vsf^0.6 (Vs in m/s). We find excellent goodness-of-fit (GOF) scores for f<1.0 Hz, in particular at deep-basin sites, that degrade when frequencies increase to 2.5Hz, suggesting an inadequate description of the finite-fault source for f > 1Hz. Poor fits (under prediction of metrics) are often found at hard-rock sites even for f < 1 Hz, likely due to too large values of the shallow Vs in the CVM. The statistical distributions of small-scale heterogeneities generate localized 2x amplifications and de-amplifications, and tend to improve GOF scores by 5-10%. Our simulations suggest that the majority of the scattering recorded in ground motions originates as a path effect as waves propagate through the basins, while local site-specific scattering in the immediate vicinity of a station at the earth’s surface tends to play a smaller role. We find almost equal contributions from scattering in the sedimentary basins and deeper parts of the shallow crust.

Our modeling demonstrates unique amplification patterns caused by scattering due to the heterogeneous structure of the shallow crust. In particular, we find that shallow sources located on the boundary to a sedimentary basin generate bands of strong amplification aligned in the direction of the ray paths. The nature of these bands depends strongly on the incidence angle of the waves into the sediments. Moreover, this banded amplification pattern is absent for sources deeper than 1-2 km. Our results imply that surface rupture on a range-bound fault (e.g, the San Andreas fault by the San Bernardino Basin) may generate a different patterns of ground motion shaking along lines parallel to the fault as compared to profiles perpendicular to the fault.