Unique Amplification Patterns Generated by Models of Small-scale Crustal Heterogeneities

Abstract:

Currently, the most sophisticated crustal velocity and density models represent the large-scale (hundreds of meters or larger) three-dimensional structure reasonably accurately but poorly resolve the small-scale (10-100m) structure of the Earth’s crust. We model the small-scale heterogeneous nature of the earth’s crust using a fractal distribution to represent the realistic variation observed in ground motion records. To investigate the effects of wave propagation through the realistic velocity structures we use finite-difference solutions to the 3D wave equation including frequency-dependent anelastic attenuation. We simulate 0 – 2.5 Hz linear viscoelastic waves in 1D and 3D velocity structures derived from the Southern California Earthquake Center Community Velocity Model CVM-SI 4.26 with different seismic source descriptions. 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. We find that the majority of the scattering recorded in ground motions originates as a path effect as the waves propagate through the basins, while local site-specific scattering in the immediate vicinity of a ground motion record at the earth’s surface tends to play a smaller role. 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.