The effect of sedimentary basins on surface waves that pass through them

Abstract

Surface waves propagating through sedimentary basins undergo elastic wavefield complications that include multiple scattering, amplification, the formation of secondary wave fronts and subsequent wave front healing. Unless these effects are accounted for accurately, they may introduce systematic bias to estimates of source characteristics, the inference of the anelastic structure of the Earth, and ground motion predictions for hazard assessment. Most studies of the effects of basins on surface waves have centred on waves inside the basins. In contrast, the purpose of this paper is to investigate wavefield effects downstream from sedimentary basins, with particular emphasis on continental basins and propagation paths, elastic structural heterogeneity and Rayleigh waves at 10 s period. Based on wavefield simulations through a recent 3-D crustal and upper-mantle model of East Asia, we demonstrate significant Rayleigh wave amplification downstream from sedimentary basins in eastern China such that Ms measurements made on the simulated wavefield vary by more than a magnitude unit. We show that surface wave amplification caused by basins results predominantly from elastic focusing and that amplification effects produced through 3-D basin models are reproduced using 2-D membrane wave simulations through an appropriately defined phase velocity map. The principal characteristics of elastic focusing in both 2-D and 3-D simulations include (1) retardation of the wave front inside the basins; (2) deflection of the wave propagation direction; (3) formation of a high-amplitude lineation directly downstream from the basin bracketed by two low-amplitude zones and (4) formation of a secondary wave front. We illustrate with several examples how the size and geometry of the basin affects focusing. Finally, by comparing the impact of elastic focusing with anelastic attenuation, we argue that on-continent sedimentary basins are expected to affect surface wave amplitudes more strongly through elastic focusing than through the anelastic attenuation except near the edge of the basin or for basins with extreme aspect ratios.