Rotational motion and axial strain near a strong lateral discontinuity

SUMMARY Ground-shaking site effects, which are anomalous earthquake ground motions at the Earth’s surface, are caused by the generation of specific wavefields in specific near-surface structures. The best known are effects in surface sedimentary layers, sedimentary basins and valleys, which are related to wave trapping in softer materials. To a lesser degree, topographic effects, associated with irregular surface geometry and often also with collocated softer or cracked material, have also received much attention and gave rise to specific provisions in some building codes (e.g., EC8). By contrast, site effects related to sharp lateral boundaries between media with very different stiffness have been much less investigated, despite consistent macroseismic observations of a significant increase in damage intensity on “stripes” located along strong lateral discontinuities. In this study we consider a semi-infinite layer as a canonical model for many real situations with sediments relatively sharply laterally bounded or with abrupt change of sediment thickness. We investigate numerically simulated particle velocity, rate of rotational motion and rate of axial strain for the vertically incident SH, SV and P waves. We use a novel type of characterization of transfer properties – time-frequency transfer functions for the particle velocity and rates of rotational motion and axial strain. We found that the rates of rotational motion and axial strain display sharp peaks at the lateral discontinuity mainly in the case of the incident SV and P waves. Particle-velocity components exhibit a maximum close to the discontinuity, but at variable distances depending on central frequency and incident wave type (“basin-edge” effect). At larger distances from the discontinuity, the decay of rotational motion and axial strain is controlled by the attenuation in sediments. The sharp peaks of the rates of rotational motion and axial strain across the lateral discontinuity, together with the well-known basin-edge effects for translational motion, are found to be the key site effects of the laterally bounded sedimentary layer.