Effect of a Strong Lateral Discontinuity on Translational and Rotational Motion – Comprehensive Analysis in the Time, Frequency and Time-frequency Domains

Ground-shaking site effects cause localized anomalous macroseismic effects and are often responsible for the greatest damage observed during earthquakes and great loss of life. The best known are effects in surface sedimentary layers, sedimentary basins and valleys.

Less known and thus less investigated are specific site effects such as those related to lateral boundaries of sedimentary layers or sudden changes of the thickness of sedimentary layers. Pioneering numerical-modelling studies by Moczo and Bard (1993) and Kawase (1996) indicated interesting and important phenomena in the semi-infinite horizontal layers for the SH motion and P-SV motion, respectively, in relation to observed macroseismic effects.

We present a unified analysis of seismic motions due to incidence of plane SH, SV and P waves, by investigating simultaneously the translational motion, rotational motion and axial strain rate in the time, frequency and time-frequency domains. We pay a special attention to the effect of damping by considering frequency-independent as well as frequency-dependent attenuation in the sediment layer. We identify the main anomalous characteristics of seismic motion at the free surface of the sediment layer. Receivers located at short distances from the discontinuity undergo significantly larger translational motion than predicted by the local 1D response, and large rotational motion and axial strain rates. At longer distances (up to forty times the layer thickness), significant deviations from the pure 1D behaviour can be seen especially on the rotational motion and axial strain rate, and on the duration of translational motion as well.