Variability of seismic ground motion in complex media: the case of a sedimentary basin in the Friuli (Italy) area

Abstract

A computational hybrid technique is presented to estimate ground motion in complex two-dimensional, anelastic media. The technique combines modal summation and finite-difference methods, and it can take into account the source, path and local soil effects to calculate the local wavefield due to a seismic event. The hybrid technique is applied to study wave propagation in a sedimentary basin in the Friuli region during the September 11, 1976, Friuli aftershock (16h 35m 04s). Special emphasis is given to the understanding of the different features of ground motion in sedimentary basins. The most important effects that can be observed are the excitation of local surface waves at lateral heterogeneities, and local resonances. Within the sedimentary basin studied, the coda of the transverse component is mainly composed of the local, fundamental-mode Love wave, whereas the P-SV-wavefield shows dominant contributions of the higher modes of Rayleigh waves. These differences in wave composition lead, in general, to different dispersion characteristics and attenuation phenomena for SH- and P-SV-waves. A parametric study demonstrates the sensitivity of the computer ground motion to small changes in the subsurface topography of the sedimentary basin, and the velocity and quality factor of the sediments. The results obtained for one- and two-dimensional structural models show that only two-dimensional structural models are suitable for the prediction of complete seismic ground motions in sedimentary basins, because they can account for the generation of local surface waves. To establish the validity of the numerical results, they are compared with observed ground motion. The relative amplitudes, durations, and frequency content of the different components of the synthetic signals agree well with the observations.