VARIABILITY OF SEISMIC GROUND MOTION IN COMPLEX MEDIA - THE CASE OF ASEDIMENTARY BASIN IN THE FRIULI (ITALY) AREA

A computational hybrid technique is presented to estimate ground motio n in complex two-dimensional, anelastic media. The technique combines modal summation and finite-difference methods, and it can take into ac count the source, path and local soil effects to calculate the local w avefield due to a seismic event.The hybrid technique is applied to stu dy wave propagation in a sedimentary basin in the Friuli region during the September 11, 1976, Friuli aftershock (16h 35m 04s). Special emph asis 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 heteroge neities, and local resonances. Within the sedimentary basin studied, t he coda of the transverse component is mainly composed of the local, f undamental-mode Love wave, whereas the P-SV-wavefield shows dominant c ontributions of the higher modes of Rayleigh waves. These differences in wave composition lead, in general, to different dispersion characte ristics and attenuation phenomena for SH- and P-SV-waves. A parametric study demonstrates the sensitivity of the computed ground motion to s mall changes in the subsurface topography of the sedimentary basin, an d the velocity and quality factor of the sediments. The results obtain ed for one- and two-dimensional structural models show that only two-d imensional structural models are suitable for the prediction of comple te seismic ground motions in sedimentary basins, because they can acco unt for the generation of local surface waves. To establish the validi ty of the numerical results, they are compared with observed ground mo tion. The relative amplitudes, durations, and frequency content of the different components of the synthetic signals agree well with the obs ervations.