Comparison between 1-D and 2-D site effects modelling of Thessaloniki, in: the effects of the surface geology on seismic motion

ABSTRACT: The site effects in the city of Thessaloniki are estimated using synthetic seismograms obtained at different sites of well known geology. First, 1-D local soil profiles are used to construct strong motion syn-thetics based on modal summation method for frequencies up to 10 Hz. As input, four point sources are used, located at different distances and azimuths from the stations. Ratios of response spectra of the local 1-D over the regional 1-D seismograms are calculated and a theoretical mean amplification is estimated. To estimate the ground motion in the laterally heterogeneous part of the structural model we use different 2-D cross-sections modeling the subsurface beneath the receivers and compute synthetic accelerograms up to frequen-cies of 6 Hz both for P-SV and SH waves. For this purpose a hybrid method is applied which combines the modal summation and finite difference techniques. A comparison between the 2-D site effects with the mean site amplification estimated by the 1-D approach shows that in cases of complex geometries the use of at least 2-D geometries in site effects estimation is mandatory.