PART I: Theoretical Site Response Estimation for Microzoning Purposes

Abstract

We estimate the theoretical site response along seven cross sections located in the city of Thessaloniki (Greece). For this purpose the 2-D structural models used are based on the known geometry and the dynamic soil properties derived from borehole measurements and other geophysical techniques. Several double-couple sources have been employed to generate the seismic wavefield, and a hybrid method that combines the modal summation with finite differences, has been deployed to produce synthetic accelerograms to a maximum frequency of 6 Hz for all components of motion. The ratios between the response spectra of signals derived for the 2-D local model and the corresponding spectra of signals derived for the 1-D bedrock reference model at the same site, allow us to estimate the site response due to lateral heterogeneities. We interpret the results in terms of both geological and geometrical features of the models and of the characteristics of the wave propagation. The cases discussed confirm that the geometry and depth of the rock basement, along with the impedance contrast, are responsible for ground amplification phenomena such as edge effects and generation and entrapment of local surface waves. Our analysis also confirms that the peak ground acceleration is not well correlated with damage and that a substantially better estimator for possible damage is the spectral amplification.

Spatial distribution of site effects and wave propagation properties in Thessaloniki (N. Greece) using a 3-D finite difference method

“…Comparison for the AA(LEP-OBS) cross-section shows that amplifications from 3-D synthetics for the central part of the crosssection (where deeper sedimentary formations are found) are significantly higher for higher frequencies (>2.5 Hz). Moreover, the two distinct high-amplification zones observed in most of the scenarios studied could not be identified in the results of Triantafyllidis et al (2004a), even for scenarios (1a) and (1b) where the source used in this study lies almost along the profile, similar to the 2-D synthetics. For cross-section A3(AGK-KAL) this study results show more clearly the high amplification zone, as well as the bedrock geometry impact on site amplification.…”

“…The synthetics computed for the six seismic scenarios were compared along three cross-sections shown in the map of Fig. 4(a) (red, green and purple lines) with the corresponding 2-D results of Triantafyllidis et al (2004a) and Raptakis et al (2004b). The Fourier amplitude spectra (FAS) ratio for the 3-D over the corresponding 1-D bedrock reference model [(FAS3D)/(FAS1D)] was estimated for a large number of receivers along the cross-sections for all scenarios and components of ground motion.…”

“…Results from Triantafyllidis et al (2004a), produced with a hybrid method (Fah 1992), are presented in the form of Pseudospectral Acceleration ratios [(PSA2D)/(PSA1D)]. The use of the [(PSA3D)/(PSA1D)] ratio, computed from PSA records for 5 per cent damping, was also initially employed in this study, however, comparisons (not shown here) between [(FAS3D)/(FAS1D)] and [(PSA3D)/(PSA1D)] ratios exhibit almost identical spatial variation and only small differences of the predicted ratios.…”

“…Vertical components also show minimal amplification levels for all six scenarios examined. Triantafyllidis et al (2004a) compared theoretical and experimental estimations of site effects using 2-D numerical simulations and studied the frequency variation of spectral acceleration response ratios SA(2-D/1-D), which are presented in Fig. 8 for the same crosssections of this study, namely AA(LEP-OBS), A3(AGK-KAL) and A5(AMP-LAB).…”

“…The thickening of sedimentary formations towards the centre of Thermaikos gulf (W-SW direction) is delineated with the Neogene sediment isodepth lines from Lalechos & Savoyat (1979). or numerical (e.g. Triantafyllidis et al 1998Triantafyllidis et al , 1999Triantafyllidis et al , 2004aRaptakis et al 1998Raptakis et al , 2004a, mainly based on 1-D or 2-D simulations of seismic motion. Recently, Skarlatoudis et al (2010), using a 3-D finite-difference (FD) scheme for the broader area of Thessaloniki, showed that for several selected sites the previous 1-D and 2-D simulations could not adequately describe the site-response, in comparison with the 3-D wave propagation results.…”