Shear wave velocity structure in western Thessaloniki (Greece) using mainly alternative SPAC method

The aim of the paper is the definition of the material parameters of sediments in the western part of Thessaloniki useful in the prediction of strong ground motion. The region of interest is elongated in the E–W direction and is confined between the Kalochori suburb and the harbor of Thessaloniki, while in the N–S direction it is extended between the coastline and the region of sediments and outcrop rock boundary. Many geological and seismotectonic studies, as well as geophysical surveys with electrical soundings and geotechnical boreholes, contribute to our understanding of the general sedimentary structure. Nonetheless, these studies could not provide any information regarding the material stiffness in terms of shear wave velocity; the most useful parameter in site response studies.     

Determination of S-wave velocity structure using microtremors and spac method applied in Thessaloniki (Greece)

Array measurements of microtremors at 16 sites in the city of Thessaloniki were performed to estimate the V_s velocity of soil formations for site effect analysis. The spatial autocorrelation method was used to determine phase velocity dispersion curves in the frequency range from 0.8–1.5 to 6–7 Hz. A Rayleigh wave inversion technique (stochastic method) was subsequently applied to determine the Vs profiles at all the examined sites. The determination of V_s profiles reached a depth of 320 m. Comparisons with V_s values from cross-hole tests at the same sites proved the reliability of the SPAC method. The accuracy of the V_s profiles, the ability to reach large penetration depths in densely populated urban areas and its low cost compared to conventional geophysical prospecting, make Mictrotremor Exploration Method very attractive and useful for microzonation and site effects studies. An example of its application for the site characterization in Thessaloniki is presented herein.