Abstract

Ground-motion amplification by local geology conditions is investigated near Thessaloniki, Greece, using spectral ratios of S waves from local and regional earthquakes. We present results for two different experiments using an array of five digital three-component seismographs. In the first, instruments were installed across Vasilika sedimentary valley, southeast of Thessaloniki (three on sediments and two on rock sites outside the valley). Empirical transfer functions relative to one of the rock sites show significant amplification for low frequencies (between 1 and 6 Hz) at the stations on the sediments and at high frequencies (between 8 and 15 Hz) for a station on a weathered rock site. Our spectral ratios for local earthquakes have shapes similar to those for regional earthquakes but the amplification levels are different. In the second experiment, the seismographs were installed at five sites within the city of Thessaloniki, on four different geological formations. According to our results, site response does not vary significantly between gneiss, quartzites, and recrystallized limestone. Two stations on the same Neogene formation (a series of red, silty clays with mica and calcareous concretionary bodies) show relative amplification which attains a factor of 5 relative to the rock site, but the shapes of the transfer functions are different between the two stations. Finally, we show that our results correlate well with macroseismic information from the 1978 seismic sequence in Thessaloniki, consisting of both regional isoseismal maps and damage distribution within the city. This substantiates the use of weak-motion records to estimate local ground-motion amplification in earthquake engineering.

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