Site effects and microzonation in the city of Thessaloniki (Greece) comparison of different approaches

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Abstract
Ten seismological stations were installed for 3 months in the city of Thessaloniki (Greece) to record earthquakes and ambient noise over areas of different geotechnical characteristics. About 40 local earthquakes were recorded with a good signal-to-noise ratio, in more than two stations. We analyze the records with three different techniques: (a) the classical spectral ratios to a reference station, (b) the receiver functions, and (c) the horizontal-to-vertical (H/V) spectral ratios on noise recordings. The results are compared in terms of predominant frequencies and amplification levels. The three methods are found equally able to reveal the fundamental frequency. Regarding local site amplification, they provide significantly different results with a general trend of the two H/V ratio methods (b and c) to underestimate the amplification levels compared to that inferred from the classical spectral ratio technique (a). The results are then used in relation with the microzonation of the city of Thessaloniki. A good correlation is found between the local site amplification and the type and age of the local geological formation as well as with the depth of bedrock. Finally, we compare our observations with the damage distribution in the city, which was hit by a magnitude 6.5 earthquake in 1978. The amplification levels from spectral ratios obtained at each site exhibit a good agreement with the map of damage intensity.