Preliminary seismic microzonation in a mountainmunicipality of Tuscany (Italy)

In this paperwe describe a microzonation surveycarried out at a mountain Municipality (Piteglio, NorthernApennines). We complemented the available data by conducting106 microtremor measurements for Horizontal-to-Vertical Spectral Ratio (HVSR) analysis. Most of the dataconcern the three major rural villages (Piteglio-Prataccio,Popiglio and Prunetta). Furthermore, we installed a fieldmobile seismic station at two of the major villages, in orderto obtain earthquakes recordings.The HVSR analysis shows amplification effects due to eluvialor colluvial deposits overlying the seismic bedrock,even for areas previously mapped as bedrock outcrops.Most zones characterized by sandstone outcrops showpeaked HVSR traces. This observation could be also interpretedin terms of a topographic effect. In contrast, areascharacterised by chaotic materials (olistostrome) generallyexhibit a flat HVSR. Other areas of seismic amplificationwere identified on alluvial and landslide deposits.Results byHVSR analysis arewell correlated with availabledata from geo-technical and geophysical surveys, thus allowingto invert HVSRs for the physical properties of thesubsoil. Subsoil properties can thus be extrapolated alsoin areas for which only measurements of seismic noise areavailable. Moreover, noise HVSRs agree well with thosefrom earthquakes recorded at the different selected sites.