Site features responsible for uneven seismic effects in historical centre of Melfi (Basilicata, Southern Italy)

This paper deals with a multi-disciplinary set of tools aimed to analyse the factors controlling the macroseismic effects caused by an earthquake of the past in a historical town. To this end we illustrate a case study based on the analysis of the damage caused by the 1930 Irpinia earthquake (Me=6.7) in Melfi, a town in southern Italy characterised by high seismic hazard. This paper benefits from a previous research that delineated the Melfi ‘scenario’ map. To understand the anomalies of damage, we used an integrated approach that combines geological and geophysical surveys and unpublished primary technical-historical documents. Both to infer the stratigraphic set-up and to estimate the thickness of shallow deposits, geological surveys were integrated by geo-electrical prospecting and drillings. To evaluate the role played by site features on the seismic ground motion, we performed thirty Horizontal-to-Vertical Spectra Ratio measurements obtained by microtremor recordings. Moreover, the influence of the soil profile on Peak Ground Acceleration and Housner Intensity was investigated by 1D numerical modelling and the earthquake slope response to seismic shaking was characterised by the azimuthal variation of Noise StandardSpectra Ratio. Finally, a combination among all these data and technical surveys coeval to the earthquake allowed us to create a complete picture of the possible causes of uneven damage. By doing so, the research supplies some insights towards prediction of future site intensities