High-resolution local seismic zonation by cluster and correlation analysis

Site response analysis is essential for seismic hazard and risk assessment and for providing useful data for land-use planning. However, current regional site amplification models do not always have the resolution required for sites such as alpine valleys, in which site response is characterized by complex effects. In addition, even in local studies, site response is usually computed at a limited number of sites because it requires the recording of earthquakes through the installation of seismological stations. To spatially extend the site response to a denser grid of points inside the investigated area, we used k-means cluster and correlation analyses and Voronoi tessellation. The method was applied to the evaluation of the site response of the lower Sarca Valley on the northern shore of Lake Garda. Earthquakes were recorded at 19 sites to calculate site response in terms of amplification and duration functions of ground motion. The results show high amplification values (up to 10) at low frequencies (at about 0.7 Hz) in the center of the valley, where the sediments reach a thickness of about 420 m. Moderate amplification values and duration lengthening of several seconds in the range of 1–10 Hz are found instead at the edge of the valley on the sedimentary deposits, while a lack of amplification is observed for the sites located on the bedrock. Both the amplification and duration functions were assigned to the area covered by the single-station noise measurements to obtain a zonation of the study area, resulting in three zones to which Fourier amplification factors can be assigned for specific frequency values. The results obtained can be used directly for hazard and risk scenarios and to improve regional maps with lower resolution at the local level.