Cluster analysis to support microzonation studies

In recent times the use of microtremor techniques for subsoil investigation increased significantly. The use of HVSR (Horizontal to Vertical Spectral Ratio)technique for seismic microzoning studies allows in many case to obtain detailed reconstruction of the roof of the seismic bedrock and to identify areas with similar seismic behaviour. Two different algorithms of clusteringhave been tested on a HVSR datasets acquired for studies of seismic microzoning in various Sicilian urban centers. HVSR data were previously properly processed to extract frequency and amplitude of peaks by a code based on clustering of HVSR curves determined in sliding time windows.To select an optimal set of time windows we have implemented a cluster procedure based on Agglomerative Hierarchical Clustering algorithms.After defining the average HVSR curves a second multi-parametric clustering procedurehas been used to group peaks tobe attributed to the same origin (stratigraphic, tectonic, topographic, anthropogenic or other sources). A nonhierarchical centroid-based algorithm has been implemented. The comparison of the HVSR pattern with the information about outcropping formations allowed to assess the geological hypotheses on the heavily urbanized investigated areas.