Contribution of HVSR measures for seismic microzonation studies

The HVSR method applied to seismic noise can be a very useful technique to map the site effects of the territory, to identify the thickness of the soft covering and so the depth of the seismic bedrock. The case of the urban area of Oliveri is presented. Because of its high seismic hazard this area has been subject of first level seismic microzonation. The town lies on a large coastal plain made of mixed fluvial/marine sediments, overlapping a deformed substrate. In order to identify points on the area probably suffering of relevant site effects and to define a preliminary Vs subsurface model, 23 HVSR measurements were performed. A clustering technique of continuous signals has been used to optimize the calculation of the HVSR curves and 42 reliable peaks, in the frequency range 0.1-20 Hz, have been identified. A second clustering technique has been applied to the set of 42 vectors, containing coordinates, frequency and amplitude of each peak, to identify subsets attributed to the same seismic discontinuities. Three main clusters have been identified. The two characterized by lower frequencies have been considered in the HVSR data inversion, as stratigraphic peaks probably caused by the seismic bedrock. Finally, the morphology of the top of the seismic bedrock has been mapped. The deepening of the seismic bedrock below the mouth of the Elicona Torrent suggests the possible presence of a buried paleo-valley.