GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 The 47 HVSR surveys considered include 32 measures carried out in the 2012 and 15 measures carried out in February 2022. These latter were realized for the geological study preparatory to the editing of the General Town Plan of the Municipality of San Vito Lo Capo. In particular 21 surveys were located within San Vito Lo Capo, 12 in Macari and 14 in Castelluzzo. The HVSR surveys were located in an almost homogeneous distribution on the most urbanized territory. All HVSR measurements, acquired with 3D seismic velocimetric sensors, were reworked and reinterpreted in the light of the available data. Data analysis and definition of the subsoil model The HVSR surveys were interpreted trying to identify the mean frequency peak within the frequency window of 0,1-20 Hz, as well as secondary peaks that may be present. In almost all HVSR measurements low frequency peaks were recognized, generally between 1 Hz and 3 Hz and other peaks at higher frequencies, around 20 Hz. The recognized peak frequencies were used to realize the frequency maps for each area, in which the spatial distribution of the HVSR surveys is reported together with peak values recognized. HVSR curves that presented stratigraphic peaks were interpreted with seismic velocity models, constraining the inversion and interpretation with the available geological and geophysical data to compute the depth of the seismic bedrock. Based on the results of the seismic tomography, which show seismic velocities greater than 800 m/s already in layers a few meters deep, the low-frequency peaks (1-3 Hz) of the HVSR curves have been considered as caused by very deep stratigraphic interfaces. Conversely the cover/bedrock interface could be responsible for the higher frequency peaks (around 20 Hz). The bedrock depth model, represented in Fig. 1, shows that investigated areas are characterized by limestone rocks that constitute the shallow seismic bedrock. Above these rocks a coverage of a few meters is present. In particular, the area of the town of San Vito Lo Capo shows very shallow seismic bedrock, excluding a restricted area characterized by the presence of an ancient quarry filled by heterogeneous material. Even the area of Castelluzzo shows few meters of coverage above the seismic bedrock. Finally, the Macari area shows greater depth variations of the seismic bedrock, reaching maximum value of 12 m b.g.l..