The integration of surface and subsurface stratigraphic data with HVSR measures for studies of seismic microzonation: the case of Oliveri (ME)

Because of its high seismic hazard the urban area of Oliveri has been subject of a study on seismic microzonation at first level (OPCM 3907/2010). The urban area develops on a large coastal plain made of Late Pleistocene and Holocene mixed fluvial/marine sediments, that mainly consist of silty sands and gravels. The eastern part of this plain is dominated by the alluvial deposits of the Elicona torrent. The coastal and alluvial sediments overlap a complexly deformed substrate made of Hercynian metamorphites (Aspromonte unit), the Capo d’Orlando Flysch, the Antisicilide Argille Scagliose and Pleistocene clays and calcarenites. This area is affected by high uplift rates as recently estimated in the adjacent Furnari zone. In order to define a preliminary subsurface model for the first level of microzonation, we performed 23 HVSR measurements, homogeneously distributed in the area with a minimum spacing of 250 m, integrating a few available well-log data. All the significant peaks of the H/V spectra in the frequency range 0.6-10 Hz have been identified. Their attribution to resonance phenomena of buried structures has been validated in agreement with SESAME criteria, sample standard deviation of the spectral coefficients and independence from the azimuth. A set of 23 vectors, containing Cartesian coordinates, central frequency and amplitude for each peak was used to identify a subsets which can be reasonably attributed to a continuous phenomenon with respect to the position, using a clustering technique. The outlier peaks were attributed to minor topographic sources or to features of the source noise and therefore they were not considered. Two clusters related to sets of points that cover a significant fraction of the investigated area were used to construct two different frequency maps by kriging interpolation. From the HVSR curves of the two clusters 1D models of S-wave velocity have been obtained. The HVSR data inversion has been constrained by stratigraphic data of a near borehole and by the related vertical seismic profile. To map the depth of the seismic bedrock, from the complete set of modelparameters we extracted only the depths of two seismic interfaces. The depth of an interface was set equal to zero where the peak related to the resonance frequency of the covering layer is absent and lithologies, mechanically compatible with seismic bedrock, outcrop. The reconstructed outline of the top of the seismic bedrock allow us to highlight its deepening below the mouth of the Elicona Torrent, thus suggesting the possible presence of a buried paleo-valley.