GNGTS 2013 - Atti del 32° Convegno Nazionale

number of clusters is not significant. The kriging algorithm applied to logarithms of the peak frequencies was used for the interpolation, obtaining a lattice of interpolated frequency values in which the equidistance between nodes is equal to 10 m. The frequency maps are plotted using a contour of logarithmic equidistance equal to eight contour lines per decade. Considering these evaluations, clusters related to datasets of HVSR measurements with significant coverage compared to the studied area were used for the construction of two different maps (Fig. 2) of the resonance frequency of the urban area of Oliveri. The frequency of 2.5 Hz that separates the two clusters was chosen taking into account also the lithological correlations of the examined area. For each measurement the amplitude of the HVSR peak is represented, if present, by means of circles of radius proportional to the owner amplitude class. Authors also chosen not to extend the interpolation (and therefore the representation for contour lines) to those areas in which measures have no significant amplitude peaks in the frequency range represented by the map. Most of the measurement points show amplification of the seismic soil motion at low frequencies (0.7 - 2.2 Hz) and therefore of negligible engineering interest if not for very big buildings. A large area (south of the town) was also highlighted, characterized by amplification at higher frequencies (greater than 2.5 Hz) more interesting for the spatial planning. Finally some measurements does not show peaks in the range 0.1 - 20 Hz, confirming the basement outcropping in the area. Seismic bedrock mapping. In almost all the HVSR measurements carried out on the alluvial material a maximum peak is evident in the range 0.7 - 1.4 Hz. This, by the inversions results, seems to be related to a variation of the shear-wave velocities that, under a depth of 40- 60 meters reach values attributable to a seismic bedrock (about 800 m/s). Analyzing the 1D inverse models derived by inverting HVSR measurements, estimates of the depth of the seismic bedrock have been obtained. These, together with other reliable data like drilling and other geophysical surveys, were used to construct the map of the thickness of the sedimentary cover (Fig. 3, left) and the map of the altitude of the top of seismic bedrock above sea level (Fig. 3, right). The estimates of the depth of the bedrock were interpolated by Fig. 2 – HVSR peak frequency maps for 0.1 Hz < f < 2.5 Hz (left) and for 2.5 Hz < f < 20 Hz (right). 238 GNGTS 2013 S essione 2.2