GNGTS 2019 - Atti del 38° Convegno Nazionale

350 GNGTS 2019 S essione 2.2 well-bedded Triassic dolomitic bedrock through an unconformity surface. The analysis of weak noise for TP02 shows a flat HVSR curve and no significant peaks according to Sesame project criteria (Sesame, 2004) in a broad frequency band (Fig. 2b, 2c). Such result was expected because of the outcropping bedrock. Only a peak at about 15 Hz is very stable, but its average height is just below 2, while some peaks between 2 Hz and 5 Hz appear only occasionally. On the contrary, the analysis performed on strong noise shows very different HVSR curve, which is characterized by two peaks at frequency of 1.6 Hz and 2.5 Hz, height more than 4, both well polarized (about 80 degrees, Fig. 5d, 5e). The origins of these peaks, their frequency value and their E-W polarization agree with a topographic amplification of the horizontal ground motion in the direction normal to the N-S oriented crest. Fig. 3 shows the results of the analysis of seismic data recorded with a Lennartz LE3Dlite seismometer at site ROI that is a permanent station of the RSU (Rete Sismica Unical). This site is located on a N-S ridge, characterized by a complex of Paleozoic acidic intrusive rocks (granodiorite and granite) often deeply weathered and tectonized, widely outcropping in the surrounding area. The analysis on weak noise shows HVSR curves characterized by irregular shape with a broad double peak between 6 Hz and 10 Hz of height 3 (Fig. 3b). Instead, in the HVSR curves of strong noise the double peak between 6 Hz and 10 Hz increases its height up to 7, and another well defined peak rises at frequency of 3.3 Hz with an average height of 5. Fig. 3 - Results of the analysis of seismic noise at site ROI. Plot a shows the rms computed for 6 days of signals bandpass filtered in three different frequency bands: 0.2 - 2 Hz (red line), 2.2 - 4.5 Hz (magenta line) and 6 - 10 Hz (blue line). Plots b and c show the plain and directional H/V spectral ratio of weak signal computed in the 8 hours time window shown by background color in plot a. Plots d and e show the plain and directional HVSR of strong signal.