Evidence of topographic effects through the analysis of ambient noise measurements

The aim of this experiment was to estimate the seismic response in the area of downtown Syracuse (Siracusa), Sicily,and to test the reliability of ambient noise recordings, processed through HVNR techniques, to estimate topographic effects. The dynamic site properties and, in particular, the shear wave velocity in the upper 30 m (V(S, 30)) of the carbonate sequences were investigated through non-invasive techniques such as multichannel analysis of surface waves (MASW) and refraction microtremor (ReMi). The combined use of both techniques allowed us to compare and check the obtained results while also going over all the limitations of each methodology. Ambient noise was recorded in fourteen sites, processing the signals through spectral ratio techniques. Directional effects were also investigated. The findings of our present study point out a good reliability of the HVNR technique for evaluating the influence of topography on the local seismic response. The H/V spectral ratios show dominant frequency peaks in the range of 1.0–3.0 Hz that are in good agreement with the theoretical resonance frequency of the hill. Both the directional resonance and the polarization analysis confirm the presence of a directional effect having an azimuth 90–100 degrees, transverse to the major axis of the ridge. Our results support the use of this quick and inexpensive technique and further confirm the importance of estimating topographic effects to reduce the potential risk of building damage as a result of ground motions.