Topographic effects in the local seismic response at a test site in Catania (Italy)

A field experiment using noise and moderate magnitude seismic events was performed on a small hill, named S. Sofia, located in the northern part of Catania (Italy), where the university campus is erected. Aim of this study was to estimate the seismic response of the investigated area and to test the reliability of the use of ambient noise recordings for the definition of possible topographic effects. The most commonly adopted techniques to evaluate site response properties, such as SSR (standard spectral ratio), HVSR (horizontal to vertical spectral ratio) and HVNR (Nakamura's method) were adopted. Techniques evaluating directional effects were also used as reconnaissance procedure for the identification of directional site response. Two sites, located on the top and along the slope of the hill, were equipped with broadband seismometers, monitoring the area for about two years. The results coming out from both SSR and HVSR analysis point out that more pronounced spectral peaks are observed at the station located along the slope of the hill, with respect to the station which is located on the top. This observation is in contrast with usually expected effects of amplification at the top of a hill. It must be however specified that the station located on the slopes is installed on the sedimentary terrains whereas the top one is settled down on a compact lava flow. The results of the investigation on the characteristics of the site response at the S. Sofia hill, set into evidence that the complexity of the near-surface heterogeneities, as well as the morphology have a strong influence on the local amplification of the ground motion and on the directivity effects. Directional effects are more evident along the slope of the hill rather than at the measurement point located on the top of the hill. Such results are observed from the analysis of both earthquake and ambient noise data and point out that the predominant polarization azimuth obtained (30-60°N) is transversal to the major axis of the hill. The complexity of the litho-stratigraphic sequence and the presence of a velocity inversion, appear to strongly influence the topographic effects in S. Sofia area. Nevertheless, the results obtained through the HVNR, have shown that this technique can be conveniently used in study of directivity properties of the site response.