Seismic site effects observed on sediments and basaltic lavas outcropping in a test site of Catania, Italy

Ambient noise measurements and a set of 44 moderate magnitude earthquakes were used to study the role of local geology and morphology on the site response of a small hill in the northern part of Catania, on top of which the University Astronomical Observatory is located. The study area has a gentle topography with a flat surface at the top, and it is characterized by a complex sedimentary sequence lying between a clayey basement and an upper volcanic formation. The recorded data were processed through standard spectral ratio and horizontal-to-vertical spectral ratio techniques. Directional effects were also investigated by computing the spectral ratios after rotating the horizontal components of motion and performing polarization analysis. Results of noise and earthquakes analysis, although showing significant differences in amplitude, are comparable in frequency, especially in the sedimentary terrains. On the lava flows, spectral ratios show significant amplification of the vertical component, which appear related to a higher P velocity contrast with underlying soft sediments. Directional effects were identified in two frequency bands (0.2–0.4 and 1.0–10.0 Hz). The effects observed at the lower frequency interval are rather stable, and it spreads out in all the studied area. At higher frequencies, directional effects are variable and mostly observed on the slopes rather than at the hill top. Our findings appear linked to the complex wave field generated by the lithologic heterogeneities existing in the area which seem to be related to the alternation of sediments and basaltic lavas.