Amplification of ground motion in fault and fracture zones: observations from the Tremestieri fault, Mt. Etna (Italy)

A series of ambient noise measurements have been performed to evaluate their use in detecting the possible influence of tectonic structures on local amplification of seismic waves. The area studied is located on the southeastern flank of Mt. Etna, a few kilometres north of the town of Catania. Using a mobile station, data were collected along three profiles which cross both a fault and an eruptive fracture. In addition, ambient noise was sampled at two permanent stations installed in close proximity to the fault, and at another station located within the city limits of Catania and considered to be a reference site. Analysis using spectral ratio techniques shows there is a tendency towards amplification at the sites located near the fault. In H/V spectral ratios such amplification is observed in the frequency range 4.0–8.0 Hz and the amplitudes of the dominant spectral peaks decrease rapidly over the first few tens of meters from the discontinuity. A similar tendency was observed for the sites located on the eruptive fracture zone. Dominant spectral peaks at 1.8 Hz are also common in the study area. The theoretical transfer function, obtained through 1-D modelling, indicates that such peaks can be related to the local stratigraphy. The mean H/H standard spectral ratio of microtremor evaluated at the permanent station located on the fault also shows an amplification peak at 4.0 Hz. Azimuthal analysis shows that the resonance at 4.0 Hz is directional, with the maximum in the azimuth range N30°E–N80°E.