Evidence of directional site effects on fault zones : observations from south-eastern Sicily and Malta

Abstract

It is well known that faults are generally characterized by a highly fractured low-velocity zone bounded by higher-velocity undamaged rock that behaves as a wave guide trapping incident seismic energy and therefore causing local amplifications (e.g. Li et al., 2000). The local seismic response in the vicinity of fault zones was recently studied in the Etnean area (e.g. Di Giulio et al., 2009). A systematic polarization of horizontal ground motion was observed near the investigated faults. The observation of a similar behaviour when using both earthquake and ambient noise recordings suggests the use of microtremors for investigating ground motion polarization properties across the main faults in the volcanic area, as well in SE Sicily and Malta. Noise measurements were performed along transects perpendicular to the fault strike, at increasing distance from it. Time series of ambient noise, having a total duration of thirty minutes, were processed through HVNR technique. Directional site effects were analyzed both by rotating the horizontal components of motion and computing the horizontal to vertical spectral ratio, as well as through a direct estimate of the polarization angle using the covariance matrix method.