Empirical evidence of orthogonal relationship between directional site effects and fracture azimuths in an active fault zone: The case of the Mt. Etna lower eastern flank

Mt. Etna, in eastern Sicily, is a composite active volcano whose eastern flank is affected by several normal and strike-slip active faults. In this study geological-structural field survey and ambient vibration measurements are performed in the lower eastern flank of Mt. Etna in order to define the role of the tectono-stratigraphic setting in the seismic site response. The sites of measurements are mainly distributed inside the few hundreds of meters long S. Caterina Graben, which is located between two main seismogenic active faults of this volcano sector, the Acireale and the Fiandaca faults. Our study aims at defining the contribution of the fault related active deformation in determining the local ground motion amplification, also providing a subsoil geological model based on the integration of geological and geophysical investigation. Therefore, structural measurements were made in seven sites, revealing coseismic and aseismic extensional fractures mainly striking NNW. The seismic site response was studied processing the ambient vibration measurements through Horizontal-to-vertical spectral ratio. The measured fundamental frequency falls in the range 1.0-4.0 Hz, that we linked to the presence of a velocity contrast at depth of 80-200 m by calibrating a frequency depth function. Moreover, the seismic site effect is due to the stratigraphic setting and is markedly directional, showing maximum amplification along the direction orthogonal to the strike of the observed isooriented extensional fracture field produced by the activity of the S. Caterina Graben. It is noteworthy that, far from the graben on sites of measurements located near the Fiandaca Fault, we observed directional effects parallel to the direction of maximum extension inferred for this tectonic lineament. Our results suggest a linkage between directional site effects and local stress tensors and fractures field orientation.