Geological modeling of Altavilla Milicia (Sicily) using HVSR data

Abstract

At today the use of inversion of HVSR curves is mainly limited to derive average parameters of the shear wave velocity, although recently they have been used also for a detailed reconstruction of the roof of the seismic bedrock (Di Stefano et al. 2014). Since ambient vibrations may contain waves travelling in all directions, as body waves and Rayleigh and Love waves, a limit of this method lies in the uncertain composition of seismic noise, in the lack of knowledge about the microseismic field and in the subjective choices regarding the data processing. This work aims to verify the potential and limits of the HVSR inversion for the purposes of geological reconstruction of the subsoil in heavy urbanized areas, where few stratigraphic and geophysical constraints are available, and the only information is practically derived from the geological maps. The test site is the area of Altavilla Milicia (Sicily), characterized by outcrops of marine terraces and continental Pleistocene deposits that do not indicate directly the oldest geological formations, which, however, outcrop in the surrounding areas. Microtremor measurements were carried out on 20 recording points equally spaced of about 300 meters along two alignments: the line P in NW-SE direction near parallel to the coast and the line Q in NNE-SSW direction crossing the town. The HVSR inversion has been carried out using the Neighborhood Algorithm (Wathelet et al. 2004) implemented by the Geopsy software. Because of the lack of boreholes, starting models for inversions were chosen by assigning to each seismic layer ranges of thickness and velocity typical of the lithology expected, as deduced from the geological formations outcropping in the area and from previous geophysical surveys. The 1D seismic models obtained by results of HVSR inversion were correlated and laterally interpolated in order to construct two-dimensional geophysical sections, considering typical seismic velocities for each lithology concerned. In the study area, the outcropping Plio-Pleistocene clastic succession covers the Numidian Flysch silico-clastic deposits and the substrate related to slope to deep water Imerese Succession. The study area is characterized by a Miocene compression and later Plio-Pleistocene transcurrent and extensional tectonics (Catalano et al. 2013). The profiles display seismic layers mostly dipping towards N. Profile P (NW-SE-trending) shows displacements whereby seismic layers are abruptly downthrown towards N. This can be interpreted as high angle transcurrent and extensional faults, that downthrow a backlimb of a major fold towards the coast. Results of this test demonstrate that HVSR inversion can be a valuable method for the reconstruction of the geometry of the main subsurface structures, if supported by the right geological-structural information.