Seismo-stratigraphic model of “La Bandita” area in the Palermo Plain (Sicily, Italy) through HVSR inversion constrained by stratigraphic data

Abstract

Ambient noise Horizontal-to-Vertical Spectral Ratio (HVSR) technique is commonly used approach to obtain 1D models of the shear-wave velocity in the shallow surface of an investigated area. However, obtained models can have a wide margin of uncertainty if inversions have not been appropriately constrained by detailed stratigraphic information. An application of HVSR inversion constrained by lithostratigraphic data is presented in order to verify the effectiveness of this technique for purposes of geological and geophysical reconstruction of a sedimentary basin in a densely urbanized area. This is often the case of seismic microzonation studies, in which almost all the information derives from near surface stratigraphic drillings, since other geophysical methods are logistically difficult to carry out. In our work, we used stratigraphic constraints derived from 93 superficial bore-holes whose depth rarely exceeds 30 m. In an area called La Bandita, located in Palermo Plain (Sicily, Italy), a geophysical survey was performed by means of 55 microtremor recordings. Part of these was distributed randomly, while others very close to the available stratigraphic perforations. The reconstruction of the stratigraphy in the studied area has been obtained by a review of the main stratigraphic sequences and by a consequent stratigraphic three-dimensional modelling. HVSR curves have been interpreted taking care the thicknesses of the near surface successions derived by the stratigraphic 3-D model. The results, in terms of vertical profiles of the shear-wave velocity, have been interpolated to obtain a 3D seismic model. This has been used to extract basic information to identify and reconstruct the seismic bedrock and the main geological boundaries that were not directly identifiable by means of only stratigraphic logs. It results that the bedrock is affected by a fault system that generated adjacent depressions where Quaternary successions deposited.