Geophysical and geologic surveys of the areas struck by the August 26th 2016 Central Italy earthquake: The study case of Pretare and Piedilama

A MW6.0 earthquake struck the Central Italian Apennines on August 24th 2016, followed by an important seismic sequence characterized by hundreds of aftershocks. The earthquake caused fatalities and partially destroyed several towns surrounding the epicentral area. In this context the Italian Centre for Seismic Microzonation and its applications was involved for the seismic characterization of the struck area. Using ambient vibrations spectral ratios at seventy-five sites and shear wave velocity profiles obtained through surface waves dispersion properties we assessed seismic site response properties in the Pretare and Piedilama villages located in the territory of Arquata del Tronto. The survey performed inside the valley in which the two villages are located set into evidence that the spectral ratios often show a predominant frequency ranging between 3.0 and 5.0 Hz or several peaks that can be related to some alternating layers with different velocities. Conversely, along the valley flanks, where the geological substratum outcrop, spectral ratios tend towards low amplitude values. The soft sediments inside the valley are characterized by shear wave velocity lower than 350 m/s, whereas the geological substratum has shear wave velocity ranging between 600 and 1200 m/s. The integrated interpretation of both geophysical and geological data, made possible to infer subsoil models of the investigated areas identifying the depth of the seismic bedrock and characterizing the buried morphology.