Evaluation of site effects at Amatrice (central Italy) after the August 24th, 2016, Mw 6.0 earthquake

This work presents results obtained from numerical site response analyses in the area of Amatrice (central Italy) with a main focus on the town's historical centre, where a more significant differential damage was observed after the August 24th, 2016, Mw 6.0 earthquake and the following aftershocks. Geological field investigations were performed and used to define a series of detailed geological cross-sections across the hill, which is the distinctive morphological feature of the town. Following insights from geological cartography, four continuous coring boreholes were drilled to obtain a new refined subsoil model of the area at depth; three of these boreholes were used to perform down-hole tests to constrain the shear-wave velocity (Vs) of the geological units. The subsoil model also included the results of the available seismic arrays and 80 single station ambient vibration stations, which allowed the calibration of the dynamic model, enhancing its robustness. The analyses allowed for a comparison of available empirical amplification functions and the simulated transfer functions of this study. Empirical transfer functions are represented in this case through the available Standard Spectral Ratio curves computed from recorded earthquakes of the 2016-2017 seismic sequence. They show a broad peak centred between 2 and 3 Hz, corresponding to the predominant frequencies of the hill, and a flat response outside the hill, in good agreement with the modelled transfer functions determined following two-dimensional (2D) approach. Conversely, one-dimensional (1D) transfer functions could not describe the amplification in the same range, confirming the evidence of overlapping topographic and stratigraphic effects. In the historical part of the town, only ambient vibration data were available. Notwithstanding that only a comparison of numerical resonance frequencies with the experimental frequencies highlighted using the horizontal-to-vertical spectral ratio (HVSR) method on ambient vibration data is possible in this case, reasonable agreements were found in the considered frequency range 1-10 Hz. In-depth characterisation of the subsoil of the historical centre of Amatrice provided a useful tool for challenging the problem of the realistic ground motion estimates, by evaluating possible local effects which could enhance or modify the input motion.