Based on a National Government Decree, detailed seismic microzonation studies were carried out after the seismic sequence of Central Italy (2016–2017) in support of post-earthquake reconstruction. During these activities mass rock creep deformations of slopes were observed in the municipality of Accumoli (Latium Region), involving two urbanized areas on the left bank of the Tronto River. The geological and geomorphological surveys as well as the geomechanical characterization of rock masses revealed a deformation process of flexural toppling which involves the sub-vertical thick sandstone layers down to 50 m of depth. Geophysical investigations, consisting of downhole tests, active surface-wave testing and seismic ambient noise measurements, were carried out in the areas involved in the deformational processes. The results highlighted that the rock mass involved in the flexural toppling exhibits peaks of the horizontal-to-vertical spectral ratios (HVSR) in a frequency band ranging from 1.5 up to 2.5 Hz. In one of the two areas, H/V calculated on weak-motions recorded during the seismic sequence within the deforming rock mass, showed a clear peak in the H/V frequency range 1.5–2.0 Hz, the same range pointed out by the ambient vibration analysis. Due to local morphological, lithological and bedding conditions, such an evidence cannot be related to a simple 1D resonance due to the seismic impedance contrasts originated from the local stratigraphy. Nevertheless, it seems more reliable that the HVSR peaks can be related to the 3D volume of intensely jointed rock mass, in agreement with the field evidence of flexural toppling, as these peaks result confined within the deforming mass. In addition, the lack of a clear polarization of the particle motion in the seismic ambient noise dataset suggests that the observed effect should not be linked to the interaction of seismic surface waves with persistent discontinuities in an anisotropic rock mass. According to the National guidelines for seismic microzonation studies, these rock masses have not been identified as unstable, i.e. involved in conventional landslide processes, but have been identified as seismic amplification prone microzones with associated amplification factors of at least 1.5 in the period range 0.1–0.5 s. The identification of such microzones within areas involved in gravity-induced slope deformations is a significant prerequisite for planning strategies of post-earthquake reconstruction. © 2019 Elsevier B.V.

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