GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 2.2 287 Differently, the area of the Tavoliere plain is characterized by a diffuse presence of pronounced low frequency resonances (< 1 Hz) related to the impedance contrast between the deepest foredeep deposits, consisting of Early Pleistocene clays (“Argille Subappennine”), and the underlying Cretaceous carbonate rocks of the Apulian platform. The resonance frequencies show regional variations: for instance at Foggia, in the central part of the plain they are observed at 0.2-0.4 Hz, whereas at Trinitapoli, few km away from the Adriatic coast, they are found at 0.6-0.7 Hz. This variation can correspond to a decrease of the depth of the calcareous substratum which, according to the borehole stratigraphy (Foggia_003: cf. Cavinato et al. , 2010) is at about 550 m under Foggia, whereas it could be approximately halved under Trinitapoli. The above-mentioned resonance frequencies are considered as the fundamental site resonance frequencies (labelled as F0), but they fall outside the most relevant frequency range for the SM mapping (0.9 – 10 Hz). Thus, they can be of interest just for the definition of the geological model of the study area. However, there are also secondary resonances (labelled as F1/F2 for increasing frequencies), which appear in relation to the presence of layering within the foredeep deposits (e.g., between the “Argille Subappennine” and more recent alluvial deposits), or even, within both of these deposits. These additional resonances show a more irregular spatial distribution, as effect of local variations of the deposits’ composition (e.g. for the presence of sandy or conglomeratic bodies). There is scarcity of direct information on the compositional variations of the deposits, because the available boreholes are generally shallow (in the order of 20 m). Thus, the resonance frequencies revealed by ambient noise analysis are valuable to distinguish areas with potentially different site response properties. As an example, a diffuse “sampling” of the resonance frequencies at Foggia pointed out the presence of zones with the first secondary resonance frequency (F1) between 0.6 - 0.9 Hz, 5 -10 Hz, 10 – 20 Hz or over 20 Hz, and in some cases with the presence of peaks within more than one of these ranges (Fig. 2). Even where these resonances are scarcely significant Fig. 2 - Examples of HVSR results obtained at Foggia by Cardillo, 2022. Sites differing by the range of the most clear secondary resonance frequency F1 are labelled with different colours. In some cases, few peaks can be recognised within different frequency bands, in relation to the resonances caused by laterally discontinuous layering observed within the foredeep deposit at different depths.