Risposta sismica locale in condizioni geologiche complesse: il caso studio della Piana di Cassino

Integrating detailed geological models into civil and environmental engineering projects is fundamental for an informed infrastructure design and an accurate seismic risk assessment, particularly in complex geologic, tectonic and hydrogeological settings. This work illustrates the methodological approaches and practical applications for developing a geological model of the Cassino Plain, an intermontane basin of central Italy characterised by a significant seismic hazard and complex hydrogeological conditions. The area is affected by major infrastructures and plants, important healthcare and education services and a high population density. The study involves comprehensive geophysical, geological, hydrogeological and geotechnical analyses, combining borehole data, seismic ambient noise measurements, and advanced geostatistical techniques to delineate the bedrock’s morphology and reconstruct a geological reference model. The Cassino Basin, formed during Pliocene-Pleistocene tectonic activity, features a complex horst-and-graben arrangement with varying thicknesses of Quaternary alluvial sediments resting on carbonate and arenaceous bedrock (Saroli et al., 2020). Past seismic events highlight the need for precise identification of the seismic bedrock to evaluate local seismic amplification phenomena effectively. This study integrated almost 400 boreholes, some of which extending to depths down to 350 m b.g.l., and more than 260 seismic ambient noise measurements to create a detailed 3D geometrical model of the bedrock. Seismic ambient noise data, analysed via Horizontal-to-Vertical Spectral Ratio (HVSR), revealed pronounced impedance contrasts and provided precise mapping of resonance frequencies, which correlated very well with the bedrock depth available from borehole data. Advanced geostatistical interpolation was utilised to transform scattered geophysical measurements into a coherent, spatially continuous map. This enabled the identification of the seismic bedrock surface and facilitated the estimation of sediment thickness through empirically derived relationships between resonant frequency and soil thickness (D’Amico et al., 2008; Van Noten et al., 2022). Virtual boreholes have been generated to augment physical subsurface investigations, which, together with real boreholes, significantly enhanced the resolution and reliability of the geological reference model of the basin. Available boreholes were further exploited to classify the Quaternary filling below the city centre, defining at least four lithological layers, characterised by specific geotechnical properties, and two different aquifers: a shallow multi-level hydrostratigraphic aquifer and a deep pressurised aquifer, which are mutually interconnected. Starting from geological sections representing the complexity of the basin, a seismic stratigraphic model was reconstructed; this model, despite the limitations imposed by basin-scale modelling and the possibilities offered by calculation codes, well represents the seismic behaviour of the area in the event of an earthquake. Six seismic cross-sections were built across the basin and along those sections six numerical analyses of local seismic response in a two-dimensional configuration and 117 analyses in a one-dimensional configuration were performed. The results were compared with the geological and structural settings of the area and a better correspondence of 2D analyses was verified. A comparison was also performed with respect to the results of the Level 3 Seismic Microzonation studies conducted in 1D mode on 4 seismostatigraphic vertical profiles; also in this case the 2D numerical simulations demonstrate a better correspondence to the complex geological conditions of the Cassino basin. As part of the research activities, seismic noise measurements were performed for the first time on a sample of 12 buildings, in order to determine their fundamental vibration periods and derive an empirical correlation between this parameter and the height of each building (Gallipoli et. al, 2010). The correlation was then extended to the rest of the buildings in the urban area of Cassino. These values, compared with the fundamental periods of the foundation soils, provided an evaluation of the proneness of these structures to undergo the phenomenon of soil/building resonance. The integrated model supports civil engineering by enhancing site-specific seismic hazard assessment and soil-structure interaction analysis. It proves the value of multidisciplinary geological modelling in reducing uncertainties and seismic risk in complex settings.