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Le cavità antropiche sotterranee poco profonde sono diffuse in molte aree urbanizzate di diverse regioni italiane, la cui stabilità rappresenta un potenziale rischio per edifici, infrastrutture e la popolazione stessa. Diverse sono le cause che possono inficiare la stabilità delle cavità: a partire dal loro scavo, la stabilità iniziale può peggiorare nel tempo a causa di diverse cause esterne (e.g. sovraccarichi imprevisti, importanti variazioni del livello di falda, etc...) che possono provocare danni progressivi fino al collasso globale. Anche le azioni sismiche rappresentano una fonte di instabilità per le cavità sotterranee come documentato da diversi studi di letteratura (Evangelista et al., 2016; Santo et al., 2011) e da osservazioni storiche (CFTI5Med, http://storing.ingv.it/cfti / cfti5 / (Guidoboni et al., 2019); CEDIT, http://www.ceri.uniroma1.it/index_cedit.html, (Fortunato et al., 2012)). Nel presente lavoro viene focalizzata l'attenzione sul comportamento delle cavità in condizioni sismiche, proponendo una metodologia per valutarne la stabilità. La proposta è stata applicata ad uno specifico tipo di roccia tenera, ovvero il Tufo Giallo Napoletano in cui sono ricorrenti cavità antropiche scavate per diverse esigenze nel corso degli anni e con caratteristiche differenti (es. forma, dimensione, copertura, sovraccarichi), come documentato da diversi studi della letteratura tecnica (Evangelista et al., 2000; Evangelista et al., 2002; Scotto di Santolo et al., 2015a; Scotto di Santolo et al., 2015b; de Silva et al., 2018; de Silva et al., 2020).

The seismic subsoil response in terms of amplification or attenuation of the ground motion is the result of a complex combination of factors, including the vertical and horizontal subsoil heterogeneities (Fabozzi et al., 2021). In volcanic areas in particular, the vertical subsoil heterogeneities are well identified by characteristic superposition of stiffer volcanic horizons on softer levels, giving rise to stiff-soft alternating layers, also in the form of multiple Vs inversions with the depth. This condition is typical of sheet-like blankets of lava or pyroclastic deposits, extensively covering the sedimentary substratum, frequent in the peripheral areas of large basaltic stratovolcanos or in areas adjacent to large explosive acidic volcanic edifices. The aim of the present work is to study the effect of such vertical heterogeneities on the seismic site response. With this end, in correspondence of volcanic areas identified by means of a preliminary geological screening in the Italian territory, subsoil properties relevant for seismic site response analyses were extracted from the Italian database of the seismic microzonation studies (DB-SMs in DPC, 2018), which is available at www.webms.it and is developed and maintained by CNR IGAG (National Research Council of Italy, Institute of Environmental Geology and Geoengineering, www.igag.cnr. it). The collection of input data was used for an extensive one-dimensional equivalent linear numerical site response analyses, in order to evaluate the influence of stiffness inversions on ground motion at surface. In particular, different idealized subsoil 1D models of the identified geological areas were defined in terms of variation of layers thickness, shear wave velocity and nonlinear properties. The effect of the variability of these parameters on the seismic site response in terms of amplification factors (ICMS, 2008) was studied parametrically.

The shear wave velocity, Vs, profile of the subsoil is one of the most important controlling parameters of the seismic site response (Rathje et al., 2010), i.e. the motion modification during the wave propagation through the stratigraphic series, from the bedrock upward to the ground surface. Despite the high heterogeneities and complexities of the geological conditions, generally the shear wave velocity increases with depth due to the effects of geologic age, cementation and overburden stress. This is the condition indicated by the Italian Building Code (ItBC2018) for the application of the simplified approach, based on subsoil categories, to define the seismic action at the ground surface. They exist, however, particular geological settings for which this condition does not occur and specific seismic site response analyses are reqired by the ItBC2018. This is the case, for instance, of the shear wave velocity inversion, when the top layer is stiffer (i.e. exhibiting higher Vs) than the one below (i.e. exhibiting lower Vs) overlaying the bedrock. The present work identifies the most common Italian geological settings where Vs profile inversions are possible, associating typical mechanical and geotechnical properties to each layer. Based on this preliminary subsoil models, 1D site response analyses have been then performed to evaluate the influence of the stiffness inversion on ground shaking in terms of shear strain profiles and acceleration response spectra at the ground surface. In particular, stochastic 1D analyses were carried out via Monte Carlo method assuming assigned statistical distribution for Vs profiles and nonlinear G/G0-? and D-? curves.

The identification of areas susceptible to different co-seismic instabilities is an important issue of the seismic zonation at urban scale finalized to the territory planning and its protection. Among the co-seismic permanent deformations caused by seismic shaking, the fractures, the landslides, the settlements due to liquefaction or compression/densification can be recognized. The seismic compression or densification is a phenomenon producing permanent ground settlements in dry cohesionless soils (clean sands and sands with fine content) inducing damages to structures, infrastructures and lifelines, accordingly with well documented post-earthquake damages of past events. The susceptibility to this co-seismic instability in presence of dry clean sand, silty sand and sandy silty has been evaluated in the present work through the evaluation of the expected permanent ground settlements by means of non-simplified uncoupled methods computing volumetric strains from cyclic shear strains evaluated by means of site response analyses. This procedure was integrated into a parametric study of 1D seismic site response analyses varying relative density (or shear wave velocity) and thickness of compressible layers, intensity of input ground motion, depth of the seismic bedrock. The results have been then processed to define simplified charts differentiated for three different levels of input peak ground acceleration values and for the three considered lithologies (clean sands, silty sands and sandy silts). These latter are mainly finalized to be used at urban scale, in the perspective of Seismic Microzonation (SM) studies requiring input-data commonly available in level 2 and 3 studies that have a strategic application in land use planning in the perspective of the territory protection. A specific methodology was proposed by means of guideline based on a procedure with increasing complexity: 1) preliminary screening; 2) level 1 analyses; 3) level 3 analyses. The areas potentially susceptible to seismic compression identified in this preliminary phase are to be studied in the level 1 of SM, that identifies attention zones by checking the presence of predisposing conditions to the phenomenon. In the level 3 of SM, the susceptible zones and respect zones are identified through the estimation of the settlements by means of the charts proposed in the present work and the seismic site response analysis, respectively.

Rapporto interno

New palaeoseismic trenching across the main splay of the Fucino fault system provides evidence for a High Middle Age surface-faulting episode conceivably associated with a disruptive earthquake, similar to the one that occurred in 1915 (Mw 7.0). The existence of this event, which has already been suggested by some previous studies, implies a shortening of the recurrence interval for Mw 7.0 earthquakes with respect to current knowledge. If we assume that the palaeoseismic Holocene record is complete, this shortening is focused in the historical period, when the Fucino structure sourced three strong earthquakes in only 1.4 ka. A similar clustering of energy release in the recent past is consistent with both palaeoseismological studies on other faults affecting the Apennine divide, and the high GPS strain rates observed in the same chain sectors.