ASSESSING DIRECTIONAL AMPLIFICATION OF SEISMIC NOISE IN FAULTED ROCK MASSES FOR SITE RESPONSE INVESTIGATION IN COMPLEX GEOLOGICAL SETTINGS

Studying seismic response in fault zone is a challenge issue to understand wave propagation in complex media. This has crucial implication for assessing seismic hazard for urban settlements located in proximity of regional fault-zone structures, especially for Mediterranean countries where much of urban centers are edified on tectonic areas Although an increasing number of case-histories documents the fault-zone related site effects and damage patterns after seismic events (Cultrera et al., 2003; Calderoni et al., 2010; Pagliaroli et al. 2015; Di Naccio et al. 2017 and references therein), the relationships between wave modifications and the internal fault architecture (including the discontinuities orientation) has not considered adequately so far. In such complex geological settings, a clear identification of the main causes of amplification is not simple, principally due of the subsoil structural complexity that can alter the expected ground motion. Recent papers have demonstrated that large systematic amplifications cannot be exhaustively explained by topographic effects, but they can be affected by the contribution of site effects related to fault zones. In particular, it is well documented that the presence of pervasively fractured rocks may lead an amplification of seismic motion and a directional site resonance in the region surrounding a fault zone, as a consequence of the high crack density leading to a stiffness anisotropy of the rock mass. In this view, our work concerns site effects investigation on a pervasively faulted limestone sequence cropping out in central Apennines (Italy). The study area is part of a structural ridge composed of Mesozoic limestones that experienced polyphasical tectonic activity during the Apennines building in the Neogene. Our multidisciplinary approach includes geological-structural and geophysical techniques performed across a c. 50 m-thick fault zone. The geological-structural analysis was devoted to individuate and describe the mechanical discontinuities, in terms of their spatial distribution, of the different structural domains characterizing the fault architecture (a fault core, two damage zones and a surrounding protolith). We performed some in-situ tests using a Schmidt hammer with the aim to evaluate the variation of rock geomechanical parameters within the different structural domains. Then, we evaluated the fracture intensity in each structural domain by using the circle-inventory method. The geophysical survey consisted of 22 noise measurements carried out along two ~80 m-long transects crossing the fault zone, spreading the measurement stations within the different fault domains. Seismic noise measurements were then computed using Horizontal-to-Vertical spectral ratios (HVSR) technique and results were used for documenting wave modification along and across the fault-zone (core zone and damage zones) and in the surrounding protolith. Results highlights the variability of directional amplification of seismic noise within the different fault domains. In particular, the proposed geologicalgeomechanical- geophysical dataset suggests a conceptual model of 3D-compartmentalised fault zone whose fault core and damage zones display distinctive seismic noise behaviour in terms of amplification and polarisation. Our results are consistent with those of recent studies that outlined a structural control on seismic response by the attitude of fault-related structures (Pischiutta et al. 2012; 2013; 2017). Eventually, our results have implication in terms of seismic response of fault zones and mitigation of seismic hazard in areas associated to tectonic activity. 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