Analysis of active and fossil seismic structures: a multidisciplinary study for the seismic risk assessment in low-seismicity regions

Understanding the source processes and wave propagation in heterogeneous rock media is one of the most challenging frontiers to improve the seismic risk assessments in densely populated areas. In this framework, the sector of the Voltri Massif (NW Italian Alps) forming the hinterland of the city of Genoa is a natural laboratory to investigate (i) the interaction between rock faulting and fluid circulation during (potential) paleo-seismic activity and (ii) the detection, location, and source characterization of micro-earthquakes along tectonic lineaments developed inland and offshore the city area (i.e., in the Ligurian Sea). Our multi-scale and multidisciplinary study is part of the PNRR research program RETURN (“Multi-risk science for resilient communities under a changing climate”): it will include the structural and petrographic characterization of fault rocks (i.e., serpentinite breccias), the quantification of serpentinite carbonatization and its impact on the fault strength, and the analysis of the network of inland-offshore tectonic lineaments. This work, coupled with the analysis of historical seismic clusters, is crucial to identify suitable areas for the deployment of high-resolution seismometers and for tracing the spatial-temporal evolution of micro-earthquakes and their static and dynamic source parameters.

The detailed structural mapping of selected fault zones has revealed a complex, multi-stage deformation history, with older ductile structures (paragenesis: antigorite + ilmenite ± chlorite ± pyrite ± chalcopyrite, likely ascribed to the alpine-subduction and collision stages) cut by steeply dipping fault planes NNE-SSW striking. These latter are subparallel with the (low magnitude) seismic clusters detected in the area and develop multiple, anastomosed fault cores consisting of serpentinite-rich ultracataclasites, locally bound by chrysotile-rich shear bands. The faults damage zones textures (e.g., breccias and microbreccias), the paragenesis of newly formed shear bands and associated veins (chrysotile + chlorite) and the orientation of these faults (NNE-SSW striking, subparallel to the Miocene-age lineaments detected in the Gulf of Genoa) suggest recent tectonic reactivation at the regional scale.

Future developments of the research project will include more detailed, high-magnification microscopy of selected samples (e.g., raman, field emission SEM, EBSD and microprobe), regional scale morphotectonic characterization by satellite image analysis, and integration of field and seismic data. This will clarify the link between inland-offshore tectonic lineaments and the (low magnitude) seismicity of the area.