Geophysical investigations for the identification of active seismic faults below alluvium for seismic hazard assessment

The existence of active faults hidden below Quaternary alluvium is a common geological scenario for intermontane basins, such as the areas struck by the recent earthquakes in Central Italy, and is of great importance for seismic hazard evaluation. Finding hidden faults is a challenging task from the geophysicist's point of view since the goal is twofold: to identify the seismic bedrock at a certain depth; and to detect lateral variations or dislocations that may indicate the presence of a fault. We propose a mixed approach encompassing at first single-station seismic noise measurements, to detect sudden lateral variations in the bedrock surface in a fast and cost-effective way, which might serve as a proxy for the potential identification of fault zones. Then, more accurate electrical resistivity tomography investigations are carried out only at selected sites as indicated by the preliminary noise analysis, as electrical methods cannot effectively be employed at a large scale for time and economic limitations. Surface-wave dispersion analysis is jointly interpreted together with ambient noise data to improve the seismic characterization of the alluvium, giving further insight on the assessment of the depth to bedrock. The proposed approach can be an effective way to manage and investigate a large portion of the territory within a sensible budget, as commonly needed in seismic hazard assessment and microzonation studies. We present a real-world application to the San Vittorino Plain (Central Italy) close to the epicentre of the 24 August 2016 Amatrice earthquake, where the geological faulted bedrock is covered by alluvial sediments of the Velino River up to a maximum estimated thickness of 150–200 m. Although engineered for the post-earthquake reconstruction emergency, the approach employed in our study can be adopted in other areas of similar geology, to ease the application of seismic microzonation in time of seismic silence as a tool for long-term land planning and management.