GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 91 the complexity of the fault zone. Vp and Vs models exhibit a lateral velocity variation in correspondence of the fault. Near-surface soils (Vp < 400 m/s) thicken just ahead the fault scarp and a low-velocity wedge (Vp < 700 m/s) is imaged in the shallow portion of the hanging wall. On the Vp models, the fault zone is characterized by a relatively high-velocity (1400-1500 m/s) body that dips SW. HighVp/Vs ratios (> 3) and the derived Poisson’s coefficient indicate that this region represents a nearly-saturated medium, probably related to the increase of permeability within the fault zone due to fracturing and shearing. This appears in agreement with the low- resistivity signature in the ERT surveys by Villani et al. (2017), which hypothesized a fluid- filled fault zone made of fine granular sediments. Conversely, the low-velocity (Vp < 700 m/s) shallow wedge imaged down to 10-15 m to the west of the fault-zone is associated to low Vp/Vs ratios (around 2) and high resistivity values. We relate this low-Vp, low-Vp/Vs zone to loose/ compact sand and gravel with low saturation degree that fill the fault hanging-wall. The results from the K-means cluster analysis also identify a homogeneous cluster in correspondence of the saturated fault zone. Acknowledgements. We thank Vincenzo Sapia for the ERT data processing and Stefano Bernardinetti for his precious comments during the cluster analysis processing. This survey was financially supported by the INGV Project MIUR-FISR 2016-2017 4-D - Structure of Central Italy and active geodynamics process - Task1: 3-D Structure of Central Italy . Fig. 3 - A) Line drawing and geological interpretation of depth converted reflection section; B) Vp tomographic model from SIRT algorithm.