GNGTS 2018 - 37° Convegno Nazionale

150 GNGTS 2018 S essione 1.1 Our goals are: 1) shallow characterization of fault zones bounding the PGC basin (and possible subsurface faults); 2) mapping of the pre-Quaternary carbonatic top-bedrock along a key transect crossing the surface ruptures of the Norcia earthquake (profile B-B’ in Fig. 1). Due to the complex tectonic setting and the >1 Myr-long extensional activity in the study area, we expect significant variations in the top-bedrock depth, which can be interpreted in terms of fault throws. We adopted ERT as a high-resolution geophysical technique capable of providing accurate images of fault zones. We assume that the top-bedrock is in most cases characterized by markedly contrasting properties with respect to the overlying continental infill (coarse-grained alluvial to fine fluvio-lacustrine sediments). Therefore, it can be detected as ( i ) a sharp electrical resistivity change between a deeper highly-resistive basement and a relatively conductive cover and/or ( ii ) a seismic impedance contrast between vertically stacked materials with different elastic moduli and seismic velocities (Di Giulio et al. , 2016). We calibrated our geophysical data with available shallow boreholes (Ge.Mi.Na., 1963) and geological data (Pierantoni et al. , 2013). Data analysis and interpretation. The three ERT profiles define the shallow structure of three important normal fault zones down to 50-100 m depth, and in particular: the two basin-bounding faults (F1 and F3), and the fault that ruptured the surface during the Norcia earthquake (VF). The TDEM surveys, performed using 50-m-sized square loop, provide 1-D resistivity models that help mapping the top-bedrock surface along two transects. They show important topographic changes that we relate to the presence of buried normal fault zones. The deepest part of the basin, between the inferred faults F5 and F6, is nearly 300 m. Those results are corroborated by the HVSR results. The estimated peak resonance frequency display changes in the 0.4-1.5 Hz range that closely follows the inferred trend of the top-bedrock depth. The results from shear wave splitting indicate the occurrence of two main fast axes directions, trending N150° and N30°, respectively. We hypothesize that the PGC basin structure is characterized by several fault splays distributed in a >3 km-wide deformation zone (Fig. 2). The easternmost splays of the VBFS that ruptured the surface during theAmatrice and Norcia earthquakes (Cordone del Vettore Fault) are Fig. 2 - A) 1-D resistivity curves, ERT sections and seismic stations used to constrain the geological section B-B’; b) interpretative geological section.