GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 89 >30 m total vertical displacement achieved since the Middle Pleistocene, based on the offset of electrical units across the VF fault trace, whereas a 2.3-2.8 m-high fault scarp at the surface was generated in the last ~12 kyr. Subsequent geophysical investigations by Villani et al. , (2019) point out a high structural complexity of the VF shallow subsurface and evaluate 100-120 m total throw accrued by the VF fault in the last 0.4 Myr. In this study, we investigate in detail the ultra-shallow structure of the VF fault, combining reflection seismology, seismic refraction tomography and multichannel analysis of surface waves (MASW) with the available electrical resistivity data from Villani and Sapia (2017). The seismic survey. The seismic profile was acquired across the coseismic surface rupture of the VF fault and coincides with the ERT profile acquired by Villani and Sapia (2017). The profile was acquired using a 5 kg hammer source; at each shot point we stacked 2 blows and source move-up was 2 m. The seismic data were recorded by fixed array of 96 vertical geophones with a 4.5 Hz eigen-frequency, placed at 2 m intervals along the 190-m-long dense, wide-aperture seismic profile. Seismic data analysis and inversion. After a pre-processing flow, aimed at improving resolution and signal/noise ratio of seismic reflection data, the picking of the maximum of semblance on Common Mid Point (CMP) super-gathers was used in our attempt to define a velocity model (VNMO) for CMP ensemble stack. The final stack velocity macro-model (VNMO) from the CMP processing was smoothed and used for post-stack depth conversion. Two different seismic refraction analysis was performed. The input data consists of 8640 first arrival travel-times handpicked on 90 Common Shot Gathers (CSG) and checked for consistency according to reciprocity rules. We applied a non-linear multi-scale tomography that does not require a starting reference model and is able to cope with very heterogeneous media as fault zones and tectonic basins (Improta et al. , 2002, 2003, 2010; Improta and Bruno, 2007; Fig. 1 - A) geological sketch map of the Pian Grande di Castelluccio basin (modified after Pierantoni et al. 2013). The violet rectangle encloses the survey area showed in figure 1B. B) Detail of the seismic survey.