GNGTS 2017 - 36° Convegno Nazionale

150 GNGTS 2017 S essione 1.2 use of multi-attribute display, that provides unprecedent images, by combining physically independent waves quantities (e.g. amplitude, phase and frequency), that highlighted peculiar geophysical signatures of geological units and structural features. Results and conclusions. In this work, we provide some new seismic images that may contribute to better constrain the subsurface geological interpretation of the area affected by the destructive 2016-2017 earthquake sequence. We focused in particular on the sector affected by the Mw 6.5 mainshock. Some features embedded on the original seismic lines have been enhanced. A strong reflection generated by a sub-horizontal horizon below Castelluccio di Norcia/Mt. Vettore sector should correspond to the Top Basement (Porreca et al. , 2017). The attributes, computed on NOR3 (beneath Acquasanta and Montagna dei Fiori anticlines, Fig. 1a) and NOR2 (beneath Norcia and Castelluccio di Norcia basins, Fig. 2a), show clear low-angle, west- dipping, high reflective horizons, enhanced in particular by amplitude attributes (e.g. Energy, Fig. 1b). Such reflectors represent strong acoustic impedance contrasts, interpretable as well- known reference horizons in central Apennines (Carbonates and Marne a Fucoidi Fm., Porreca et al. , 2017). These reflector geometries help draw also the location of the low angle main thrusts. Even the location and geometries of the seismogenic normal faults can now be inferred more easily: high angles structures are shown close to the surface, as indeed visible on the outcrops. Then, their tracking in depth is aided by exploiting the discontinuity of some thin reflectors sets, limited in length and enhanced by attributes: such faults show a progressively angle decrease in depth (Porreca et al. , 2017). The computed attributes reveal unprecedent details on the organization of the fault zone patterns, not visible in the standard unmigrated and migrated data. Such features are particularly emphasized by the Cosine Phase and Pseudo- Relief attributes (Figs. 1c and 2b, overlapped in transparency on the standard data), as well as by the Energy attribute. Those fault zones are particularly evident on the sides of the two main basins (Norcia and Castelluccio di Norcia, details in Fig. 2d). Their display highlights high complexity due to minor subparallel fault patterns surrounding the main W-dipping structures and their antithetics. The frequency attributes highlight the depositional characteristics of the basins, and clearly display at high resolution the units infilling the two aforementioned basins. These are characterized by higher frequency contents mainly close to the surface (green colors on Fig. 2c), whilst more in depth the blue colors (low frequency content) are predominant, indicating massive bedding geometries (Top Basement). It is interesting to notice how the high frequency sectors of the basins are laterally limited by the normal faults alignments (e.g. in Fig. 2, mainly beneath the Castelluccio basin). Here, sharp lateral transitions to lower frequencies (blue colors) are apparent and enhanced by the Pseudo-Relief displayed in transparency. In Fig. 2 - Attribute analysis on the seismic line NOR2 (Norcia and Castelluccio di Norcia area); a) standard amplitude section; b) NOR2 Pseudo-Relief, displayed in transparency over the standard amplitude seismic; c) Envelope Weighted frequency displayed in transparency over the NOR2 Pseudo-Relief; d) details of Figs. 2b and 2c showing the Castelluccio Basin.