GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 Our 3D results show an apparent dissimilarity between the pre-sequence and sequence, where we can specify an increase of scattering with time after the mainshock attributed to tectonic features (e.g., thrusts and faults) and lithological differences. The significant transformations in scattering are observed around 4 and 6 km depth, west of the Norcia basin and of the Monti Sibillini thrust (MST - Figure 1), as can be interpreted that this thrust acts as a barrier between high and low scattering zones (Figure 1), where high scattering follows the MST thrust’s curvature at those depths. During the sequence, peak delay also shows a high scattering area corresponding to the Amatrice-L’Aquila zone, where a seismic sequence (Mw 6.3) occurred in 2009. The spatial variations are detecting another significant tectonic element, the Acquasanta thrust. However, this thrust, located on the east of Monti Sibillini, presents low scattering anomalies in its footwall during a pre-sequence epoch, generally in areas where a mainshock is going to occur (in the thrust roots), replaced by a high scattering anomaly during the sequence. The anomaly is also flat at the top, following the shape of the top of the Triassic deposits layer mapped by Buttinelli et al. (2021). Further in time, the propagation of scattering along the Acquasanta thrust suggests an increment of fracturing mainly due to the mainshock of Norcia. Overall, the main conclusions can be summarized from this study are as follows: 1) Change in space and time from low to high scattering can be interpreted as micro-fracturing processes in the tectonic areas during the seismic sequence, as also observed in the 2D images of Gabrielli et al. (2022). 2) Tectonic features, such as the Monti Sibillini and Acquasanta, act as structural and rheological barriers, creating contact between different geological domains (e.g., Umbria-Marche and Laga Formation domains). 3) The difference in lithologies causes a different response in fracturing and, therefore, in scattering pre- and during the sequences. 4) Low scattering can be ascribed to an increase in pore pressure associated with fluid circulation before the mainshocks, and that possibly triggered the events, creating fractures and increasing scattering afterward.