Analysis of possible relations between deep fluid diffusion and seismic sequences in Central Italy

Seismic sequences are a complex phenomenon and the processes governing their evolution continue to be actively investigated. At the base of their investigation, three main triggering mechanisms are believed to govern the spatio-temporal evolution of seismic events: static triggering (e.g., Stein & Lisowski, 1983), dynamic triggering (e.g., Gomberg et al., 2001) and fluid diffusion triggering (e.g., Noir et al., 1997, Shapiro et al., 2003). For the North-Central Apennines, the presence of over-pressurized fluids (CO2) at seismogenic depths is reported by many studies (e.g., Baccheschi et al., 2020; Chiodini et al., 2021), and also different studies (e.g., Antonioli et al., 2005; Cabrera et al., 2022; Malagnini et al., 2012) investigate whether it is possible to correlate pore fluid diffusion with the two major seismic sequences that during past times struck the Central Italy, the 2009 L’Aquila sequence and the 2016-2017 Amatrice-Visso-Norcia (AVN) sequence. Here, I wanted to push further this approach in comparison with previous studies by also modeling the anisotropic diffusivity and the centroid migration pattern. I used the isotropic/anisotropic diffusivity and the centroid migration to understand the relation between the diffusive process, the spatio-temporal evolution of seismicity and the control played by the geological setting and the geometry and structure of the active fault systems. The results obtained from the analysis of the 2009 L’Aquila and the 2016-2017 AVN highlighted a contrasting behavior between the two seismic sequences. Both sequences resulted in high values of isotropic seismic diffusivity, ranging between 9 and 39 m²/s (isotropic permeability of 1.31e-12 – 4.64e-12 m²), but with an opposite response to the anisotropic diffusion modelling. The 2009 L’Aquila seismicity shows a general isotropic seismic diffusion pattern, with low difference between maximum and minimum anisotropic diffusivity and maximum anisotropic diffusivity of 7-169 m²/s, anisotropic permeability of 1.04e-12 – 2.45e-11 m². While the AVN shows a strongly anisotropic seismic diffusion pattern with the anisotropic diffusivity of 42-326 m²/s and anisotropic permeability of 6.10e-12 – 4.73e-11 m². For most of the aftershock sequences analyzed, the data fit is weak, with both isotropic and anisotropic permeabilities being extremely high and several orders of magnitude greater than typical rock permeabilities (10e-15 – 10e-19 m²), even for fractured rocks. This thesis provides a direct comparison between the two sequences, applying for the first time anisotropic diffusion models to the 2009 L’Aquila sequence, and centroid migration analysis to the 2016–2017 Amatrice–Visso–Norcia (AVN) sequence. These combined approaches have the potential to offer new insights into the complexity of earthquake triggering mechanisms in Central Italy. In particular, the analysis presented in this thesis suggests that fluid diffusion, if present, played a secondary role compared to static and dynamic stress transfer processes in the propagation of aftershocks.