GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.3 ______ ___ GNGTS 2023 Crustal Permeability Changes Inferred From Seismic Attenuation L. Malagnini, T. Parsons, and I. Munafò For over four decades, seismic attenuation in the shallow crust has been linked to fluid viscosity, and, at least indirectly, to rock crack density and bulk permeability (i.e. crack interconnection). Such a vision has never been challenged. In their seminal paper, which in turn was based on the theoretical results by O’Connell and Budiansky (1977, 1978), Winkler et al. (1979) wrote: “ … mechanisms such as fluid flow must dominate seismic attenuation in the upper crust. ”. More recently, Malagnini et al. (2022, 2019) and Malagnini and Parsons (2020) conceptually exploited the viscous dissipation model and proposed a method to investigate the time variability of seismic attenuation by directly linking their observations of time fluctuations of the attenuation parameter to fluctuations in rock bulk permeability. They based their conclusion on: i) −1 ,( ) ( ) the work by Roeloffs (1998), who interpreted in the same terms the variability in the water level of a well near Parkfield induced by a suite of regional events; ii) laboratory experiments (Liu and Manga, 2009); iii) numerical simulations (Barbosa et al., 2019); iv) on their own observations that the same earthquakes mentioned in Roeloff’s paper induced sudden changes of the seismic attenuation at shallow crustal depths. Here we measure variations of seismic attenuation over time, providing unique insights into the dynamic state of stress in the Earth’s crust at depth. We analyze two different datasets from two energetic seismic sequences: the one that struck the Central Apennines in 2016-2017, and the one occurred at Ridgecrest, California in 2019. Both datasets were high-quality enough to allow us to obtain high-resolution time histories of seismic attenuation (frequency band: 0.5-30 Hz). The Central Apennines In the Central Apennines, negative dilatation caused by cumulative earthquake stress drop reduced both permeability and seismic attenuation, whereas strong-motion surface waves produced an increase in crack density at shallow depths, as well as in permeability and seismic attenuation. In the aftermath of the multiple main shocks of the Italian sequence, we find that occurrences of M ≥ 3.5 earthquakes vs. time and distance are consistent with fluid diffusion; in fact, diffusion signatures may be associated with changes in seismic attenuation during the first days of the Amatrice, the Visso-Norcia, and the Capitignano sub-sequences. In the immediate aftermath of a mainshock in Central Apennines, the competition between shallow rock damage and negative dilatation at depth affects the intermediate frequencies of the sampled bandwidth, where we observe a short-lived increase of the parameter , which is −1 ,( )