GNGTS 2022 - Atti del 40° Convegno Nazionale

34 GNGTS 2022 Sessione 1.1 INFLUENCE OF FLUID MIGRATION AND FRACTURING IN THE CENTRAL ITALY SEISMIC SEQUENCE FROM SCATTERING ATTENUATION AND INTRINSIC ABSORPTION S. Gabrielli 1 , A. Akinci 1 , F. Napolitano 2 , E. Del Pezzo 3,4 , L. De Siena 5 1 Istituto Nazionale di Geofisica a Vulcanologia, Roma, Italy 2 Università degli Studi di Salerno, Dipartimento di Fisica “E.R. Caianiello”, Fisciano, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy 4 Istituto Andaluz de Geofisica, Universidad de Granada, Granada, Spain 5 Institute of Geosciences, Johannes Gutenberg University of Mainz, Germany Quantifying seismic attenuation parameters in the crust, such as scattering and absorption, is fundamental in understanding the behavior of seismic events and, therefore, seismic hazard assessment. Between August and October 2016, the Central Apennines (Italy) was struck by a cascading seismic sequence known as the Amatrice (Mw 6.0) - Visso (Mw 5.9) - Norcia (Mw 6.5) sequence, which has been considered connected to the fluid migration in the faults network, as suggested by previous studies (Miller et al. , 2004; Di Luccio et al., 2010; Malagnini et al., 2012; Chiodini et al., 2004, 2020; Chiarabba et al. , 2020b). Seismic attenuation imaging is a powerful tool to define the influence and abundance of fluids in a seismic sequence. Indeed, an increase in seismic attenuation can be associated with high fluid content and enhanced of permeability and fracturing. Here, we present the separation of scattering and absorption contribution to the total attenuation of coda waves of the Central Italy seismic sequence, providing 2D images at different frequency bands of these quantities. We used two datasets: the first one comprising ~3100 waveforms recorded before the sequence (March 2013-August 2016) and the second one comprising ~13900 waveforms (ML > 2.8) from the Amatrice-Visso-Norcia sequence. The 2016-2017 sequence was further divided into three-time phases, each one related to a mainshock: 1) Amatrice sequence, including ~3600 waveforms recorded between 24/08/2016 and 25/10/2016 2) Visso sequence, including ~1600 waveforms recorded between 26/10/2016 and 29/10/2016 3) Norcia sequence, including ~8500 waveforms recorded between 30/10/2016 and 18/01/2017 The subdivision of the seismic sequence can highlight changes in attenuation in space and time, allowing us to interpret the possible patterns of fracture development and fluid migration in the seismogenic zone. Methods. Scattering (Q s ) and absorption (Q i ) can be mapped in space using two seismic attributes: 1) Peak delay time, defined as the time difference between the S-wave arrival and the maximum amplitude of the event (Takahashi et al. , 2007); 2) Coda-waves attenuation (Q c -1 ), where coda waves are the wave trains following the S wave. When coda waves enter the diffusive regime (at late lapse times), coda attenuation can be considered equal to absorption (Qc ≈ Qi - Shapiro et al. , 2000) Peak delay is associated with random inhomogeneities, measuring multiple forward scattering. One of the first applications of peak delay was in volcanic areas (Northeast Japan - Takahashi et al. , 2007). In tectonic zones, such as the Pyrenees (Calvet et al., 2013) and the Vrancea region (Romania - Borleanu et al. , 2017), high peak delays detected fractured volumes. High peak delay values are also visible in the carbonate successions of the Pollino seismic gap (Southern Apennine - Napolitano et al. , 2020).