Dual Role of Fluids in Activating Extensional Seismicity at Middle and Lower Crust Depth: the Deformation Corridor Throughout the Southern-Central Apennines (Italy)

Our study extensively explores the analysis of successive swarms and seismic sequences that followed the 2009 L'Aquila mainshock (Mw 6.3) in the southern-central Apennines of Italy—a region historically recognized for seismic events, some reaching ~M7.The study area is characterized by active fault alignments well-documented in the geological literature. Bounded eastward by the Fucino-Marsicano-Barrea SW-dipping faults and westward by the southern termination of the Ernici NE NNE-dipping fault systems, it is also crosscut by the Villavallelonga-Pescasseroli–San Donato-Val Comino and the innermost right-stepping en-echelon Balsorano-Posta Fibreno sets, as well as the Sora fault.Despite its active fault systems, the study area exhibits a low level of seismicity, prompting extensive investigation in previous experiments conducted by temporary seismic networks (Bagh et al., 2007; Romano et al., 2013) and more recent contributions by Frepoli et al. (2017). Notwithstanding, the paucity of seismicity, coupled with the deep geometry of faults and their association with historical earthquakes, remains a topic of ongoing debate in the literature.Through the application of template matching techniques (Vuan et al., 2018) to enhance the available seismic catalog, we unveil previously undetected low-level seismicity with a completeness magnitude ML ~ 0.0. The space-time evolution of the intense seismicity, along with the characterization of each seismic episode, migration velocity, and Vp/Vs ratios, combined with the 3D distribution of seismicity and geological data, revealed both a clear tectonic influence and the role of fluids in seismic processes. This exploration illuminated previously unknown geometric aspects and provided the first evidence of the WNW-ESE deformation zone in the southernmost segment of the Villavallelonga fault at depths ranging from 11 to 15 km. Our results indicate that deeper seismicity (>16 km) suggests an ascending trend of possible mantle-derived CO2.These findings significantly contribute: (1) to the understanding of tectonic seismic swarms in extensional domains, (2) to provide insights into fluid involvement in seismic processes, and (3) to the discussion of the seismotectonic setting in high seismic-risk areas. The acknowledgment of the value of microseismicity for regional seismotectonic studies serves as a compelling conclusion, underscoring the broader significance of these findings. Bagh, S., Chiaraluce, L. et al. 2007. Background seismicity in the Central Apennines of Italy: The Abruzzo region case study. Tectonophysics, 444,80–92.Frepoli, A, Cimini, G.B et al., 2017. Seismic sequences and swarms in the Latium-Abruzzo-Molise Apennines (central Italy): New observations and analysis from a dense monitoring of the recent activity, Tectonophysics, 312-329.Vuan, A., Sugan, M., et al., 2018. Improving the Detection of Low‐Magnitude Seismicity Preceding the Mw 6.3 L’Aquila Earthquake: Development of a Scalable Code Based on the Cross Correlation of Template Earthquakes. BSSA 108, 471–480.Latorre, D., Di Stefano, R., et al., 2023. An updated view of the Italian seismicity from probabilistic location in 3D velocity models: The 1981–2018 Italian catalog of absolute earthquake locations (CLASS): Tectonophysics, 846, 229664.Romano, M. A., de Nardis, et al., 2013. Temporary seismic monitoring of the Sulmona area (Abruzzo, Italy): A quality study of microearthquake locations. NHESS, 13, 2727–2744.