GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Monitoring tectonic environments with DAS: the case study of the Irpinia Near Fault Observatory G. Festa 1,2 , C. Strumia 1 , F. Scoto di Uccio 1 , A. Trabatoni 3 1 Department of Physics ‘Etore Pancini’, University of Napoli Federico II, Italy. 2 Isttuto Nazionale di Geofsica e Vulcanologia, Italy. 3 Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, IRD, Géoazur, France. DAS systems are interrogators connected to a terminaton of a fber optc dark cable, that sense the fber by sending a laser pulse and recovering the back-scatered light. When the fber is at rest, the patern of the light is the same afer each pulse; during the occurrence of an earthquake, because of the deformaton of the cable, the backscatered light changes its patern and the resultng phase diference can be connected with the strain rate along the directon of the cable (Hartog, 2017). Thus, interferometric analysis provides tme series with spatal sampling along the fber even of few meters, making the DAS recording similar to seismic arrays, with the main advantages of being cheaper, easier to plugin into available dark fbers, and working in harsh environments, such as at the seafoor or in volcanic areas (e.g., Sladen et al., 2019; Current et al., 2021). On the other hand, DAS systems collect a huge amount of data that require proper management and advanced processing tools. We tested the DAS system along a short cable in the Irpinia Near Fault Observatory (INFO), an on- feld laboratory of 31 seismic statons that monitors the seismicity along the Campania-Lucania Apennines (Iannaccone et al., 2010; Chiaraluce et al., 2022), across the fault system that generated the 1980, M 6.9 Irpinia earthquake. INFO has been operatonal in the last 15 years and has recorded more than 3000 earthquakes, with magnitude of completeness of 1.1 (Vassallo et al., 2011). We performed a DAS survey lastng 5 months along a 1.1 km-long fber, buried in a dry lake at depths ranging from 30 cm to 1.5m, collectng contnuous strain rate data at a spatal sampling of 2.4 m and sampling frequency of 200 Hz. The system recorded several earthquakes spanning diferent magnitude and locaton, providing an inital dataset to investgate DAS records and related source parameters (Trabatoni et al., 2022). This experiment also provided the basis for the future installaton of a long-term monitoring system, grounded on 3 DAS interrogatng fbers of diferent extent across the near-fault observatory. Seismic phases related to earthquakes were analyzed with the help of seismic refracton experiments and numerical simulatons. In this partcular context, DAS records appear to be mainly sensitve to waves guided horizontally by the low-velocity structure beneath the site. Because of the near-to-vertcal incidence of waves, seismograms appear to be depleted of P waves, and dominated by low velocity S and surface waves. Moreover, the wave patern is similar for all