Enhancing Earthquake Location in the Central Apennines (Italy): A Hybrid Approach Combining Arrivals from Line-Sensor Telecom Fiber Interferometry and Traditional Point-sensors

The integration of existing telecommunication fiber-optic infrastructure into seismic monitoring networks offers a transformative opportunity to densify observations in seismically active regions. We present the results of a multi-year monitoring experiment (2021–2026) utilizing a 39-km telecom fiber link from the Italian telecommunication company Open Fiber between Ascoli Piceno and Teramo in the Central Apennines, Italy. The system employs an ultra stable laser to measure seismic-induced deformation of the fiber, operating on a dedicated wavelength in coexistence with commercial data traffic.A significant challenge in utilizing fiber-optic data for earthquake location is the transition from traditional point-sensor geometry to distributed sensing. To address this, we implemented a hybrid localization approach using a modified version of the NonLinLoc (NLL) algorithm. We move beyond traditional discrete measurements (point sensors) by treating the cable as a continuous "line sensor." Following the NLL algorithm, the most effective strategy is translating both point and line geometries into a unified framework of 3D travel-time maps. Once the sensors are translated into these maps, their combined use for location becomes independent of the sensor type, allowing for a seamless merging of traditional seismic station data and fiber-optic pickings. We applied this methodology to the real seismic catalog recorded from the fiber's installation in mid 2021 until January 2026 in the Ascoli-Teramo area, a region where the Italian seismic network is relatively sparse. Specifically, we analyzed signals from: 1) several small seismic sequences occurring at short distances (up to approximately 20 km) from the fiber cable, including the Civitella del Tronto (TE) sequence that followed a Mw 3.9 event (September 22, 2022); and 2) more distant earthquakes (ranging from approximately 20 to 50 km from the fiber) with local magnitudes exceeding ML 2.5, distributed along the Central Apennines axis. For events where the fiber signal allowed for the correct identification of P- and S-wave arrival times, we applied the NLL algorithm using the integrated network. In this work, we present several of these examples and associated tests to discuss how the inclusion of fiber-derived arrival times can provide further hypocentral constraints. This study aims to highlight the scalability of fiber interferometry combined with non-linear inversion as a robust tool for seismic surveillance in populated and high-risk tectonic environments.