Multi-fiber Distributed Acoustic Sensing for Urban Seismology in Athens, Greece

Distributed Acoustic Sensing (DAS) offers a promising approach for dense seismic recording in urban environments by repurposing existing telecommunication infrastructure. Athens presents an ideal setting for such an approach, as Greece is one of the most seismically active countries in Europe, and the Athens metropolitan area — home to nearly four million inhabitants — lies within a geologically complex basin whose vulnerability was demonstrated by the destructive 1999 Mw 5.9 Parnitha earthquake. Seismic hazard assessment requires accurate subsurface velocity models, but acquiring the data to build them in dense urban areas remains challenging.We present results from a multi-fiber DAS experiment conducted in Athens, Greece, from 16 May to 30 June 2025, using four telecommunication fibers provided by the Hellenic Telecommunications Organisation (OTE). Two Sintela ONYX interrogators simultaneously interrogated the four fibers, which fan out from an OTE building with lengths of approximately 24, 38, 42, and 48 km, providing extensive azimuthal coverage of Athens. This makes the study one of the largest urban DAS campaigns ever performed.Data were acquired in two configurations, a lower spatial resolution mode optimised for earthquake recording (~26 days) and a higher resolution mode for ambient noise interferometry (~19 days). To detect seismic events, we applied bandpass filtering followed by phase-weighted stacking across channels to enhance coherent arrivals. An STA/LTA (short-time average/long-time average) trigger was then used to identify candidate events. During the acquisition period, the National Observatory of Athens (NOA) recorded 2,645 events across the broader seismic network, of which 548 were detected on at least one fiber (368, 343, 328, and 322 on fibers 1–4, respectively). Detection capability depends on distance and magnitude — we achieve near-complete detection within ~20 km, while many events of ML ≥ 2 were recorded at distances exceeding 200 km. The array also captured small local events absent from the NOA catalogue, likely corresponding to local seismicity below the detection threshold of the sparser regional network. Characterising this unobserved local seismicity is one of the objectives of ongoing work.For events within 50 km of the interrogator site, we pick P- and S-wave arrivals to constrain body-wave travel times. These picks are used to locate events in the NOA catalogue, which enables us to compare with network-derived hypocentres and allows us to assess potential improvement from the dense DAS coverage, before applying the approach to smaller events detected only by DAS. The travel-time data will also serve as input for 3D eikonal traveltime tomography to image subsurface velocity structure beneath metropolitan Athens.