Speaker
Description
Dense instrumentation is essential for urban seismic monitoring, yet deploying traditional seismometer networks within city environments poses significant logistical and financial challenges. Distributed Acoustic Sensing (DAS) offers an alternative by repurposing existing telecommunication infrastructure as continuous arrays of seismic receivers. Athens presents a particularly suitable study site: the Greek capital lies within a tectonically active sedimentary basin where more accurate subsurface velocity models are needed to better characterize local seismic hazard. We present a multi-fiber DAS experiment conducted in Athens in partnership with the Hellenic Telecommunications Organization (OTE), in which four fibers spanning 24 to 48 km from a central location were simultaneously interrogated using two units, achieving wide azimuthal coverage across the metropolitan area. This constitutes one of the most spatially extensive urban DAS deployments to date. Two sets of acquisition parameters were used, optimized for earthquake detection and ambient noise interferometry workflows. Event detection via phase-weighted stacking and STA/LTA triggering identified 548 National Observatory of Athens (NOA) catalogue events on at least one fiber, as well as numerous sub-catalogue events. P- and S-wave arrival times from ~29 NOA events are used as input to 3D Eikonal traveltime tomography, employing a second-order fast marching method with L-BFGS inversion and L2 damping regularization, yielding a refined velocity model of the Athens basin. This model is used to relocate NOA catalogue events, enabling an assessment of hypocenter constraint quality relative to catalogue solutions, and to locate DAS-only events. The newly located events are then incorporated into an iterative inversion to further refine the velocity model and improve tomographic resolution.