Automatic Earthquake Catalogs From a Permanent DAS Offshore Network

Applying Distributed Acoustic Sensing (DAS) to sense offshore telecom cables offers a unique opportunity to expand permanent seismological networks far offshore with an unprecedented instrumentation density but presents challenges in managing and processing data. This study introduces a workflow for automating earthquake catalog generation by combining DAS arrays with regional on-land seismic networks. We apply it to the ABYSS offshore permanent observatory in central Chile, which began continuous recording in October 2023 on three 150-km segments of a regional dark-fiber telecommunication network. Our workflow includes state-of-the-art components originally developed for conventional seismic sensor data. We first show that PhaseNet, pretrained on conventional seismic stations, can perform automatic phase picking on DAS strain-rate data converted to velocity. In a second step, we demonstrate that the GaMMA associator, tuned with parameters adapted to our dense data set, can cluster picked arrivals of small and moderate-sized earthquakes. We then adapt NonLinLoc to compute hypocenter locations using numerous phase picks, combining DAS and onland seismic data for improved accuracy. Finally, we show that reliable local magnitudes can be estimated on DAS recordings using the Richter scale. The earthquake catalog is stored in an SQL database using QuakeML formalism and is accessible via a user-friendly webpage. By applying the workflow to onshore station recordings, we evaluate the benefits of adding permanent seafloor DAS instrumentation, demonstrating that offshore DAS significantly improves the detection of small-magnitude earthquakes and reduces the offshore magnitude of completeness to 1.3.