A workflow for building an automatic earthquake catalog from DAS data recorded on offshore telecommunications cable in central Chile

Distributed Acoustic Sensing (DAS) is of critical value for the offshore expansion of seismological networks. The work presented here is part of the 5-years ERC ABYSS project, which aims at building a permanent seafloor seismic observatory leveraging offshore telecommunication cables along the central coast of Chile. 

The ABYSS project near-real time data collection started the 30^th of September 2023 using three ASN (Alcatel Submarine Networks) OptoDAS units to sense three segments over two offshore telecommunications cables connecting the cities of Concón to La Serena and La Serena to Caldera. The DAS data covers over 500 km of cable, comprising 26,664 virtual sensors sampled at 62.5 Hz and 100 Hz. These data are synchronized once a day with a storage server located in France, the volume of which is anticipated to reach an estimated 608 TB by the end of the project. We developed an automatic workflow to detect an average of 100 daily local, regional and teleseismic events with magnitudes down to ML = 0.5, over 59 GB of data per day after compression.

As a first step, we perform automatic seismic phase arrival picking using PhaseNet pretrained on conventional seismological stations, followed by phase association with GaMMA. We then apply a correction of the phase picks to account for shallow sedimentary layers and invert for the event hypocenters with NonLinLoc software. Finally, we estimate the Richter local magnitude based on peak ground displacements. The results show that DAS data combined with data from the national onland seismic network greatly increases the accuracy of the earthquake hypocenters. Once the earthquake catalogs are built, we can perform a relative relocation of the earthquakes with HypoDD software using cross-correlation and/or the catalog results. With this workflow we show that conventional tools used in seismology can be used on DAS data with few adjustments. Furthermore, the size of our catalog, enriched with numerous undetected offshore events is a significant improvement over the existing regional catalogs, which may aid future studies of the Chilean margin subduction zone seismicity.