Azimuthal sensitivity and spatio-temporal decimation of data from distributed acoustic sensing on submarine cables for offshore earthquake early warning systems.

Offshore earthquakes can cause widespread destruction due to directly propagating seismic vibrations and/or the generation of tsunamis. Early warning of offshore earthquakes is vital, but their location complicates identification of clear early warning (EW) signatures. Distributed acoustic sensing (DAS), applied to offshore fibre-optic cables offers the prospect to improve early warning of offshore events. We use a DAS dataset acquired from a 30 km submarine cable spanning a complex bathymetry, offshore Sicily, to contribute to a proof-of-concept using five regional earthquakes to investigate factors impacting incident body wave detection capabilities. We demonstrate observations for P- and S-waves incident parallel and perpendicular to the cable, and that cable coupling to the surrounding medium exerts far more control on signal quality than incidence azimuth. Compared to the nearest land station, an EW signal for a Mw5.8 earthquake was triggered 1.59 seconds earlier using P-wave arrivals on the fibre, and 3.89 seconds earlier using S-waves. We show that effective warnings can be made despite spatially decimating data by factors of 10 and 100 for P-waves and S-waves, respectively. This data volume reduction allows DAS on legacy seafloor cables to be integrated with EW systems for offshore earthquakes and other geohazards which threaten coastal populations.