Observations of P-wave phase arrivals from major earthquakes with the IRIS fiber-opticsubsea cable connecting Iceland and Ireland

Approximately 70% of the Earth's surface is covered by the oceans. Installing permanent seismic sensors on the seafloor is both difficult and costly, resulting in a gap in the the global seismic monitoring. Recently, the potential of using the existing network of submarine fiber-optic cables for the observation of seismic waves has been investigated, several works demonstrating the feasibility of using trans-oceanic subsea cables as seismic sensors (f.ex. Marra 2018,2022; Zhan, 2021; Mazur 2024). In this work, we use a distributed fiber optic sensing (DFOS) prototype capable of measuring the integrated strain between each repeater along the entire length of a fiber optic cable (the repeaters are typically placed 100 km from each other). This instrument is used on the IRIS telecommunication cable, an operational subsea cable connecting Iceland to Ireland, transforming 17 spans of the cable into an array of 17 individual seismic sensors. Signals from several large earthquakes can be observed on the recorded data from the cable and surface waves as well as multiple seismic body wave phases can be tracked across the spans.

To assess the capability of the monitoring system to detect the P-wave phase, we use an STA-LTA algorithm to automatically detect the arrival of P-waves in the data and we compare the phase detections to those predicted by travel-time curves from a catalogue of major earthquakes from the USGS database (with a magnitude above 6 and a distance of 30 to 100° from the fiber-optic subsea cable). We manage to retrieve 40% of our earthquake catalogue with our detection algorithm. However, a large part of our detections are not pickings of the P-wave phase.