Observations of earthquakes on multiple spans of the IRIS subsea fiber-optic cable connecting Iceland and Ireland

 

About 70% of the Earth's surface is covered by ocean, making observations of seismic waves difficult and expensive in vast regions.  Recently, seismic waves have been observed using transmission fiber-optic sensing on trans-oceanic subsea cables, making them an exciting potential addition to global seismic networks (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 (typically 100 km) along the entire cable length, with each span acting as an effective sensor.  This division allows for a separation of signals from different parts of the cable, leading to higher S/N ratios on quiet deep-water segments and more easily interpreted waveforms.  We show signals from several large earthquakes registered on 17 spans of the IRIS subsea telecommunications cable, that connects Iceland and Ireland.  Multiple seismic body-wave phases, as well as longer period surface waves, can be tracked across the spans, or the array of effective sensors.  Comparing the observed phase arrivals to those predicted by travel-time curves, we show how different phases are visible on different segments as expected by the varying orientation of individual segments relative to the seismic wave field.  Furthermore, we simulate the observed waveforms using a spectral-element wave propagation software (SPECFEM3D_GLOBE), following the theoretical development of Fichtner et al 2022, and compare to the observed waveforms.