T-wave localisation with offshore Distributed Acoustic Sensing arrays

Offshore earthquake pose significant hazard to coastal communities, both in the form of strong ground motions induced by seismic waves and the potential triggering of tsunamis. Because seismic energy rapidly attenuates with increasing distance, damaging ground motions are typically restricted to the near-epicentral area. On the other hand, tsunamis can travel vast distances, crossing entire ocean basins and posing hazard far beyond the epicentral region. From the perspective of tsunami alerting, it is challenging to accurately detect and characterise distant seismic sources to evaluate whether a tsunami could have been triggered. A possible solution to this, is to leverage the recordings of seismo-hydro-acoustic signals known as T-waves. Like tsunami waves, T-waves experience minimal attenuation, and so they can be clearly recorded over vast distances. In this study, we use seafloor fibre-optic cables combined with fibre-optic sensing (Distributed Acoustic Sensing; DAS) to detect T-waves and to locate their origins using array processing techniques. We demonstrate this principle with fibre-optic cables located off the southern French shore, analysing T-waves produced by earthquakes offshore Algeria. While DAS-based T-wave analysis does not replace conventional tsunami alerting systems, it can make a substantial technological contribution at practically zero deployment and maintenance cost.