Harnessing converted phases for rapid magnitude estimation and early warning with distributed acoustic sensing offshore Chile

Abstract Distributed Acoustic Sensing provides a transformative view of seismic wavefields, offering spatially continuous recordings that enable tracking of seismic phases along fibre optic cables. Offshore, fibres often cross sedimentary layers that generate phase conversions dominating early waveforms, increasing signal complexity and hindering rapid magnitude estimation for Earthquake Early Warning. Here, we analyse the peak amplitudes of seismic signals in a wide magnitude range (2.5≤ M ≤7.4), recorded by three interrogators along a 400km-long fibre array offshore Chile. We show that the direct P-phase is quickly dominated by secondary phases, limiting its use for source size estimation of moderate-to-large earthquakes. Conversely, we report that converted P-to-S waves carry a signature of the source similar to that of the S-phase within a few seconds from the first arrivals, offering a valuable proxy for rapid magnitude estimation. This finding reinforces the potential of Distributed Acoustic Sensing for Early Warning when sensing offshore cables.