Harnessing secondary phases for rapid magnitude estimation with DAS offshore Chile and its implications for Early Warning

Distributed Acoustic Sensing provides a transformative view of seismic wavefields, offering spatially continuous recordings that enable tracking of seismic phases along fiber optic cables. In offshore environments, fibers often cross sedimentary layers that generate phase conversions dominating DAS early waveforms, increasing signal complexity and hindering rapid magnitude estimation for Earthquake Early Warning. Here, we analyzed the peak amplitudes of seismic signals in a wide magnitude range (2.5≤M≤7.4), recorded by three DAS systems along a 400km-long fiber 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 few seconds from the first arrivals, offering a valuable proxy for rapid magnitude estimation. This finding reinforces the potential of DAS for Early Warning when sensing offshore cables.