Earthquake size from first seconds of DAS records

While several studies have shown the possibility to estimate earthquake magnitude from analysis of the S-phases recorded along fiber optic cables using Distributed Acoustic Sensing (DAS), understanding the source information hidden in the first seconds of these seismic recordings is still an open question requiring further investigation. In fact, in the case of submarine cables, with the fibers located closer to the epicenters, DAS could also be an asset for Early Warning of offshore earthquakes. 

In this study, we explore the possibility of measuring the size of the earthquake from the first few seconds of the signal received by the DAS by relating measurements of peak amplitudes to earthquake magnitude. We analyze a dataset of over 100 events (2.5 < M < 7.4) recorded along three submarine dark fibers running parallel to the Chilean margin, between Concon and La Serena, forming an approximately 450-km-long linear array.  

Unfortunately, this sensing technique suffers several limitations that complicate the recording of the first direct seismic arrivals. These include a lower signal-to-noise ratio (SNR) compared to traditional seismometers, and a high sensitivity of the measured parameters to local medium heterogeneities. Additionally, the longitudinal sensitivity of DAS makes it challenging to detect P-waves when using horizontally deployed cables, which are common when telecom fibers are utilized. Finally, modern DAS interrogators struggle to record strong ground motions due to phase wrapping of the backscattered light within the fixed interval [-π, π], resulting in the saturation of DAS recordings during intense shaking. 

Despite these challenges, we show that the P wave is poorly informative about the seismic source due to the influence of a shallow sedimentary layer, which generates a dominant PS-converted phase in the early DAS data. However, we demonstrate that this converted phase can be effectively used to robustly estimate earthquake sizes up to magnitude 7 within few seconds from the recording of an event. Furthermore, we derive amplitude attenuation laws as a function of distance and magnitude, overcoming the limitations of saturation by leveraging records from large events occurring hundreds of kilometers away from the array. 

Overall, this work highlights the continued potential of DAS-based seismic monitoring infrastructures while providing valuable insights for the development of a new generation of DAS-based Earthquake Early Warning Systems.