Accounting for shallow sedimentary layers for accurate earthquake localization using submarine Distributed Acoustic Sensing

Distributed Acoustic Sensing (DAS) technology facilitates the instrumentation of areas that are challenging to access with conventional instruments. In Chile, the presence of offshore submarine telecommunication cables offers a unique opportunity to instrument a major subduction zone close to the trench. Here we report an analysis of DAS data collected during a one–month campaign, sensing a commercial telecom cable connecting Concón to La Serena positioned several dozen kilometers off the coast.  

The earthquake recordings displayed P and S arrivals along with an additional Ps arrival, which is the result of the conversion of the P-wave at the bedrock/sediment interface. These three phase arrivals were identified and manually picked taking advantage of the spatial continuity of DAS measurements. To correctly account for the presence of the sediment layer in the localization procedure we introduced sedimentary corrections, which are a modification of the conventional station corrections. Instead of introducing an arbitrary constant time delay for each station and each phase, the corrections are derived from a physical first order modeling of the wave propagation in the sediments. The estimation of sedimentary parameters relies on: (i) the observed delay between the transmitted P-phase and the converted Ps-phase that give an indication of the sediment thickness; (ii) an inversion of the P- and S-wave speed in the sediments which is made possible thanks to the high sensor spatial density.   

We show that sedimentary corrections: (i) can represent most of the observed pick residual bias while only requiring the inversion of two global parameters (compared to station correction that requires three parameters per station); (ii) allow one to retrieve the sediment thickness and wave speed values that are consistent with common values for sediments; (iii) reduces the residuals of the earthquake hypocenter localization. The proposed correction method should improve the hypocenter estimation quality, facilitating the analysis of geological structures, and will contribute to a more detailed view of seismic activity in the studied area.