Imaging the Sediment Cover Offshore Central Chile With Surface-Wave Dispersion and P-Wave Conversion Using Distributed Acoustic Sensing

Distributed acoustic sensing (DAS) is an attractive solution for ocean-bottom seismological instrumentation, providing dense and long-range measurements of ground deformation along submarine fiber optic cables. By sensing a 145 km telecom cable in central Chile, we determined the sedimentary cover structure leveraging both surface wave dispersion and the time difference of arrival ($\Delta t$) between the transmitted P and converted S waves from the bedrock-sediment interface. We estimated the shear wave velocity (Vs) profile and sediment thickness by performing a joint inversion of Scholte-wave dispersion curves and $\Delta t$ along the fiber. The dispersion curves were extracted from the seismic coda of local and regional earthquakes using frequency-wavenumber analysis. The additional information provided by the observed $\Delta t$ improved depth inversion constraints, enhancing the stability of the retrieved sedimentary cover thickness. Our results show significant variations in thickness and Vs along the cable, providing new insights into the structure of sedimentary deposits along the margin. The influence of tectonic structures, such as faults and basins, is reflected in the observed velocity contrasts and sediment accumulation patterns. Southern structures, such as the Valparaiso Forearc Basin and canyons, are not characterized by significant sedimentary deposits (<500 m.s⁻¹). Beyond the Punta Salinas Ridge, the sediment velocity increases slightly while crossing basin-like features. The proposed methodology, combining surface waves dispersion and P-to-S conversion, confirms the effectiveness of DAS for high-resolution subsurface imaging. These results highlight the importance of accounting for spatial variability in sedimentary velocity when modeling seismic hazards and tectonic dynamics in subduction zones.