Shallow crustal imaging with distributed acoustic sensing (DAS) offshore central Chile

In subduction zones, shallow crustal faults accommodate a fraction of the tectonic deformation and modulate the distribution of marine sediments. The interplay between active faults and sedimentary basins influences seismic hazards and the potential for submarine landslides. Imaging these active structures is crucial for constraining their geometry, physical properties, and contributions to regional geodynamic processes. Distributed acoustic sensing (DAS) offers an opportunity to passively image shallow offshore sediments at high spatial resolution, by converting preexisting submarine telecommunication cables into dense seismic arrays. Using hundreds of kilometers of submarine fibers, DAS enables ambient seismic noise tomography with resolutions of a few hundred meters near the coastline, sufficient to resolve detailed sedimentary velocity structures beneath the cables. Beyond velocity imaging, identifying strong impedance contrasts enables the localization of diffracting structures, such as faults and sedimentary basin edges.In this study, we present a set of complementary imaging approaches based on both ambient noise and earthquake records, which we use to investigate shallow marine sediments offshore central Chile. Our analysis uses over two years of continuous DAS recordings from three submarine cables located within the study area. Firstly, we apply wavefield separation to the earthquake recordings within a local back-projection framework in order to image fault-related structures at sub-kilometer scales, identifying scattered wavefields that are consistent with fault zones intersecting the cables. Secondly, we use the autocorrelation and cross-correlation of ambient seismic noise to image strong impedance contrasts and reveal sedimentary basin edges along the three cables. Thirdly, we analyze high-resolution power spectral density using earthquakes, ambient noise, and autocorrelation functions to investigate the relationship between high-frequency resonances, shallow sedimentary deposits, local attenuation, and basin-edge effects. These are all key factors in quantifying site response offshore. Finally, we validate our interpretations using numerical wave propagation simulations, which show good agreement with the observed DAS data.Together, these methods reveal sedimentary accumulations within basins and fault-related structures that are consistent with regional geological constraints. Although variability in coupling and the use of two-dimensional models limit full structural characterization, our results demonstrate the ability of DAS to resolve fine-scale offshore structures and highlight its potential for studying offshore faulting, sediment dynamics, and site effects along the central Chilean margin.