Imaging Submarine Fault Zones Using Reflected S‐waves Extracted by Ambient Noise Interferometry From Distributed Acoustic Sensing

Abstract

The northern margin of South China Sea has experienced strong tectonic deformation since the Late Mesozoic, and formed a complex fault network. Along this margin, previous studies have mainly imaged offshore faults in the Shantou region at >30 m water depth. However, the precise fault location and geometry at <30 m water depth (nearshore) are poorly resolved due to lack of geophysical observations, preventing a better assessment of marine geohazards. Here, we retrieve high‐resolution zero‐offset S‐wave reflections from interferometric autocorrelation of ambient noise recorded by Distributed Acoustic Sensing (DAS) along a 29‐km submarine cable deployed in the nearshore. The meter‐scale reflections delineate stratigraphic boundaries, fault structures, and potential magmatic intrusions, where fault locations are validated by independent constraints including a 2D shear‐wave velocity model, spurious energy in the cross‐correlation functions, and earthquake wavefield analyses. We precisely locate the nearshore extension of the Changle‐Nan'ao Fault and resolve an ∼8‐km‐wide graben bounded by the Nanpeng Island Outer Fault and a newly mapped fault, beneath the graben the igneous intrusion is identified, indicating strong tectonic and magmatic interaction. This structurally weakened zone has localized extension which may contribute to seafloor instability. By using DAS ambient noise autocorrelation, we successfully retrieved geologically interpretable reflections, demonstrating the potential of using existing fiber‐optic cables to provide cost‐effective, high‐resolution structural constraints in complex tectonic settings.