Redistribution of Basal Forces and Shielding Model for Seismic Signals of Debris Flows Over Loose Thin‐Layer Sediments

Abstract Debris flows over riverbeds generate intense basal force fluctuations that radiate seismic signals, offering a key tool for remotely monitoring their dynamics. In steep, highly erosive mountainous channels, bedrock is often covered by a thin layer of loose sediments, which significantly reduce seismic energy. In this study, we developed an extended model for seismic signals of debris flows over thin‐sediment bed, explicitly accounting for the redistribution of basal forces of debris flows by these loose sediments. Integrating large‐scale flume experiments with discrete element simulations, we quantitatively revealed the shielding mechanisms of thin‐layer sediments on basal fluctuating forces and seismic signals, and clarified the force‐chain evolution and energy dissipation within the granular medium during the transmission of basal impact forces from the sediment surface to the underlying bed at the particle scale. The applicability of the extended model was additionally validated through a field monitoring case. Compared to bedrock, sediment layers attenuate basal force fluctuations and shift seismic signals to lower energy. The normalized intensity of basal force fluctuations increases linearly with the dimensionless sedimentary grain size, whereas the normalized seismic power follows a parabolic increase. Unlike conventional views that treat sediments as attenuating media along wave propagation paths, we interpret loose thin‐layer sediments as an attenuation layer that reduces the transmission of debris‐flow impact forces to the underlying bedrock. This study not only reveals the microscopic mechanism by which thin‐layer sediments shield seismic signals but provides a theoretical basis for inferring debris‐flow dynamics from seismic signals in complex natural environments.