Liquefied Deposition Layers Modulate Seismic Wave Propagation in Surge-type Debris Flows

Surge-type debris flows advance through successive surges, during which in-channel deposition layers progressively develop between surges and continuously modify basal conditions. Seismic observations from Jiangjia Ravine show that ground vibration amplitudes systematically weaken as surge sequences evolve, even when successive surges exhibit similar flow magnitudes, implying a breakdown in the conventional scaling between flow intensity and seismic response. This phenomenon is interpreted as a consequence of the progressive buildup and partial liquefaction of inter-surge deposition layers, rather than the influence of static, pre-existing bed deposits. To represent this process quantitatively, we introduce an effective transmission parameter (ξ) into a fluvial seismology framework and establish a sigmoid relationship between ξ and the normalized thickness of the deposition layer (H*). Incorporating this relationship substantially enhances the ability to reproduce observed variations in seismic power spectral density (PSD) across surge sequences and offers a transferable means of capturing subsurface flow–bed coupling. These results highlight the importance of dynamic bed evolution in controlling debris-flow-generated seismic signals and provide new insights for improving real-time monitoring and early-warning strategies in sediment-laden mountain catchments.