Surge-type debris flows propagate as a sequence of surges, forming gradually thickening in situ deposition layers between surges that dynamically alter channel-bed conditions. Seismic recordings from Jiangjia Ravine reveal a progressive attenuation of ground motion amplitude with surge sequence, despite comparable flow magnitudes-indicating a decoupling between flow scale and seismic response. We attribute this to the accumulation and liquefaction of inter-surge deposition layers, rather than pre-existing deposits. To quantify this mechanism, we adopt an effective transmission parameter (xi) within a fluvial seismology-based framework, and propose a sigmoid function linking xi to normalized deposition layer thickness (H*). This formulation significantly improves the prediction of seismic power spectral density (PSD) across surges and provides a transferable approach to characterize subsurface flow-bed interactions. Our findings underscore the critical role of bed structure evolution during flow in modulating debris-flow-induced seismic signals, with implications for real-time monitoring and early warning in sediment-rich catchments.