Linking characteristics of debris flows to their high frequency seismic signature: insights from field measurements and model predictions

Measuring debris-flow properties remains a significant challenge in studies of natural hazards. Recent works suggest that the seismic signals generated by debris flows can help analyze flow dynamics, but theoretical details for estimating bulk flow properties from seismic signals are not fully understood or comprehensively tested. Here, we invert basal force fluctuations on the torrent bed using high frequency seismic signals generated by 6 well-documented debris flows at Illgraben, Switzerland. Verified against independent measurements, our seismically-derived basal force fluctuations match well with the measured basal fluctuations at a force plate and correlate with the bulk flow properties, including flow depth and weight. We propose a physical model employing the multi-particle force chains and random single-particle impacts within a debris flow to simulate the generation of high frequency seismic signals. We find that the random impacts of single particles and of multi-particle force chains are active at the same time, and together they control the debris-flow’s basal force fluctuations. For different events and different positions within events, the relative contributions of single particle impacts and of multi-particle force chains dominating the basal fluctuations vary significantly and control the non-linear relation between the high-frequency seismic signal strength and the bulk flow characteristics. According to our model, fluctuating basal forces are strongly controlled by particle sizes and flow depth. Our results open new perspectives for the understanding of high frequency seismic signals generated by debris flows and the estimation of bulk flow characteristics, such as flow depth and weight.