Debris flows induce seismic ground unrest detectable over large distances. However, recent theoretical description of debris-flow seismograms has yet to be implemented in alarm systems. Here, we test the implications of a particle impact model for debris-flow detection and warning using seismic data. We analyze 14 debris flows at Illgraben, Switzerland, using instantaneous weight measurements, a seismic network and video footage. A novel approach to simulate seismic wave propagation estimates both vertical and horizontal particle impact forces on the ground. Scaling between impact forces, weight, and particle sizes suggests that seismic data identify destructive debris flows with large boulders. Impact force spectra change with bed topography and increase during flow propagation of large events, but decrease for smaller events. This can be explained with the known conveyor mechanism concentrating boulders at the flow front. Our results highlight opportunities to use seismic data for debris-flow research and warning.