Infrasound Unmasks Flow Turbulence as an Additional Seismic Source in Debris Flows

Debris flows radiate both seismic and infrasonic waves. According to previous studies, seismic energy is generated by the solid particle collisions with the riverbed and is dominated by the boulder-rich front, while infrasound is produced by turbulence-induced waves at the flow surface. To further investigate this complex radiation processes, we present the seismo-acoustic analysis of a debris-flow event at Illgraben (Switzerland). Array processing shows that infrasound is preferentially radiated at channel irregularities, acting as predominant acoustic sources because of the intense flow turbulence. The high crosscorrelation observed between the recorded infrasonic and seismic signals suggests that, in addition to the dominant source related to particle impacts, a minor seismic component is produced by the flow waves developing at topographic steps.