Seismic evidence of the transition from debris flow to hyperconcentrated flow at Mount Rainier, USA

Debris flows are a seasonal hazard in Tahoma Creek in Mount Rainier National Park, Washington, United States. We present seismic recordings spaced every 100–200 m along the path of a debris flow triggered by a glacial outburst on 15 August 2023. These recordings capture changes as the debris flow deposits ∼24,000 cubic meters of material along an ∼800 m stretch of channel, approximately 2.5 km from the glacial source. As the flow deposits material, the seismic signal onset shape associated with the debris flow snout recedes, resulting in an elongation of the delay between the signal onset and the peak amplitude from 100 to 300 s. In parallel, the peak frequency increases by more than 20 Hz, and energy below 20 Hz fades, indicating deposition of larger particles and changes in sediment concentration within the flow. We interpret these changes to correspond to the transition from debris flow to hyperconcentrated flow, a distinction based on sediment concentration by volume and on rheological criteria such as yield strength. We find that the frequency content of the seismic signal depends on grain size and sediment concentrations within the flow, among other factors, complicating the starting assumptions of many physical models of debris flows.