Infrasonic and Seismic Analysis of Debris‐Flow Events at Illgraben (Switzerland): Relating Signal Features to Flow Parameters and to the Seismo‐Acoustic Source Mechanism. Issue 6 (21st June 2022)

• Record Type: Journal Article
• Authors: Belli, Giacomo; Walter, Fabian; McArdell, Brian; Gheri, Duccio; Marchetti, Emanuele
• Is Part Of: Journal of geophysical research. Volume 127:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-21
• Subjects: infrasound -- geophysical monitoring -- mass movements -- radiation processes -- fluid dynamics -- seismic waves; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006576
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Debris flows are among the most dangerous phenomena in mountain environment. Recently the use of seismic and infrasonic recordings has proven to be a powerful tool for studying and monitoring debris flows. However, open questions remain about how signal characteristics refllect flow parameters and fluid dynamic processes. We present a seismo‐acoustic analysis of the debris flow activity between 2017 and 2019 at the Illgraben catchment (Switzerland). Seismic and infrasonic amplitudes (maximum root mean square amplitude [RMSA]) and peak frequencies are compared with flow measurements (front velocity, maximum flow depth and density). Front velocity, maximum depth, peak discharge and peak mass flux show a positive correlation with both infrasonic and seismic maximum RMSA, suggesting that seismo‐acoustic amplitudes are influenced by these flow parameters. Comparison between seismo‐acoustic peak frequencies and flow parameters reveals that, unlike seismic signals, characterized by a constant peak frequency regardless of the magnitude of the flow, infrasound peak frequency decreases with increasing flow velocity, depth and discharge. Our results suggest that infrasound and seismic waves are generated by different source processes, acting at the flow free surface and at the channel bed respectively. We propose that infrasound is likely generated by waves and oscillations that develop at the surface of the flow, which, according to fluid dynamics, are mostly generated wherever the flow encounters significant channel irregularities. Furthermore, the observed positive correlations between seismo‐acoustic signal features and flow parameters highlight the potential to use infrasound and seismic measurements for debris‐flow monitoring and risk management. Plain Language Summary: Debris flows represent a serious hazard in mountain environments and significant efforts are undertaken to develop automatic warning systems. We analyzed infrasonic (low‐frequency sound) and seismic (ground vibrations) signals generated by the debris‐flow activity at the Illgraben (Switzerland) between 2017 and 2019. Seismic and infrasonic signal features are compared with flow parameters. Positive correlations emerge between flow parameters and the amplitude of infrasonic and seismic signals, suggesting that the seismic and infrasonic radiation by debris flows is influenced by flow depth and discharge. Moreover, our results indicate that seismic and infrasonic waves are generated by different sources acting at the channel bed and at the flow surface respectively. For the seismic signal, presented results agree with previous models, which attribute the radiation of seismic waves to solid particle collisions, friction and fluid dynamic structures. Our novel observations allow to improve our understanding of the infrasound source within debris flows, being consistent with waves that develop at the surface of the flow depending on flow properties and channel irregularities. Finally, experimental results highlight how infrasound and seismic measurements could be successfully used to quantify remotely the magnitude of debris flows, thus opening new perspective for monitoring and risk management. Key Points: Infrasound by debris‐flows is generated by turbulence induced waves at the surface of the flow, mostly generated at channel steps or bends Infrasound peak frequency by debris‐flows decreases with increasing flow magnitude, while seismic peak frequency is constant for all events Infrasonic and seismic energy radiation by debris‐flows are strongly influenced by flow discharge and flow depth