Infrasound Array Analysis of Debris Flow Activity and Implication for Early Warning

Abstract

Debris flows constitute a severe natural hazard and studies are performed to investigate triggering mechanisms and to identify and evaluate early warning systems. We present a seismoacoustic analysis of debris flow activity at Illgraben, Switzerland, with infrasound data collected with a small aperture array. Events are recorded as emergent signals of long duration, with seismic and infrasound amplitudes scaling with the flow discharge. The spectral content is stable and peaking at 8 Hz for the seismic and at 5 Hz for the infrasound that suggests two separate processes of elastic energy radiation, most likely bed‐load transport for the seismic and waves at the free surface for the infrasound. Although amplitude and frequency content of the infrasound signal are well within the processing limits, most of the signal is not showing any correlation among the array elements. We suggest that this is a consequence of the contribution of multiple sources of infrasound acting with variable amplitude and phase along the surface of the debris flow. At Illgraben, coherent infrasound is recorded only from fixed sources, corresponding to check dams within the channel. Here infrasound radiation is increased and the dams turn into predominant sources of energy. This allows to unambiguously identify the occurrence of debris flow at Illgraben with the infrasound array, from a remote and safe position and with a timing that is similar to the early warning system based on in‐channel sensors. This clearly shows how infrasound arrays could be used as an efficient early warning systems.

Seismic Characterization of Debris Flows: Insights into Energy Radiation and Implications for Warning

“…Besides the frequency drop before the arrival of the main front, on Geo1 and Geo2 there seems to be a “binary” switch between the peak frequency values of 20 and 50 Hz (Figures a and b). A similar effect was observed at the Illgraben, and there it was explained by the presence of specific geomorphic features (e.g., small stream steps, channel bends, and high‐friction spots) which cause the dominant seismicity of the approaching debris flow front (Marchetti et al, ). The peak frequency of the tail of the debris flow surge is often similar to the peak frequency of the background noise (Figure ), which may suggest that the background noise is mainly caused by preevent flow discharge.…”

“…Therefore, a further advantage of a GVD is its capability to detect the occurrence of a debris flow tens of seconds earlier than other types of devices (i.e., stage sensors, pendulums, and tripwires). Among the different sensors employable to monitor debris flows, only infrasound sensors (acoustic sensors) have similar characteristics (Johnson & Palma, 2015;Kogelnig et al, 2011;Marchetti et al, 2019;Schimmel & Hübl, 2015). Given the short warning time usually available, even a gain of some tens of seconds may represent a significant improvement in the Figure 3) during the passage of the front of the debris flow occurred at Gadria on 18 July 2014, the STA (b), and the LTA (c) calculated with a moving window of 10 and 100 s, respectively, and the ratio STA/LTA (d).…”

Insights Into the Internal Dynamics of Natural Lahars From Analysis of 3-Component Broadband Seismic Signals at Volcán de Colima, Mexico

“…Recently, others have shown (e.g. Coviello et al, 2019;Marchetti et al, 2019) that the energy of the source and the channel geometry play a significant role in the frequency response. High-friction channel beds, bends, and extreme elevation changes could all cause differing frequency responses produced by the same seismic source.…”

Insights From the Particle Impact Model Into the High‐Frequency Seismic Signature of Debris Flows

“…Recent advances in environmental seismology provide unique constraints of the dynamics and timing of landslides, rockfalls, and debris flows (e.g., Barrière et al., 2015; Dammeier et al., 2016; Ekström & Stark, 2013; Hibert et al., 2011; Moretti et al., 2012), as well as sediment transport in rivers (Burtin et al., 2010; Gimbert et al., 2014; Tsai et al., 2012). Debris flows are sources of seismic and acoustic signals, which can be used to estimate location and propagation of their flow fronts (Hürlimann et al., 2019; Marchetti et al., 2019; Schimmel et al., 2018; Schimmel & Hübl, 2015, 2016; Walter et al., 2017). However, multiphase flow contributes to complex seismogenesis (e.g., Iverson, 2003) and typical high‐frequency seismic signals generated by debris flows are sensitive to small‐scale variations in ground structure (e.g., Allstadt, 2013; Hibert et al., 2017; Kean et al., 2015; Ogiso & Yomogida, 2015).…”