Analyzing Bulk Flow Characteristics of Debris Flows Using Their High Frequency Seismic Signature

Abstract

Quantifying debris‐flow characteristics remains a key challenge in natural hazard research. Recent studies suggest that seismic signals can reveal debris‐flow properties. However, the accuracy and applicability of theoretical models linking bulk flow properties to the seismic signature of a debris flow remain elusive. By extending a previously proposed model of random single‐particle impact forces to multi‐particle force chains acting on the channel bed, we describe the generation of high frequency seismic signals, and thereby estimate the bulk flow dynamics of six debris‐flow events at Illgraben, Switzerland. The theoretically predicted basal force fluctuations agree with in‐situ measurements at a basal force plate and seismic signals recorded adjacent to the channel. According to our model, both random single‐particle impacts and random impacts caused by multi‐particle force chains control basal force fluctuations. The relative contributions of single particles and of multi‐particle force chains may vary significantly for different events and different positions within events. The relative contribution of multi‐particle force chains is expected to be larger in the flow front, where particle concentrations are higher. The ratio between single‐particle and multi‐particle contributions appears to control the non‐linear relation between flow depth and the magnitude of the high frequency seismic signals. Our results suggest that fluctuating basal forces are strongly controlled by particle size and flow depth, and open new perspectives for the understanding of bulk flow characteristics, such as flow depth and weight, and of the high frequency seismic signals generated by debris flows.

Infrasonic and Seismic Analysis of Debris‐Flow Events at Illgraben (Switzerland): Relating Signal Features to Flow Parameters and to the Seismo‐Acoustic Source Mechanism

“…Based on presented results and taking into account previous models and experiments, we propose a decoupled source mechanism for seismic and infrasound radiation by the flow, with the two wavefields being generated by different processes acting at the ground and at the flow surface respectively. Results agree with previous models and observation of seismic energy radiation by rivers (Burtin et al., 2008; Schmandt et al., 2013; Tsai et al., 2012) and debris flows (Burtin et al., 2009; Kean et al., 2015; Lai et al., 2018; Zhang, Walter, McArdell, de Haas, et al., 2021), indicating that seismic waves are generated by solid particle collisions and friction with the riverbed and banks and by fluid dynamic structures.…”

“…Therefore, the analysis of the seismic-acoustic radiation would require locating the radiating source. However, many studies have shown that the flow front dominates the generated seismic signal (Coviello et al, 2019;Zhang, Walter, McArdell, de Haas et al, 2021;, while for infrasound Marchetti et al (2019) found that the maximum signal amplitude is generated when the barycentre of the extended source representing the flow is located at the minimum distance to the infrasonic sensor. Therefore, we believe that our comparison between the seismic and infrasonic signal as a whole and the hydraulic data is permissible, since the velocity is actually the front velocity, and the maximum depth is measured in correspondence of the flow front (Pierson, 2020).…”

“…Therefore, the analysis of the seismic‐acoustic radiation would require locating the radiating source. However, many studies have shown that the flow front dominates the generated seismic signal (Coviello et al., 2019; Zhang, Walter, McArdell, de Haas et al., 2021; Zhang, Walter, McArdell, Wenner, et al., 2021), while for infrasound Marchetti et al. (2019) found that the maximum signal amplitude is generated when the barycentre of the extended source representing the flow is located at the minimum distance to the infrasonic sensor.…”

“…Concerning the seismic wavefield, because of the limited constraints we provide in this study, we rely on existing models for seismic energy radiation by rivers (Burtin et al., 2008; Schmandt et al., 2013; Tsai et al., 2012) and debris flows (Huang et al., 2007; Burtin et al., 2009; Kean et al., 2015; Lai et al., 2018; Zhang, Walter, McArdell, de Haas, et al., 2021), which attribute the generation of seismic waves to solid particle collisions and friction with the channel bed and banks and fluid dynamic structures. In accordance with all these studies, our results suggest that the radiation of seismic waves by debris flows is influenced by flow discharge (Figure 5), that in turn controls the solid transport capability of the flow.…”

“…Concerning debris flows, most efforts focused on the early detection of the events (Arattano, 1999;Chmiel et al, 2021;Kogelnig et al, 2011;Lai et al, 2018;Liu et al, 2015;Schimmel & Hübl, 2016;Walter et al, 2017). For seismic waves, several theoretical source mechanisms have been proposed (Farin et al, 2019;Kean et al, 2015;Lai et al, 2018;Zhang, Walter, McArdell, de Haas, et al, 2021), mostly based on the model developed by Tsai et al (2012) for seismic noise by rivers. However, in our knowledge, clear on field validation of proposed seismic model are still missing and no accurate source models have been proposed yet for infrasound by debris flows.…”