Abstract
Abstract. While flowing downhill, a snow avalanche radiates seismic waves in the ground and infrasonic waves in the atmosphere. Seismic energy is radiated by the dense basal layer flowing above the ground, while infrasound energy is likely radiated by the powder front. However, the mutual energy partitioning is not fully understood. We present infrasonic and seismic array data of a powder snow avalanche, which was released on 5 February 2016, in the Dischma valley above Davos, Switzerland. A five-element infrasound array, sensitive above 0.1 Hz, and a seven-element seismic array, sensitive above 4.5 Hz, were deployed at a short distance (<500 m) from each other and close (<1500 m) to the avalanche path. The avalanche dynamics were modelled by using RAMMS (rapid mass movement simulation) and characterized in terms of front velocity and flow height. The use of arrays rather than single sensors allowed us to increase the signal-to-noise ratio and to identify the event in terms of back-azimuth angle and apparent velocity of the recorded wave fields. Wave parameters, derived from array processing, were used to identify the avalanche path and highlight the areas, along the path, where seismic and infrasound energy radiation occurred. The analysis showed that seismic energy is radiated all along the avalanche path, from the initiation to the deposition area, while infrasound is radiated only from a limited sector, where the flow is accelerated and the powder cloud develops. The recorded seismic signal is characterized by scattered back-azimuth angle, suggesting that seismic energy is likely radiated by multiple sources acting at once. On the contrary, the infrasound signal is characterized by a clear variation of back-azimuth angle and apparent velocity. This indicates that infrasound energy radiation is dominated by a moving point source, likely consistent with the powder cloud. Thanks to such clear wave parameters, infrasound is revealed to be particularly efficient for avalanche detection and path identification. While the infrasound apparent velocity decreases as the flow moves downhill, the seismic apparent velocity is quite scattered but decreases to sound velocity during the phase of maximum infrasound radiation. This indicates an efficient process of infrasound to seismic energy transition, which, in our case, increases the recorded seismic amplitude by ∼20 %, at least in our frequency band of analysis. Such an effect can be accounted for when the avalanche magnitude is estimated from seismic amplitude. Presented results clearly indicate how the process of seismo-acoustic energy radiation by a powder avalanche is very complex and likely controlled by the powder cloud formation and dynamics, and the process is hence affected by the path geometry and snow characteristics.
Highlights
As a first approximation, a powder snow avalanche (PSA) can be described with a two-layer model consisting of a dense basal snow layer, with densities of 100–400 kg m−3, and a powder part that develops at the head of the flow, with density of 3–30 kg m−3 (Issler, 2003)
The event appears as a cluster of detections (Fig. 6), each associated with a corresponding value of back-azimuth (Baz) and apparent velocity, calculated for each signal time window
The duration of the infrasound signal is not representative of the entire volume of snow that was transported by the avalanche
Summary
A powder snow avalanche (PSA) can be described with a two-layer model consisting of a dense basal snow layer, with densities of 100–400 kg m−3, and a powder part that develops at the head of the flow, with density of 3–30 kg m−3 (Issler, 2003). PSAs correspond to the intermittent regime (IR) identified from GEORADAR analysis at the Vallée del la Sionne test site by Kohler et al (2018). They are characterized by intense surging activity, flow heights up to 9 m, front velocities up to 60 m s−1 and develop once the snow temperature is < −2 ◦C. Carrol et al (2013) provided a detailed analytical description of the powder front of a PSA in terms of an eruption current They showed how the front evolution is mostly controlled by the amount of snow scoured from the snowpack, as the front moves downhill. They showed that a temperature of −2 ◦C of the scoured snow is a threshold value, with the cloud formation inhibited for “warm avalanches”
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.