Using Local Infrasound Arrays to Detect Plunging Snow Avalanches along the Milford Road, New Zealand (Aotearoa)

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Snow avalanches pose a hazard in alpine environments. There is a need to improve monitoring capabilities in order to reliably detect and locate avalanche activity, which will help to validate avalanche hazard assessments. Recent work has demonstrated the utility of infrasound as it can provide continuous monitoring and broad geographic coverage. Here, we present the first use of infrasound to monitor snow avalanche activity in a maritime climate along the Milford Road in Fiordland, New Zealand (Aotearoa). Size 4 (or larger) plunging avalanches frequently occur along the Milford Road, which travels through a glacial-carved valley with steep cliffs (slope angles can exceed 50°) that are over 1000 m tall. We deployed two infrasound arrays on the eastern side of the Homer Tunnel and recorded triggered and natural avalanches during our month-long field campaign. We use array processing to identify avalanche signals, calculate back-azimuths, and triangulate source locations. Source locations are well constrained for avalanches that are in-network but are worse for avalanches that occur out-of-network, likely due to topographic scattering of acoustic waves from the steep valley walls. The infrasound amplitudes are substantially larger than previously recorded at other locations with a maximum peak-to-peak amplitude of 37 Pa detected for a large triggered avalanche, which reflects the unique dynamics of the avalanches along the Milford Road. This study demonstrates the utility of infrasound for snow avalanche monitoring in maritime climates and showcases an efficient processing workflow that could be easily operationalized.