Supporting information for: A pilot experiment on infrasonic lahar detection at Mount Adams, Cascades: Ambient infrasound and wind-noise characterization at a quiescent stratovolcano: time-lapse camera images

Erosion, hydrothermal activity, and magmatism at volcanoes can cause large and unexpected mass wasting events. Large fluidized debris flows have occurred within the past 6,000 years at Mount Adams, WA, and present a hazard to communities downstream. In August 2017, we began a pilot experiment to investigate the potential of infrasound arrays for detecting and tracking debris flows at Mount Adams. We deployed a telemetered 4-element infrasound array (BEAR, 85-m aperture) ~11 km from a geologically unstable area where mass wasting has repeatedly originated. We present a preliminary analysis of BEAR data, representing a survey of the ambient infrasound and noise environment at this quiescent stratovolcano. Array processing reveals near-continuous and persistent infrasound signals arriving from the direction of Mount Adams, which we hypothesize are fluvial sounds from the steep drainages on the southwest flank. We interpret observed fluctuations in the detectability of these signals as resulting from a combination of (1) wind-noise variations at the array, (2) changes in local infrasound propagation conditions associated with atmospheric boundary layer variability, and (3) changing water flow speeds and volumes in the channels due to freezing/thawing and precipitation events. Suspected mass movement events during the study period are small (volumes <105 m3 and durations <2 minutes), with one of five visually confirmed events detected infrasonically at BEAR. We locate this small event, which satellite imagery suggests was an ice/snow avalanche, using three additional temporary arrays operating for five days in August 2018. Events large enough to threaten downstream communities would likely produce stronger infrasonic signals detectable at BEAR. In complement to recent literature demonstrating the potential for infrasonic detection of volcano mass movements (Allstadt et al., 2018), this study highlights the practical and computational challenges involved in identifying signals of interest in the expected noisy background environment of volcanic topography and drainages.

火山的侵蚀作用、热液活动与岩浆作用，可诱发规模宏大且突发的块体运动（mass wasting）事件。过去6000年间，美国华盛顿州亚当斯火山曾发生大规模流化碎屑流事件，对下游社区构成潜在威胁。2017年8月，我们启动一项试点实验，旨在探究次声阵列（infrasound array）用于探测与追踪亚当斯火山碎屑流的应用潜力。我们在一处曾多次发生块体运动的地质不稳定区域约11公里处，布设了一套遥测式4元次声阵列（BEAR，孔径85米）。本文对BEAR阵列的观测数据开展初步分析，涵盖对这座休眠层状火山（stratovolcano）周边环境的次声与噪声场勘测。阵列处理结果显示，存在近乎连续且持续稳定的次声信号从亚当斯火山方向传来，我们推测该信号源自西南侧翼陡峭水道的水流声。我们将观测到的信号可探测性波动归因于以下三类因素的共同作用：(1) 阵列处的风噪声变化；(2) 与大气边界层变化相关的局地次声传播条件改变；(3) 因冻融与降水事件导致的水道内水流流速与流量变化。研究期间疑似的块体运动事件规模较小（体积小于10万立方米，持续时长不足2分钟），5个经目视验证的事件中仅有1个被BEAR阵列以次声方式探测到。我们利用2018年8月布设的3套临时观测阵列（运行时长5天）对此次小型事件开展定位，卫星影像显示该事件实为冰雪崩。规模足以威胁下游社区的块体运动事件，其产生的次声信号强度更高，可被BEAR阵列有效探测到。作为对近期相关研究的补充——该研究证实了火山块体运动次声探测的应用潜力（Allstadt等，2018），本研究着重探讨了在火山地形与水道固有的高噪声背景环境中，识别目标信号所面临的实际与计算层面挑战。