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.