A Pilot Experiment on Infrasonic Lahar Detection at Mount Adams, Cascades: Ambient Infrasound and Wind-Noise Characterization at a Quiescent Stratovolcano

Erosion, hydrothermal activity, and magmatism at volcanoes can cause large and unexpected mass wasting events. Large fluidized debris flows have occurred within the past 6000 yr at Mount Adams, Washington, 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 four-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 min), with one of five visually confirmed events detected infrasonically at BEAR. We locate this small event, which satellite imagery suggests was a glacial 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.

Helicopter electromagnetic data map ice thickness at Mount Adams and Mount Baker, Washington, USA

Ice-thickness measurements critical for flood and mudflow hazard studies are very sparse on Cascade Range (North America) volcanoes. Helicopter electromagnetic (HEM) data collected to detect hydrothermal alteration are used to determine ice thickness over portions of Mount Baker and Mount Adams volcanoes. A laterally continuous inversion method provides good estimates of ice <100 m thick over water-saturated and altered regions where the resistivity of the basement is <200 Ωm. For areas with ice overlying fresh, resistive rocks with small resistivity contrasts between ice and rock, ice thickness is not well resolved. The ice thicknesses derived from HEM data are consistent with the previous drillhole data from Mount Adams and radar data from both volcanoes, with mean thicknesses of 57 m for Mount Adams and 68 m for Mount Baker. The thickest ice on Mount Baker rests on the gentle lower slopes whereas the thickest ice at Mount Adams lies on the flat summit. Ice volume calculations suggest that Mount Baker contains ∽710 × 106 m3 of ice in the HEM survey area, with a crude estimate of ∽1800 × 106m3 for the entire volcano. Ice volume on Mount Adams is 65 × 106m3 in parts of the HEM survey area and ∽200 × 106m3 overall.