Seismic Mapping of Subglacial Hydrology Reveals Previously Undetected Pressurization Event

Understanding the dynamic response of glaciers to climate change is vital for assessing water resources and hazards, and subglacial hydrology is a key player in glacier systems. Traditional observations of subglacial hydrology are spatially and temporally limited, but recent seismic deployments on and around glaciers show the potential for comprehensive observation of glacial hydrologic systems. We present results from a high-density seismic deployment spanning the surface of Lemon Creek Glacier, Alaska. Our study coincided with a marginal lake drainage event, which served as a natural experiment for seismic detection of changes in subglacial hydrology. We observed glaciohydraulic tremor across the surface of the glacier that was generated by the subglacial hydrologic system. During the lake drainage, the relative changes in seismic tremor power and water flux are consistent with pressurization of the subglacial system of only the upper part of the glacier. This event was not accompanied by a significant increase in glacier velocity; either some threshold necessary for rapid basal motion was not attained, or, plausibly, the geometry of Lemon Creek Glacier inhibited speedup. This pressurization event would have likely gone undetected without seismic observations, demonstrating the power of cryoseismology in testing assumptions about and mapping the spatial extent of subglacial pressurization.This work was made possible in part by hard work in the field by Margot Vore, Daniel Bowden, Galen Kaip, and the students and staff of the 2017 Juneau Icefield Research Program. We especially thank Matt Beedle for provision of the photogrammetrically-produced DEM of Lake Linda, following lake drainage. This work was also aided by the advice of Mike Gurnis and Rob Clayton. We thank Paul Winberry and two anonymous reviewers for their helpful feedback, which improved this paper greatly. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1745301. This work was made possible in part by a University of Idaho seed grant, #FY18-01. DEM provided by the Polar Geospatial Center under NSF-OPP awards 1043681, 1559691, and 1542736.Ye

Global quieting of high-frequency seismic noise due to COVID-19 pandemic lockdown measures

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics

Lemon Creek Glacier 2017 Seismic Full Resolution

Full-resolution seismic data from the July 2017 nodal deployment on Lemon Creek Glacier, Alask

Between Seismic Speed and Glacial Pace: Cryoseismic Observation of Intermediate-Scale Processes at Lemon Creek Glacier, Alaska

In this thesis, I present three studies in environmental seismology. First, I present an analysis of seismic tremor generated from subglacial water flow during the rapid drainage of an ice-marginal supraglacial lake, collected by an on-ice nodal seismic array. I find that seismic tremor indicates a partial pressurization of the subglacial hydrologic system that was not accompanied by the expected change in glacier surface velocity, suggesting that factors like glacier geometry play a significant role in whether pressurization necessarily leads to velocity change. Using seismometers in this way allows remote observation of active subglacial hydrologic systems as they vary over space and time, a vital parameter for understanding how liquid water affects glacier motion, melting, fracture, and hazards. Second, I present observations of glacier surface crevasse development over space and time, as detected by a dense array of seismometers atop the glacier. I find that icequakes associated with surface crevassing have a magnitude distribution that is swarm-like, rather than aftershock-like, and that the spatiotemporal distribution of events indicates that crevasses regularly widen, deepen, reactivate, and trigger activity at nearby crevasses through cryoseismicity. Understanding surface crevassing activity is valuable for constraining the degree to which glacier surface velocity measurements represent ice flow as a whole, and for interpreting how glacier flow responds to changes in forcing over time. Third, I present an investigation of changes in anthropogenic urban seismic noise in Los Angeles associated with changes in community behavior. I find that changes in human activity from the scale of hours to the scale of months create distinguishable differences in ambient seismic noise power that correlate well with other measures of community behavior. Characterizing anthropogenic seismic noise is beneficial for accurately interpreting measurements of transient seismic wave data collected in urban areas toward goals such as hazard mapping.</p

Lemon Creek Glacier 2017 Tremor Processing

MATLAB programs for processing the Lemon Creek Glacier seismic data for investigation of glaciohydraulic tremor. See included ReadMe_LCG17.m file for details

Lemon Creek Glacier 2017 Seismic Array

Seismic and hydrologic data from 2017 seismic node deployment on Lemon Creek Glacier, Alask