DOI: 10.7914/sn/yg_2020
title: Passive Seismic Observations of a glacier surge: Turner Glacier, Alaska, USA

description: We will deploy up to six pairs of three-component broadband passive seismic stations along the margins of Turner Glacier. Five more intermediate-period geophones will be installed on the glacier surface (stations from the University of Idaho). Each glacier-adjacent pair will be orientated normal to the glacier flow, with one station on each side of the glacier and installed on bedrock or shallow sediment. Continuous broadband seismic data will be recorded at 100 Hz for the duration of the field deployment. This configuration will allow us to replicate the surface wave anisotropy study by Zhan (2019). Zhan (2019) used seismic noise to interrogate the subglacial drainage system of a surging glacier and found that changes in mechanical coupling between the ice and the glacial bed not only affects ice flow, but also the overall (poro)elastic properties of the ice‐water‐till‐bedrock system. From observations of seismic noise and the corresponding frequency-dependent surface-wave anisotropy, Zhan (2019) was able to infer that high subglacial water pressures are maintained by the impoundment of water in transverse basal crevasses during a glacier surge. We will apply the methodology of Zhan (2019) to determine if fluid-filled transverse basal crevasses exist prior to, during, and after surging at Turner Glacier. Passive seismic data will also allow us to track variations in subglacial water discharge. Recent observations of flow‐induced seismic noise, including at Alaskan tidewater glaciers similar to Turner Glacier, have demonstrated the ability to track increases and decreases in subglacial discharge and sediment transport, changes in the efficiency of the subglacial hydrologic system, and the extent of subglacial water pressurization (Bartholomaus et al., 2015; Gimbert et al. 2016; Bartholomaus et al., in prep.). The passive seismic data will be coupled with a numerical model of noise production by subglacial water flow and sediment transport to disentangle the noise produced by water or sediment transport using a technique newly developed by co-I Beaud. Extension of these techniques to ~15 km of Turner Glacier will allow us an unprecedented view of imbalances in water and sediment transport along the glacier bed, including and extending beyond the length of the actively surging glacier. To complement our assessment of subglacial water storage and transport, we will track changes in till consolidation and failure using seismic coda wave changes over time. Coda wave methods can be used to track seismic velocity variations in the subsurface (e.g., Sens-Schönfelder and Wegler 2006; Meier et al. 2010; Mainsant et al. 2012; Hillers et al. 2014; Hillers et al. 2015; Rivet, et al. 2015; Larose et al. 2015, and references therein). In general, the observed seismic velocity changes are attributed to changes in tectonic stress, hydrologic loading, or climate variation (e.g. air temperature or pressure). Mainsant et al. (2012) used coda wave changes to observe the failure of a clay layer at depth during a landslide. They showed that the seismic velocity of the sliding material, from daily noise-based coda wave measurements, decreased continuously and rapidly for several days prior to the catastrophic failure event as the layer became oversaturated. In order to test hypothesis 1, we will monitor changes in the till layer using this technique to determine the till state prior to, during, and after surge along the length of the glacier. To further refine this methodology, we will use the glacier parameters derived from active-source seismic observations to model the frequency-dependent sensitivity of surface waves (Mainsant et al. 2012) to the till layer and incorporate a new wavelet-based coda wave change measurement technique to improve depth resolution (Mordret et al., in review). The till state will be inferred from the observed frequency-dependent seismic velocity changes, with decreases in seismic velocity directly proportional to decreases in the shear modulus of the till.