Icequake source mechanisms for studying glacial sliding

Abstract

Improving our understanding of glacial sliding is crucial for constraining basal drag in ice dynamics models. We use icequakes, sudden releases of seismic energy as the ice slides over the bed, to provide geophysical observations that can be used to aid understanding of the physics of glacial sliding and constrain ice dynamics models. These icequakes are located at the bed of an alpine glacier in Switzerland and the Rutford Ice Stream, West Antarctica, two extremes of glacial settings and spatial scales. We investigate a number of possible icequake source mechanisms by performing full waveform inversions to constrain the fundamental physics and stress release during an icequake stick-slip event. Results show that double-couple mechanisms best describe the source for the events from both glacial settings and the icequakes originate at or very near the ice-bed interface. We also present an exploratory method for attempting to measure the till shear modulus, if indirect reflected icequake radiation is observed. The results of this study increase our understanding of how icequakes are associated with basal drag while also providing the foundation for a method of remotely measuring bed shear strength.

Not all Icequakes are Created Equal: Basal Icequakes Suggest Diverse Bed Deformation Mechanisms at Rutford Ice Stream, West Antarctica

“…This is comparable to the surface ice flow direction (azimuth of 148°) measured with GPS, which suggests flow‐parallel sliding at the base of the ice stream. This agrees with previous source mechanism observations at RIS (Hudson, Brisbourne, et al., 2020; Smith, Smith, et al., 2015). Here, the P‐axes describe a gentle rotation (±11°) toward the ice stream margin on either site of the central high along with this large‐scale trend.…”

“…If the icequake source would have a significant nondouble‐couple component, a less clear separation of positive and negative polarities close to the nodal planes would be expected. In addition, results of full‐waveform modeling for one icequake at RIS show a double‐couple source to be more likely (Hudson, Brisbourne, et al., 2020). Full‐waveform modeling would allow for the resolution of different source types.…”

Distributed Acoustic Sensing (DAS) for Natural Microseismicity Studies: A Case Study From Antarctica

“…This effectively acts as a moving average measurement of strain over 10 m of fiber, in 1 m increments, with the maximum resolvable frequency signals for ice therefore being ~200 Hz and ~90 Hz, for P-and S-waves with velocities of 3841 m s -1 and 1970 m s -1 , respectively. These frequencies are greater than typical observed icequake corner frequencies at Rutford Ice Stream (Hudson et al, 2020;Kufner et al, 2021;Smith et al, 2015). The seismic data were acquired in January 2020, during the austral summer.…”

Breaking the Ice: Identifying Hydraulically Forced Crevassing

“…The icequake source mechanisms are obtained by performing a Bayesian full waveform source inversion using an identical approach to a method detailed by Hudson et al (2020). Only P wave phases on the vertical component are used since the horizontal components are generally too noisy to use, due to the instruments melting out of the glacier.…”