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.…”

“…This assumption is justified as the vertical location uncertainty ( Figure S6), including possible effects of model errors and anisotropy (Sections S1 and S2), and the uncertainty in the radar-constrained interface (King, Pritchard, et al, 2016), places all events at the interface. This agrees with full-waveform source inversions that suggest that such icequakes at RIS occur within meters of the ice-bed interface (Hudson, Brisbourne, et al, 2020). Furthermore, we note that our event catalog does not allow us to draw detailed conclusions on the shape of the individual clusters.…”

“…Magnitudes are calculated using a two-step approach. First, we determine the moment magnitude (M w ; Hanks & Kanamori, 1979) for a subset of our data (January 1, 2019, 1,520 events) from the far-field displacement of the P-wave (Shearer, 2009; implementation of Hudson, Brisbourne, et al, 2020) and assuming density (917 kg m −3 ) and seismic velocity (3.841 km s −1 ) at the source (Maurel et al, 2015;Smith, 1997). We then calibrated a local magnitude scale (ML), obtained from the maximum amplitudes in the waveforms, to the M w scale (see processing windows in Figure 2a and processing details in supplementary material S4).…”

“…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

“…It should be noted that although we use S-waves in the inversion, we plot the P-wave radiation pattern, for consistency with other studies. The double-couple (DC) upper hemisphere focal mechanism is as one might expect (Hudson et al, 2020;Kufner et al, 2021;Smith et al, 2015), suggesting horizontal slip with the slip vector aligned approximately with the ice flow direction showing the observed waveforms used in the inversion, the modeled result, and the difference, colored by normalized strain-rate. The blue scatter points on the focal mechanism plot indicate the fiber and the red solid arrow and dashed lines indicate the slip vector and its associated uncertainty, respectively.…”