Nathan Maier

Glacier internal deformation is usually described by Glen's flow law using two material parameters: the creep factor (A) and the flow law exponent (n). However, the values of these parameters and their spatial and temporal variability are rather uncertain due to the difficulty in quantifying internal strain and stress fields at natural scales. In this study, we combine 1-year-long continuous measurements of borehole inclinometry and surface velocity with three-dimensional full-Stokes ice flow modeling to infer ice rheologies and sliding velocities for the ablation zone of the Argentière Glacier, a temperate glacier in the French Alps. We demonstrate that the observed deformation rate profile has limited sensitivity to the flow law exponent (n) and instead mainly reflects an increase in the creep factor (A) with depth, with A departing from its surface value by up to a factor of 2.5 below 160 m depth. We interpret this creep factor enhancement as an effect of increasing interstitial water content with depth (from 0 % to 1.3 %), which results in an average value of A=148 MPa⁻³ a⁻¹. We further observe that internal ice deformation exhibits seasonal variability similar to that concerning surface velocity, indicating that the local basal sliding velocity exhibits no significant seasonal variation. We suggest that these changes in deformation rate are due to variations in the stress field, driven by contrasting changes in subglacial hydrology conditions between the sides and center of the glacier. Our study provides further evidence that borehole inclinometry, combined with full-Stokes flow modeling, allows for the constraining of both ice rheology and basal friction at scales that cannot be inferred from surface velocity measurements alone.

Glacier internal deformation is usually described by Glen's Law using two material parameters, the creep factor A and the flow law exponent n. However, the values of these parameters and their spatial and temporal variability are rather uncertain due to the difficulty of quantifying internal strain and stress fields at the natural scale. In this study, we combine 1-year long continuous measurements of borehole inclinometry and surface velocity with three-dimensional full Stokes ice flow modeling to infer ice rheology and sliding velocity in the ablation zone of the Argentière Glacier, a temperate glacier in the French Alps. We demonstrate that the observed deformation rate profile has limited sensitivity to the flow law exponent n and instead mainly reflects an increase in the creep factor A with depth, with A departing from its surface value by at least up to a factor of 2.5 below 160 m. We interpret this creep factor enhancement as an effect of increasing interstitial water content with depth from 0 % to 1.3 % which results in an average value of A = 148 MPa^-3 a^-1. We further observe that internal ice deformation exhibits seasonal variability similar to that in surface velocity, such that the local basal sliding velocity exhibits no significant seasonal variation. We suggest that these changes in deformation rate are due to variations in the stress field driven by contrasting changes in subglacial hydrology conditions between the side and the center of the glacier. Our study gives further evidence that borehole inclinometry combined with full-Stokes flow model allows constraining both ice rheology and basal friction at scales that cannot be inferred from surface velocity measurements alone.

Glacier internal deformation is usually described by Glen's Law using two material parameters, the creep factor A and the flow law exponent n. However, the values of these parameters and their spatial and temporal variability are rather uncertain due to the difficulty of quantifying internal strain and stress fields at the natural scale. In this study, we combine 1-year long continuous measurements of borehole inclinometry and surface velocity with three-dimensional full Stokes ice flow modeling to infer ice rheology and sliding velocity in the ablation zone of the Argentière Glacier, a temperate glacier in the French Alps. We demonstrate that the observed deformation rate profile has limited sensitivity to the flow law exponent n and instead mainly reflects an increase in the creep factor A with depth, with A departing from its surface value by at least up to a factor of 2.5 below 160 m. We interpret this creep factor enhancement as an effect of increasing interstitial water content with depth from 0 % to 1.3 % which results in an average value of A = 148 MPa^-3 a^-1. We further observe that internal ice deformation exhibits seasonal variability similar to that in surface velocity, such that the local basal sliding velocity exhibits no significant seasonal variation. We suggest that these changes in deformation rate are due to variations in the stress field driven by contrasting changes in subglacial hydrology conditions between the side and the center of the glacier. Our study gives further evidence that borehole inclinometry combined with full-Stokes flow model allows constraining both ice rheology and basal friction at scales that cannot be inferred from surface velocity measurements alone.

Glacier internal deformation is usually described by Glen's Law using two material parameters, the creep factor A and the flow law exponent n. However, the values of these parameters and their spatial and temporal variability are rather uncertain due to the difficulty of quantifying internal strain and stress fields at the natural scale. In this study, we combine 1-year long continuous measurements of borehole inclinometry and surface velocity with three-dimensional full Stokes ice flow modeling to infer ice rheology and sliding velocity in the ablation zone of the Argentière Glacier, a temperate glacier in the French Alps. We demonstrate that the observed deformation rate profile has limited sensitivity to the flow law exponent n and instead mainly reflects an increase in the creep factor A with depth, with A departing from its surface value by at least up to a factor of 2.5 below 160 m. We interpret this creep factor enhancement as an effect of increasing interstitial water content with depth from 0 % to 1.3 % which results in an average value of A = 148 MPa^-3 a^-1. We further observe that internal ice deformation exhibits seasonal variability similar to that in surface velocity, such that the local basal sliding velocity exhibits no significant seasonal variation. We suggest that these changes in deformation rate are due to variations in the stress field driven by contrasting changes in subglacial hydrology conditions between the side and the center of the glacier. Our study gives further evidence that borehole inclinometry combined with full-Stokes flow model allows constraining both ice rheology and basal friction at scales that cannot be inferred from surface velocity measurements alone.

Glacier internal deformation is usually described by Glen's Law using two material parameters, the creep factor A and the flow law exponent n. However, the values of these parameters and their spatial and temporal variability are rather uncertain due to the difficulty of quantifying internal strain and stress fields at the natural scale. In this study, we combine 1-year long continuous measurements of borehole inclinometry and surface velocity with three-dimensional full Stokes ice flow modeling to infer ice rheology and sliding velocity in the ablation zone of the Argentière Glacier, a temperate glacier in the French Alps. We demonstrate that the observed deformation rate profile has limited sensitivity to the flow law exponent n and instead mainly reflects an increase in the creep factor A with depth, with A departing from its surface value by at least up to a factor of 2.5 below 160 m. We interpret this creep factor enhancement as an effect of increasing interstitial water content with depth from 0 % to 1.3 % which results in an average value of A = 148 MPa^-3 a^-1. We further observe that internal ice deformation exhibits seasonal variability similar to that in surface velocity, such that the local basal sliding velocity exhibits no significant seasonal variation. We suggest that these changes in deformation rate are due to variations in the stress field driven by contrasting changes in subglacial hydrology conditions between the side and the center of the glacier. Our study gives further evidence that borehole inclinometry combined with full-Stokes flow model allows constraining both ice rheology and basal friction at scales that cannot be inferred from surface velocity measurements alone.

We quantify the total attenuation, κ, and the attenuation component due to near-surface site effects, κ0, in a region in northern New Mexico using data recorded by the Los Alamos Seismic Network. The area is characterized by low seismicity, where most of the well-recorded earthquakes have magnitudes between 1 and 2. This magnitude range poses a challenge for commonly used kappa methods because the high-frequency attenuation cannot be confidently isolated from the bandwidth in which the corner frequency roll-off occurs. We determine through synthetic experiments that estimates of κ within this range have quantifiable biases that depend on source (corner frequency), site (κ magnitude), and data quality characteristics (fitting bandwidth), which can be used to correct estimated κ from three commonly used kappa methods. Using 412 recorded earthquakes, we show that a bias correction results in κ distributions and κ0 estimates that are more consistent between the three methods, suggesting that the bias correction results in κ values with higher fidelity. Using the bias-corrected κ, we find κ0 between 0.038 and 0.049 s within the Valles Caldera and between 0.026 and 0.066 s on Los Alamos National Laboratory property, values near those commonly used in the western United States. We find that a main limitation in the quality of κ0 is the small number of usable waveforms at some stations, which will to improve as more earthquakes are recorded. This contrasts with other aspects, such as fitting bandwidth and source and path variability, which are unlikely to change in the future and will ultimately be the limiting factor in κ0 resolution. Overall, our results suggest that the bias-correction scheme presented here could potentially be used in other regions where small-magnitude earthquakes are prevalent. However, future work should look to verify that bias-corrected κ estimates show consistency with those retrieved from higher magnitude earthquakes.

Surface melt forces summertime ice‐flow accelerations on glaciers and ice sheets. Here, we show that large meltwater‐forced accelerations also occur during wintertime in Greenland. We document supraglacial lakes (SGLs) draining in cascades at unusually high elevation, causing an expansive flow acceleration over a ∼5,200 km² region during winter. The three‐component interferometric surface velocity field and decomposition modeling reveal the underlying flood propagation with unprecedented detail as it traveled over 160 km from the drainage site to the margin, providing novel constraints on subglacial water pathways, drainage morphology, and links with basal sliding. The triggering SGLs continuously grew over 40 years and suddenly released decades of stored meltwater, demonstrating surface melting can impact dynamics well beyond melt production. We show these events are likely common and thus their cumulative impact on dynamics should be further evaluated.

Surface melt forces summertime ice-flow accelerations on glaciers and ice sheets. Here, we show that large meltwater-forced accelerations also occur in winter in Greenland. We document supraglacial lakes (SGLs) draining in cascades at unusually high elevation, causing an expansive flow acceleration over a ~5200 km ² region during winter. The 3-component interferometric surface velocity field and decomposition modeling reveals the underlying flood propagation with unprecedented detail as it traveled over 160 km from the drainage site to the margin, providing novel constraints on subglacial water pathways, drainage morphology, and links with basal sliding. The triggering SGLs continuously grew over 40 years and suddenly released decades of stored meltwater into regions of the bed never previously forced, demonstrating surface melt can impact dynamics well beyond its production. We show these events are common and thus their cumulative impact on dynamics should be further evaluated.

Ice speeds in Greenland are largely set by basal motion¹, which is modulated by meltwater delivery to the ice base^2,3,4. Evidence suggests that increasing melt rates enhance the subglacial drainage network’s capacity to evacuate basal water, increasing bed friction and causing the ice to slow^5,6,7,8,9,10. This limits the potential of melt forcing to increase mass loss as temperatures increase¹¹. Here we show that melt forcing has a pronounced influence on dynamics, but factors besides melt rates primarily control its impact. Using a method to examine friction variability across the entirety of western Greenland, we show that the main impact of melt forcing is an abrupt north-to-south change in bed strength that cannot be explained by changes in melt production. The southern ablation zone is weakened by 20–40 per cent compared with regions with no melt, whereas in northern Greenland the ablation zone is strengthened. We show that the weakening is consistent with persistent basal water storage and that the threshold is linked to differences in sliding and hydropotential gradients, which exert primary control on the pressures within drainage pathways that dewater the bed. These characteristics are mainly set by whether a margin is land or marine terminating, suggesting that dynamic changes that increase mass loss are likely to occur in northern Greenland as temperatures increase. Our results point to physical representations of these findings that will improve simulated ice-sheet evolution at centennial scales.