Tidal modulation of ice streams: Effect of periodic sliding velocity on ice friction and healing

Abstract

Basal slip along glaciers and ice streams can be significantly modified by external timedependent forcing, although it is not clear why some systems are more sensitive to tidal stresses. We have conducted a series of laboratory experiments to explore the effect of time varying load point velocity on ice-on-rock friction. Varying the load point velocity induces shear stress forcing, making this an analogous simulation of aspects of ice stream tidal modulation. Ambient pressure, double-direct shear experiments were conducted in a cryogenic servo-controlled biaxial deformation apparatus at temperatures between −2 °C and −16 °C. In addition to a background, median velocity (1 and 10 μm/s), a sinusoidal velocity was applied to the central sliding sample over a range of periods and amplitudes. Normal stress was held constant over each run (0.1, 0.5 or 1 MPa) and the shear stress was measured. Over the range of parameters studied, the full spectrum of slip behavior from creeping to slow-slip to stick-slip was observed, similar to the diversity of sliding styles observed in Antarctic and Greenland ice streams. Under conditions in which the amplitude of oscillation is equal to the median velocity, significant healing occurs as velocity approaches zero, causing a high-amplitude change in friction. The amplitude of the event increases with increasing period (i.e. hold time). At high normal stress, velocity oscillations force an otherwise stable system to behave unstably, with consistently-timed events during every cycle. Rate-state friction parameters determined from velocity steps show that the ice-rock interface is velocity strengthening. A companion paper describes a method of analyzing the oscillatory data directly. Forward modeling of a sinusoidally-driven slider block, using rate-and-state dependent friction formulation and experimentally derived parameters, successfully predicts the experimental output in all but a few cases.

Oscillatory Loading Can Alter the Velocity Dependence of Ice‐on‐Rock Friction

“…where v m is the median driving velocity of the load point, and v a is the half-amplitude of the signal. Each experimental run was conducted at a constant temperature in the range −16.4°C 𝐴𝐴 ≤ 𝑇𝑇 ≤ −2 • C, and the load point oscillations took place at a constant normal stress of 100 kPa, 500 kPa, or 1 MPa, and consisted of applying a succession of different signals (Table 1; for details, see McCarthy et al, 2021). In general, the load point signal resulted in a periodic frictional response with the same frequency as the load point.…”

“…Finally, two experiments (C41, C44) consisted of 16 oscillation events defined by the same protocol as C34 for example, but at normal stresses σ n = 0.5, 1 MPa. All of the experiments except for C29 -C33 contained a velocity step from 1 μm/s to 10 μm/s that McCarthy et al (2021) used to determined rate and state frictional parameters. The total data set is comprised of 100 and 13 unique oscillation events, as defined by the values of v m , v a , ω, σ n , and T. Eighty-one events were conducted at 100 kPa normal stress, and 16 each conducted at 0.5 and 1 MPa normal stress.…”

“…In a companion to this paper, McCarthy et al (2021) describe ice-granite experiments in which we examine the frictional response to harmonic oscillations in the load point velocity, motivated by observations of modulation by tidal stresses in the movements of ice streams in Antarctica (Anandakrishnan & Alley, 1997;Anandakrishnan et al, 2003;Minchew et al, 2017). We found that imposed loading oscillations results in a wide range of sliding response, from steady sliding, to slow slip, to stick slip.To better understand the data set and behaviors observed by McCarthy et al (2021), and to provide tools for understanding other types of tidally influenced slip behavior, we have developed a numerical inversion scheme that determines rate and state frictional parameters directly from the frictional response to load point oscillations. The scheme is applied using a graphical user interface software package that is written in Matlab.…”

“…Often these observations are explained in terms of rate and state friction theory. In a companion to this paper, McCarthy et al (2021) describe ice-granite experiments in which we examine the frictional response to harmonic oscillations in the load point velocity, motivated by observations of modulation by tidal stresses in the movements of ice streams in Antarctica (Anandakrishnan & Alley, 1997;Anandakrishnan et al, 2003;Minchew et al, 2017). We found that imposed loading oscillations results in a wide range of sliding response, from steady sliding, to slow slip, to stick slip.…”

“…To better understand the data set and behaviors observed by McCarthy et al (2021), and to provide tools for understanding other types of tidally influenced slip behavior, we have developed a numerical inversion scheme that determines rate and state frictional parameters directly from the frictional response to load point oscillations. The scheme is applied using a graphical user interface software package that is written in Matlab.…”