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 time-dependent 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.