Experimental Evidence of Velocity-Weakening Friction during Ice Slip over Frozen Till: Implications for Basal Seismicity in Fast Moving, Soft-Bed Glaciers and Ice Streams

Observations of glacier slip over till beds, across a range of spatial and temporal scales, show abundant seismicity ranging from M-w similar to-2 microearthquakes and tremor (sub-meter asperities and millisecond duration) to M-w similar to 7 slow-slip events (similar to 50 km rupture lengths and -30 min durations). A complete understanding of the mechanisms capable of producing seismic signals in these environments represents a strong con-straint on bed conditions. In particular, there is a lack of experimental confirmation of velocity-weakening behavior of ice slipping on till, where friction decreases with increasing velocity-a necessity for nucleating seismic slip. To measure the frictional strength and stability of ice sliding against till, we performed a series of double-direct-shear experiments at controlled temperatures slightly above and below the ice melting point. Our results confirm velocity-strengthening ice-till slip at melting tem-peratures, as has been found in the few previous studies. We provide best-fit rate-and-state friction parameters and their standard deviations from averaging 13 experiments at equivalent conditions. We find evidence of similar velocity-strengthening behavior with 50% by volume debris-laden ice slid against till under the same conditions. In con-trast, velocity-weakening and linear time-dependent healing of ice-till slip is present at temperatures slightly below the melting point, providing an experimentally supported mechanism for subglacial seismicity on soft-beds. The stability parameter (a-b) decreases with slip velocity, and evolution occurs over large (mm scale) displacements, suggesting that shear heating and melt buildup is responsible for the weakening. These measurements provide insight into subglacial stiffness in which seismicity of this type might be expected. We discuss glaciological circumstances pointing to potential field targets in which to test this frozen seismic asperity hypothesis.