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Deformation and basal sliding velocity
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Sliding dominates slow-flowing margin regions, Greenland Ice Sheet

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The deformation velocity, the component of motion from shearing of the ice column, was calculated by integrating the du/dz estimated from the inclinometers from the surface to the bed. du/dz values at each inclinometer depth were averaged across all boreholes to account for any local variability present in the deformation rates. The result is a profile of mean deformation rates that is representative of the field location. There is a small offset in the inclinometer installation depth (2 to 4 m) in holes 14Sa, 14Sb, 14W, and 14N compared to the inclinometer strings installed in 2015. In the averaging process, du/dz values in boreholes drilled in 2014 were averaged with du/dz values from the closest depth interval from boreholes drilled in 2015.

Integration of the mean deformation profile yields an estimate of the deformation velocity for the field site. du/dz at the bed was set to the value calculated from the bottom inclinometer depth, and du/dz at the surface was set to zero. Measurement gaps between inclinometer depths were filled using linear interpolation. Uncertainty bounds in the deformation velocity were estimated by calculating the deformation velocity using the minimum and the maximum du/dz measured at each depth interval at any borehole location. This yields a conservative range for the error in the deformation velocity beyond that of the averaging and integrating the individual sensor error and uncertainty.

The basal sliding velocity was calculated as the residual of surface velocity and deformation velocity. The surface velocity was calculated from position data collected continuously at 15-s intervals from five Trimble Net-R9 GPS receivers installed in a diamond array (Fig. 1). The data from the GPS receiver were processed against a base station located ~22 km away off the ice sheet using TRACK version 1.29 differential kinematic processing software. The winter surface velocity at each GPS location was calculated using$us=ΔxΔt$(2)where Δt is the duration of the winter period and Δx is the change in position across the winter period. The error in the velocity is estimated to be ±1.3 × 10−6 m a−1 (SD) for the velocity at each station. The site mean velocity was calculated by averaging the mean winter velocity from all five GPS locations. For continuous velocities presented in fig. S4, the position data were smoothed using a Gaussian smoothing kernel, with a window length of 5 days, and then differentiated continuously through the winter period with Δt = 5 days.

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