SHORT-PERIOD OBSERVATIONS OF SPEED, STRAIN AND SEISMICITY ON ICE STREAM-B, ANTARCTICA

The speed of Ice Stream B, Antarctica, was measured twice a day over a 1 month study period, and found to be steady at about the +/- 3 1/2% level, the sensitivity of the measurements. The vertical strain was measured at three sites over a 1 year period at 1 h intervals with sensitivities of 2 or 0.2 ppm. The strain rate varied on all time-scales. Events of high strain rate were observed, but never at more than one site at a time. They can probably be understood in terms of local modification of the strain field associated with crevassing. Diurnal variation in strain rate was observed at one and possibly two sites during two summers. The seismicity was measured at all three sites, and diurnal and seasonal variations were prominent at all, the seismicity being much more intense in winter. Several possible causes of the diurnal variations in strain and seismicity are considered: thermal and atmospheric effects, and the effects of tides in the Ross Sea.