Motion of an Antarctic glacier by repeated tidally modulated earthquakes

Between debris-laden glacial ice and bedrock, basal seismicity can develop that yields information about bed properties(1,2), stress distribution(3), outburst flooding(4), and crevassing and calving(5-7). Basal seismicity in response to glacial motion is linked to variations in both stress and lubrication of bedrock by water and till(8,9). Here we analyse data from the Transantarctic Mountains Seismic Experiment array in 2002-2003 to investigate seismic behaviour at David Glacier, a large outlet glacier that drains 4% of East Antarctica's ice sheet into the Ross Sea. We identify about 20,000 seismic events that are larger in magnitude and duration than typical for glacial sources and repeat at regular intervals of about 25 min. These events are consistent with stick-slip behaviour of debris-laden ice moving over a single obstacle of rough bedrock, modulated by relatively small stress changes from the ocean tides. In the years before and after the interval of repeating events, seismic events with irregular and generally longer intervals were detected at the same location, and are consistent with combined stick-slip and continuous sliding of the subglacial interface. We suggest that the observed transitions in seismicity patterns capture the dynamic behaviour of the ice stream, and that-despite lower ice-flow velocities-sliding in the stick-slip regime enhances subglacial erosion.