Sustained seismic tremors and icequakes detected in the ablation zone of the Greenland ice sheet

During summer 2011, seismic activity in the ablation zone of the western Greenland ice sheet (GrIS) was monitored using a network of three-component seismometers. The seismic record includes a large variety of icequakes and seismic tremors that demonstrate a clear correlation with subglacial water flow. We verified the existence of well-known shallow icequakes (related to surface crevasse formation), deep icequakes (located at 100-160 m depth) and narrow-banded short-term seismic tremors (tens of seconds in duration). In addition, we present previously unreported long-term tremors lasting several hours. Using attenuation of the measured tremor amplitude, we locate the epicentre of this long-term tremor to a large moulin within our study area. Between 3 and 11 Hz, our continuous seismic record is dominated by this 'moulin tremor' and shows strong correlation with the water level of the generating moulin. We argue that monitoring of icequake and glacial tremor sources bears high potential for investigating glacier hydraulics and dynamics, and is thus an ideal supplement to traditional glaciological measurements.