Basal Icequakes: Insights into Stick-Slip Motion of the Greenland Ice Sheet

Friday, 19 December 2014: 10:50 AM
Roeoesli Claudia1,2, Fabian Walter3, Agnes Helmstetter3 and Eduard H Kissling2, (1)Laboratory of Hydraulics, Hydrology and Glaciology VAW, ETH Zurich, Zurich, Switzerland, (2)Institute of Geophysics, ETH Zurich, Zurich, Switzerland, (3)ISTerre Institute of Earth Sciences, Saint Martin d'Hères, France
The flow dynamics of the Greenland Ice Sheet (GrIS) and its response to climate change are still incompletely understood. At the same time, a solid understanding of the subglacial drainage system and basal motion is crucial for further ice sheet modeling and sea level rise prediction. Conditions at the ice-bed interface, however, are inherently difficult to investigate due to limited accessibility.

Seismic signals recorded by a dense local network installed in the ablation zone of the GrIS shed some light on basal sliding and sub- and englacial drainage systems. In the slow flowing part of the GrIS, some 30km north of the calving front of Jakobshavn Isbræ, we detected basal icequakes originating near the interface of ice and underlying bed. The icequake waveform's first motions point towards shear dislocations corresponding to stick-slip motion of the ice sheet, similar to what has been observed in Antarctica. Waveform modeling of direct and critically refracted waves propagating along granitic bedrock constrain source process and bedrock properties and provide an estimate of how much till is present between ice and bedrock.

We observe more than 10.000 deep icequakes that can be grouped into more than 30 clusters with up to 2000 events per cluster originating in an area of approximately 1 km in diameter. Nearly identical waveforms in each cluster indicate that repeating source processes and failure mechanisms occur in the same volume. The magnitude and occurrence frequency of events for some clusters show a close temporal correlation with water level measured in a nearby moulin and surface velocity measured by differential GPS. These observations provide quantitative insights into the interplay between sudden seismogenic episodes of basal motion and glacier hydraulics, thus filling a critical data gap in our understanding of ice sheet flow.