Meltwater influences on deep stick-slip icequakes near the base of the Greenland Ice Sheet

Abstract

We detected over 11,000 stick-slip icequakes near the base of the western margin of the Greenland Ice Sheet using a 17-seismometer array. These icequakes have negative (i.e., very small) moment magnitudes and, according to similarities in their waveforms, group into over 100 distinct clusters distributed beneath our 3 × 3 km study area. Some clusters were active for several weeks, while others have burst-like episodes lasting 1-6 days only. Some clusters correlate with subglacial water pressure measured within a nearby moulin. For these clusters, we observe high water pressure concurrent with many small yet numerous stick-slip icequakes and periods of lower water pressures with larger, less frequent icequakes. These patterns might change over time and are not common to all clusters. We explain these observations that the stick-slip icequakes are located at sticky spots at the interface of the ice sheet with the glacier bed that consists of basal till characterized by different connectivity to the subglacial drainage system. Because the till's frictional strength depends on its pore pressure, variations in subglacial water pressure can either weaken or strengthen the bed; this explains the variation in seismic moments and interevent times. Our results suggest that seismogenic stick-slip motion is an integral part of the flow mechanism in the ablation zone in western Greenland, which is highly sensitive to the configuration of the local subglacial drainage system. Stick-slip motion may therefore play a key role in the relationship between climate-induced changes of surface runoff and ice sheet dynamics.