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Keywords:

  • probabilistic icequake locations;
  • deep icequakes;
  • icequake source mechanisms

[1] In order to improve our understanding of the dynamics of potentially unstable steep glacier tongues, we monitored during summer 2010 the micro seismicity of Triftgletscher, Switzerland. Our system, comprising 8 three‒component seismometers coupled with the ice surface, was installed upstream of the glacier's tongue, which is likely to evolve toward an unstable regime. Complementary surface motion and proglacial runoff measurements allowed the icequake activity to be interpreted in terms of glacier dynamics and hydraulics. The strong contrast in seismic wave velocities due to the underlying bedrock was taken into account using a three‒dimensional (3‒D) velocity model, implemented in a nonlinear probabilistic location procedure allowing to accurately define the hypocenter uncertainty. We located 120 icequakes, with a focal depth accuracy that allowed distinguishing between shallow events (87 events) and near‒bedrock icequakes (33 events). The first motions of most of the deep events argue against pure shear sources expected in case of stick‒slip motion, and our suggested source mechanism is a superimposed tensile crack and shear dislocation. The analysis of surface strain showed that near‒surface events represent the opening and transverse extension of surface crevasses in a confluent flow regime.