Gornergletscher (Switzerland)

Sliding of glacial ice over its base is typically described by a frictionless or slowly deforming bed. This view is challenged by recent seismic observations of stick-slip motion at the ice-bed interface. We revisit a high-frequency (20–35 Hz) harmonic tremor recorded on Gornergletscher, Switzerland. In contrast to previous interpretation in terms of glaciohydraulic tremor, we present evidence for superimposed stick-slip episodes as tremor sources: we locate the tremor source with matched field processing polarity optimization, which allows for azimuthal polarity patterns associated with nonisotropic moment tensors and yields a tremor source clustering near the glacier bed. Our analysis confirms an S wave radiation pattern in agreement with a double-couple source derived from ice sliding over bedrock and explains our tremor observations in terms of glacier stick-slip motion. Adding to observations of stick-slip tremor beneath polar ice streams, this first report on stick-slip tremor beneath Alpine ice favors widespread seismogenic glacier sliding.

[a] Overview map showing Gornergletscher, Grenzgletscher, and the location of the seismic array (white triangles) used for monitoring lake Gornersee’s drainage from May 25 to July 22, 2007 (CH1903 Swiss Grid coordinates. UTM coordinates for station G7J1: 407,410 m Easting, 5,091,352 m Northing [32 T]). The red dot indicates the position of the glacier moulin and the blue line the outline of Gornersee at its highest level in 2007. [b] A day-long seismic record at station G7J1 (band pass filtered between 5 and 45 Hz) and [c] the respective spectrogram showing the observed harmonic tremor. The spectrogram was computed using a 10-s window with 50 % overlap.

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Stick-Slip Tremor Beneath an Alpine Glacier