Stick‐Slip Tremor Beneath an Alpine Glacier

Abstract

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.

Plain Language Summary: For many years, researchers have observed cryoseismic stick‐slip tremor exclusively in Antarctica. Stick‐slip tremor is due to small repeating slip events at the glacier bed as a glacier advances downstream. This type of tremor is a telltale sign of what is happening at the ice‐bed interface and indicates frictional sliding. Here, we present first evidence for stick‐slip tremor at an Alpine glacier—Gornergletscher, Switzerland. We identify indicators in the seismic signature and apply data processing techniques that reveal that the creeping glacier sole moves under the influence of gravity and irregularly rubs over a sticky area at the bed.

Key Points:

We revisit a harmonic tremor recorded by a seismic array on an Alpine glacier, which was previously interpreted as hydraulic tremor.

Applying matched field processing that accounts for nonisotropic radiation patterns suggests a tremor source at the ice‐bedrock interface.

A focal mechanism derived from ice slip over bedrock explains our results and suggests seismogenic stick‐slip motion at the glacier's base.