Melt water on the Greenland ice sheet in August 2011 (Image © Stephan Husen).

A huge moulin on the Greenland ice sheet, August 2011.

Crevasses on the Greenland ice sheet near Ilulissat, western Greenland, August 2011.

Waveform of an ice quake caused by the successive opening or closing of crevasses.

A waveform created by the calving of the Jakobshavn Isbræ (with chunks of ice breaking off into the sea), a glacier of the Greenland ice sheet. The waveform was recorded approximately 30 kilometers away from the calving front with seismometers installed on the ice.

A floating iceberg in the ice fjord near the KULLO station and the village Kullorsuaq in northeastern Greenland. Kullorsuaq is inhabited by some 400 Inuits and is one of the northernmost settlements in the world (74°N).

Waveform of an ice quake that occurred approximately 600 meters below the surface of the ice in the area where the ice reaches the glacier bed. The second, smaller deflection after approximately two-thirds of the signal belongs to the same ice quake (S-wave).

Waveform of an ice quake that occurred approximately 100 meters below the surface of the ice.

The pattern of this waveform is known as a tremor. The recording shows a period of eight hours during which the tremor was initiated. The tremor is suspected to have originated in a moulin and is most likely associated with the geometry of underground channels and the incoming water.