Tremors reveal the structure of deep shafts in the Greenland ice sheet, so why aren't we afraid?

Those who do pay attention to global warming know that Greenland is melting. The world’s second largest ice sheet is melting fast, the rate of melt is accelerating, and it is not going to stop melting anytime soon. In fact, NASA’s twin GRACE satellites measured it gushing 2 trillion tons of ice emptying both fresh melt water, and ice bergs into the North Atlantic over the past decade.

The drainage system on the surface of the ice sheet has been studied for years. The accumulated meltwater forms streams, rivers and lakes called supraglacial lakes. In the case of rivers and streams the water carves a vertical shaft into the ice called a Moulin. This narrow and tubular shaft provides the pathway for surface meltwater to reach all the way to the bottom. In the case of the surface lakes, it is believed that nearby Moulin’s accumulates water at the bottom of the ice sheet which creates a bulge that floats the ice sheet creating tension at the surface underneath the lake. The stress continues until a crack forms below the lake. The volume of water in the lake widens and extends the crack, or hydro-fracture, and the lake water surges to the bedrock. 

NASA notes that subglacial lakes form at the “base of an ice sheet or glacier because of either friction or trapped heat from bedrock below. In order for this to happen, ice needs to move quickly or be thick enough to protect the ice sheet base from cold air at the surface and trap heat coming from the bedrock below.”  The National Science Foundation team that NASA references, observed that that the “basin floor rose significantly during the next summer at the same time that nearby surface meltwater drained into cracks along the basin’s edge. This led the team to hypothesize that water from surface melting was refilling a lake beneath the ice.

Earth and Space Science news reports on a new report from Roeoesli et al,.  on how seismic waves produced by free-falling meltwater could improve understanding of glacial drainage processes.

On most days the seismometers detected tremors that lasted from 4 to 16 hours. According to seismometer data, these events made the moulin’s ice walls resonate in patterns similar to those experienced by an organ pipe with one end plugged. Just as organ pipe resonance depends on the length of the tube, the observed moulin patterns appeared to depend on the level of the collected water.

On the basis of this similarity, the scientists built a model of moulin tremor production. In the model, free-falling meltwater causes tremors when it strikes the surface of the water deep inside a moulin. The impact sends acoustic pressure waves into the walls and base of the shaft and through the ice. Resonance patterns picked up by seismometers can be interpreted to reveal the moulin’s size, water level, and water depth.

Other scientists have previously used seismic data to examine the flow of glacial meltwater, but this marks the first time they have been used to model a glacial moulin. However, this model approximates a moulin as a simple cylinder, and scientists suspect that many moulins actually consist of an alternating series of shafts and wide pools. Future research could refine seismic techniques to detect more complex moulin structures and examine how moulins affect glacial movement. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2015JB012786, 2016)

From the abstract.

Through glacial moulins, meltwater is routed from the glacier surface to its base. Moulins are a main feature feeding subglacial drainage systems and thus influencing basal motion and ice dynamics, but their geometry remains poorly known. Here we show that analysis of the seismic wavefield generated by water falling into a moulin can help constrain its geometry. We present modeling results of hour-long seimic tremors emitted from a vertical moulin shaft, observed with a seismometer array installed at the surface of the Greenland Ice Sheet. The tremor was triggered when the moulin water level exceeded a certain height, which we associate with the threshold for the waterfall to hit directly the surface of the moulin water column. The amplitude of the tremor signal changed over each tremor episode, in close relation to the amount of inflowing water. The tremor spectrum features multiple prominent peaks, whose characteristic frequencies are distributed like the resonant modes of a semiopen organ pipe and were found to depend on the moulin water level, consistent with a source composed of resonant tube waves (water pressure waves coupled to elastic deformation of the moulin walls) along the water-filled moulin pipe. Analysis of surface particle motions lends further support to this interpretation. The seismic wavefield was modeled as a superposition of sustained wave radiation by pressure sources on the side walls and at the bottom of the moulin. The former was found to dominate the wave field at close distance and the latter at large distance to the Moulin.