Seismic moulin tremor

• Published in: Journal of Geophysical Research: Solid Earth
• Main Authors: Roeoesli, Claudia, Walter, Fabian, Ampuero, Jean-Paul, Kissling, Edi
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Geophysical Union 2016
• Subjects: seismic tremor; moulin; Greenland Ice Sheet; englacial hydraulics; Greenland; glacier; Ice Sheet
• Online Access: https://doi.org/10.1002/2015JB012786

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. © 2016 American Geophysical Union. Received 2 JAN 2016; Accepted 10 JUL 2016; Accepted article online 13 JUL 2016; Published online 8 AUG 2016. This project was funded by Swiss Federal Institute of Technology Zürich (ETH) under grant ETH-27 10-3. The salary of F.W. was partially funded by the European Union Seventh Framework Programme (FP7-PEOPLE-2011-IEF) under grant agreement 29919 and by the Swiss National Science Foundation (GlaHMSeis Project ...