Basal icequakes during changing subglacial water pressures beneath Gornergletscher, Switzerland

Abstract

Using dense networks of three-component seismometers installed in direct contact with the ice, the seismic activity of Gornergletscher, Switzerland, was investigated during the summers of 2004 and 2006, as subglacial water pressures varied drastically. These pressure variations are due to the diurnal cycle of meltwater input as well as the subglacial drainage of Gornersee, a nearby marginal ice-dammed lake. Up to several thousand seismic signals per day were recorded. Whereas most icequakes are due to surface crevasse openings, about 200 events have been reliably located close to the glacier bed. These basal events tend to occur in clusters and have signals with impulsive first arrivals. At the same time, basal water pressures and ice-surface velocities were measured to capture the impact of the lake drainage on the subglacial hydrological system and the ice-flow dynamics. Contrary to our expectations, we did not observe an increase of basal icequake activity as the lake emptied, thereby raising the subglacial water pressures close to the flotation level for several days. In fact, the basal icequakes were usually recorded during the morning hours, when the basal water pressure was either low or decreasing. During the high-pressure period caused by the drainage of the lake, no basal icequakes were observed. Furthermore, GPS measurements showed that the glacier surface was lowering during the basal seismic activity. These observations lead us to conclude that such icequakes are connected to the diurnal variation in glacier sliding across the glacier bed.

Subglacial till behaviour derived from in situ wireless multi-sensor subglacial probes: Rheology, hydro-mechanical interactions and till formation

“…We suggest that, during the winter, surface water, generated from rainfall or surface melt, takes a long time to pass through the glacier since the majority of englacial pathways have closed and the glacier is snow covered (Roberts et al, 2006;Purdie et al, 2008). The water that does reach the subglacial environment may cause an increase in glacier velocity (Iken et al, 1983;Mair et al, 2001;Bartholomaus et al, 2008;Walter et al, 2008). This may be particularly relevant during the winter when the subglacial drainage system will be poorly developed and of a low capacity (Willis, 1995;Rose et al, 2009).…”

Moment Tensor Inversions of Icequakes on Gornergletscher, Switzerland

“…Several types of source mechanisms have been postulated or assumed, such as surface crevasse formation (e.g., Neave and Savage, 1970;Deichmann et al, 2000), stick-slip motion (Weaver and Malone, 1979;Roux et al, 2008), resonant water-filled cavities (Métaxian et al, 2003), or bottom crevasse formation due to increased basal drag during low subglacial water pressures (Walter et al, 2008). Whereas these conclusions were drawn on the basis of hypocentral locations, temporal variations in activity, or frequency content of the seismograms, we are unaware of any publication on full-waveform inversions for the source parameters for these classes of events.…”

“…The high-pass corner frequencies of the sensors were between 1 and 28 Hz, and the instruments were operated at high-sampling frequencies (1000-4000 Hz) in trigger mode. For a detailed description of the instrumentation and seismic data as well as detection and location algorithms, the reader is referred to Walter et al (2008).…”

“…The data used for the present analysis were acquired as part of an investigation of the yearly subglacial drainages of Gornersee, a nearby ice-marginal lake (e.g., Huss et al, 2007;Sugiyama et al, 2007;Walter et al, 2008). The instrumentation for the 2004 and 2006 seismic arrays consisted of 14 and 24 seismometers, respectively, most of which were installed at the glacier surface (Fig.…”

A wireless sensor network system deployment for detecting stick slip motion in glaciers

“…Geophones have previously been used to monitor glacial movement; one example is [5] in which the data from geophones with a frequency response peak of 12Hz are continually sampled. Whilst in an ideal situation all data could be stored for later analysis, limitations of the deployment environment mean that this is not possible.…”