Joint geodetic and seismic analysis of surface crevassing near a seasonal glacier-dammed lake at Gornergletscher, Switzerland

Abstract

Seasonal lake Gornersee forms at the confluence of Gornergletscher and Grenzgletscher, Switzerland, and experiences outburst floods annually in midsummer. To study the interplay between lake drainage, glacier movement and crevasse activity, high-frequency seismometers and GPS receivers were deployed in networks near Gornersee during the summer ablation seasons of 2004, 2006 and 2007. We use a Rayleigh wave coherence method to locate 3289, 7939 and 4087 icequakes, respectively, primarily along well-defined surface crevasses. We calculate two-dimensional strains from triads of GPS stations and find mean differential strain rates of ~300 × 10−6 d−1 with diurnal variations up to 800 × 10−6 d−1. Crevasse icequake activity and glacial velocity are highest during early season, then decrease as meltwater channels erode and subglacial water pressure decreases. Glacial response to Gornersee drainage varied year-to-year, with icequake activity promoted at some crevasses and inhibited at others, suggesting syn-drainage icequakes may be indicative of local drainage patterns and small-scale features of the stress field. Diurnal pulses in icequake activity exhibit peak activity at different times of day in different locations, coincident with a southeast-to-northwest trending concentrated shear zone near the Gornergletscher–Grenzgletscher confluence, likely due to differences in the timing of peak strain rate in these regions.

Breaking the Ice: Identifying Hydraulically Forced Crevassing

“…All icequake crack orientations in Figure 2c agree with the principal stress directions calculated from the observed surface velocities, as shown by the orange vectors in Figure 2c, as suggested in previous studies (Garcia et al, 2019; Harper et al, 1998). The apparent closing crack observation for the icequake at 64.327°N, 17.21°W may be supported by the presence of tensile stresses in both principal stress directions.…”

“…In any case, an opening or closing crack of a specific orientation is required to represent the observations adequately, as inferred from previous seismic observations (Mikesell et al, 2012;Neave & Savage, 1970;Roux et al, 2010;Walter et al, 2009). All icequake crack orientations in Figure 2c agree with the principal stress directions calculated from the observed surface velocities, as shown by the orange vectors in Figure 2c, as suggested in previous studies (Garcia et al, 2019;Harper et al, 1998). The apparent closing crack observation for the icequake at 64.327°N, 17.21°W may be supported by the presence of tensile stresses in both principal stress directions.…”

Stick‐Slip Tremor Beneath an Alpine Glacier

“…As in previous years, the 2007 Gornersee drainage affected glacial microseismic activity near the glacier surface and its bed (Garcia et al, 2019;Heeszel et al, 2014;Walter, 2009;Walter et al, 2008). During the 2007 lake drainage, a multihour harmonic tremor signal was discovered on July 13, 2007 (Figures 1b and 1c).…”

“…Beneath Antarctica's fast flowing Whillans Ice Stream, stick-slip tremor accompanies regular large-scale slip episodes, during which the ice stream undergoes most of its displacement (Winberry et al, 2013). No such surging behavior was observed for the tremor presented in this study as ice velocities measured at several coordinates on Gornergletscher's surface (Garcia et al, 2019) did not reveal an anomaly such as a widespread or unusually large flow acceleration ( Figure S11). Our tremor seems more similar to recent observations at a Greenland ice stream, where stable or discrete stick-slip sliding transitions into stick-slip tremor (McBrearty et al, 2020).…”

“…We next determine the polarization of the coherent signals, which our MFP algorithm optimizes to determine variations in tremor signal polarity. First, the horizontal components of the borehole sensors are aligned using linearly polarized P waves of surface crevasse icequakes that have been located based on surface waves (Garcia et al, 2019). We used 10 events, which occurred within 1-2 days of the tremor episode and do not observe systematic drifts in geographical orientation of the horizontal axes, which would be indicative of sensor rotation within a vertical borehole shaft.…”