Spatial and Temporal Aspects of Alpine Icequakes During Three Seasons of Glacier-Dammed Lake Drainages: Gornergletscher Switzerland

Using high-density seismometer arrays we examine the temporal and spatial signature of icequake activity on Gornergletscher, Switzerland, during drainages of a nearby glacier-dammed lake in the summer months in 2004, 2006 and 2007. Our seismometer arrays had apertures of approximately 300-400m and consisted of 8-24 3-component seismometers installed at the glacier surface. In addition to the seismic measurements, we monitored surface displacements, subglacial water pressures and lake level. During the three summers we observed three different kinds of lake drainages: rapid subglacial (2004), slow supraglacial (2006) and a combination supraglacial, subglacial and englacial (2007). Unlike in 2006, large calving events preceded the 2004 and 2007 drainages. In 2004 the lake drainage initiated within a day or less after the calving event, whereas in 2007 the lag time between the calving event and drainage onset was between 5 and 7 days. Within each season, we recorded up to several thousand icequakes per day. The large majority of these seismic events constitutes fractures and crevasses opening near the glacier surface. There was a substantial increase in icequake productivity coincident with the lake drainages in 2004 and 2007. The differences in lag times between calving and lake drainage in 2004 (< 1 day lag) and 2007 (~5-7 days lag) indicate that acceleration of icequake detections occurred in response to the lake drainages rather than the calving events. This may be inherent to the event detection method, which is sensitive to changes in seismic background noise. Using a recently developed automatic surface icequake location technique based on Rayleigh wave coherence (Roux et al., 2010) we locate about 20-40% of the icequake catalog. We observe different spatial patterns of icequake locations: lineation along surface crevasses, diffuse scattering of icequakes and regions exhibiting activity voids. With the onset of the lake drainage in 2007, icequake activity appears more prolific in regions close to the lake, exhibiting a decrease in activity at more distant locations from the lake. Within a surface crevasse field in the vicinity our network, the onset of the lake drainage inhibits seismic emission. This is similar to observed changes in seismic activity during the 2004 lake drainage. We suggest that the observed temporal/spatial shift in surface seismicity is a response to changes in the glacier’s stress regime induced by two main aspects of the lake drainage: first, basal motion enhanced by drainage water input and, second, the detachment of the lake’s ice dam during the calving event preceding the lake drainage.