[1] During the 2004 outburst flood of Gornersee, Switzerland, we observed surface microseismic activity (so-called icequakes) near the glacier-dammed lake on Gornergletscher. Using surface waves, we located near-surface tensile fracturing events by inverting time delays using a quasi-Newton scheme. We located 8520 near-surface events with uncertainties of less than 10 m. Seismic activity increased during the lake drainage, and the icequake epicenters migrated downglacier. Furthermore, we identified events whose fault plane orientations are nearly perpendicular to the local pattern of surface crevassing. These observations can be explained by the rotation of principal strains at the glacier surface deduced from local ice flow measurement which occurred during the flotation of the ice dam at the onset of the lake drainage. In general, we suggest that our technique is a useful tool to locate large data sets of microseismic events and can be used to monitor the temporal evolution of fracture processes and their dependence on glacier flow and glacier hydrology.Citation: Roux, P.-F., F. Walter, P. Riesen, S. Sugiyama, and M. Funk (2010), Observation of surface seismic activity changes of an Alpine glacier during a glacier-dammed lake outburst,

[ 1 ] To investigate the triggering and the drainage mechanisms of ag lacier-dammed lake outburst, we conducted high-frequency measurements of the ice surface motion in the vicinity of Gornersee, an ice marginal lake on Gornergletscher,S witzerland. During the outburst event in July 2004, the ice surface within ad istance of 400 mf rom the lakeshore moved vertically upward by up to 0.1 m. This vertical surface motion cannot be explained by vertical straining of ice which was measured in one of the boreholes; therefore, we suggest the separation of the glacier sole from the bed was caused by subglacially drained lake water.Our observation indicates that the lake water drained as a sheet-like flow through the space created by the basal separation. The upward surface motion was greater in the region where the ice flotation level was exceeded by the lake level, implying that the ice barrier was breached when the lake water hydraulically connected to the bed and lifted up the glacier.I na ddition to the centimeter-scale vertical ice motion, three survey stakes located within 100 mfrom the lake showed extraordinarily large vertical displacement of 0.5-3.0 ma ssociated with abrupt changes in horizontal flow direction. Ap lausible interpretation is that the marginal ice wedge bent upward because of the buoyancy force generated by the drained water.Such bending is possible if subglacial and englacial fractures formed at about 200 mf rom the glacier margin and acted as ah inge. The newly formed and preexisting englacial fractures probably took the role of inducing englacial water drainage which preceded the outburst.

We present the thermal distribution in the confluence area of Gorner-and Grenzgletscher, Valais, Switzerland. The area was mapped by ice-penetrating radar at 1-5 and 40 MHz. The higherfrequency data reveal a thick surface layer of low backscatter in the center of the Grenzgletscher branch. Based on datasets of borehole-temperature measurements and flow velocity, we interpret this as a thick layer of cold ice, advected from the accumulation region of Grenzgletscher. Along seven profiles the base of the low-backscatter zone can be found at a maximum depths between approximately 100 and 200 m. Laterally, the layer extends some 400 m, ∼1/3 of the width of the Grenzgletscher branch. The lower boundary of the low-backscatter zone is systematically higher than the cold-temperate transition surface found in the boreholes. This discrepancy is attributed to the direct sensitivity of radar backscatter to liquid-water inclusions, rather than to the temperature distributions as observed in boreholes. We present the current state of the cold layer and discuss its influence on other glacier characteristics.

[1] High frequency ice flow measurements during speed-up events in Gornergletscher, Switzerland, revealed intriguing ice motion which has never been reported in detail before. During the summer 2005, more than a 100% flow speed increase was observed three times at four GPS stations installed across Gornergletscher. The speed-ups were accompanied by a decimeter scale surface uplift. Two of the events were triggered by intensive surface melt and rainfall, while the third one was due to the outburst of Gornersee, a glacier-dammed lake located 2 km upglacier. An interesting observation was ice motion deviating toward the side margins during the events. As the glacier accelerated, a transverse (cross glacier) velocity component was generated, turning the flow direction away from the central flow line toward the margins. When the glacier decelerated, the transverse velocity component reversed so that the ice flowed back to the azimuth of the initial flow direction. In the most significant case, the trajectory of the survey stake deviated from the original track by 0.2 m in the transverse direction. We hypothesize that the observed lateral ice motion was caused by locally elevated subglacial water pressure. When the ice sole decoupled from the bed at a part of the glacier, a point source of vertical displacement was transmitted to the surface through viscous ice. This caused the transverse as well as vertical surface motion, as observed in ground motion during magma intrusion. The hypothesis was tested with a two-dimensional ice flow model applied to the transverse glacier cross section. The model confirmed that the surface ice would move toward the margins as observed in Gornergletscher, if subglacial water pressure exceeded the ice overburden pressure over a limited part of the bed.Citation: Sugiyama, S., A. Bauder, P. Riesen, and M. Funk (2010), Surface ice motion deviating toward the margins during speed-up events at Gornergletscher, Switzerland,

We report measurements using a portable real aperture radar (Gamma Portable Radar Interferometer (GPRI)) for interferometric imaging of the surface ice motion on Gornergletscher, Switzerland, during the drainage of the adjacent ice-marginal lake Gornersee. The GPRI tracked the surface ice motion in line of sight over an area of $3 km 2 down-glacier of Gornersee almost continuously during the drainage event. The displacement maps derived from the acquired interferograms capture the spatial distribution of the surface ice motion. Due to fast acquisition times of the microwave images, the GPRI was able to record sub-daily variations of the ice displacements, most likely caused by the impact of the Gornersee drainage on the ice motion of Gornergletscher. In situ point measurements of the ice displacement agree reasonably well with the results obtained by the GPRI and highlight the use of the GPRI for high-resolution measurements of glacier surface ice motion.