On the stability of sleep glacier tongues: a combined seismological and ice dynamical study performed on Triftgletscher

ISSN:0374-005

Seismic activity and surface motion of a steep temperate glacier: a study on Triftgletscher, Switzerland

The tongue of Triftgletscher, Switzerland, is particularly susceptible to major break-off events due to its steep slope. In order to detect precursors of such an event, we monitored the local seismic activity and detected 2426 icequakes with sources located in an area ranging between 2050 and 2350 m a.s.l. Events triggered by cracks and icefalls were recorded, but no precise distinction between the two sources was possible using duration or frequency criteria. Clusters of seismic activity were located and confirmed by visual observations. We performed a surface motion analysis and found that surface motion was driven significantly by runoff changes at a timescale of 2-3 days. By means of a statistical analysis, a power-law behaviour of the released seismic energy distribution was discerned at certain times during the period investigated. Variations in power-law exponent values indicated that low- and high-energy events predominantly occurred during phases of enhanced and reduced surface motion, respectively. Substantial releases of seismic energy likely to signal the glacier recoupling were detected during phases of decreasing runoff. Clues to potential seismic precursors of break-off events are discussed

Seasonal variations of glacier seismicity at the tongue of Rhonegletscher (Switzerland) with a focus on basal icequakes

Fractured systems developing at the base of temperate glaciers play a significant role in the transport of subglacial water. Their influence on dynamics and hydraulics of glaciers and the seasonal alterations they experience during the hydrological year, remain however, poorly understood, as they lay beyond the reach of most conventional glaciological techniques. In this work, we investigate the seasonal evolution of glacier seismicity in the ablation zone of Rhonegletscher (Switzerland) and focus on basal fracture processes. To do this, we use seismic measurements performed between June 2012 and July 2013 and complementary measurements of surface motion and changes in melt water input inferred from degree day modeling. Results of a spectrogram analysis highlight predominant seismic emissions until end of October 2012 and from mid April 2013 in line with measured surface velocities, and indicate a rapid adaptation of the basal drainage system at initiation and end of the winter period. The analysis of icequakes emitted from two active regions shows that the basal events, due to opening and closing of cracks within fracture networks, persist after winter initiation, and points out the year-long preservation of the associated sources. In this regard, we deduce that basal crack networks aseismically evolve during the winter period.ISSN:0022-1430ISSN:1727-565

Monitoring unstable parts in the ice-covered Weissmies northwest face

The glacierized northwest face of Weissmies in the Saas valley (Switzerland) recently became unstable due to climate-induced glacier thinning of the supporting Triftgletscher below. In the case of a large break-off of ice, human infrastructure in the Saas valley is exposed to the danger of an ice/snow avalanche. A monitoring campaign was initiated with the goal of detecting precursory signals to the break-off. Interferometric and Doppler radar, optical imaging as well as GPS sensors provide measurements of surface displacements. Infrasound and seismometer arrays monitor acoustic and seismic emissions of ice avalanches and englacial fracture development. Here we discuss the monitoring methods and the results obtained so far. The unstable glacier mass did not undergo a large-scale break-off event, in fact it decelerated during the unusually warm summer months. An explanation remains elusive but likely involves subglacial processes and bedrock topography. Nevertheless, our results allow us to draw important conclusions regarding the suitability of different approaches to monitoring unstable glaciers