Time forecast of a break-off event from a hanging glacier

A cold hanging glacier located on the south face of the Grandes Jorasses (Mont Blanc, Italy) broke off on the 23 and 29 September 2014 with a total estimated ice vol- ume of 105 000 m 3 . Thanks to accurate surface displacement measurements taken up to the final break-off, this event was successfully predicted 10 days in advance, enabling local au- thorities to take the necessary safety measures. The break- off event also confirmed that surface displacements expe- rienced a power law acceleration along with superimposed log-periodic oscillations prior to the final rupture. This pa- per describes the methods used to achieve a satisfactory time forecast in real time and demonstrates, using a retrospective analysis, their potential for the development of early-warning systems in real time

Icequakes coupled with surface displacements for predicting glacier break-off

A hanging glacier at the east face of Weisshorn (Switzerland) broke off in 2005. We were able to monitor and measure surface motion and icequake activity for 25 days up to three days prior to the break-off. The analysis of seismic waves generated by the glacier during the rupture maturation process revealed four types of precursory signals of the imminent catastrophic rupture: (i) an increase in seismic activity within the glacier, (ii) a decrease in the waiting time between two successive icequakes, (iii) a change in the size-frequency distribution of icequake energy, and (iv) a modification in the structure of the waiting time distributions between two successive icequakes. Morevover, it was possible to demonstrate the existence of a correlation between the seismic activity and the log-periodic oscillations of the surface velocities superimposed on the global acceleration of the glacier during the rupture maturation. Analysis of the seismic activity led us to the identification of two regimes: a stable phase with diffuse damage, and an unstable and dangerous phase characterized by a hierarchical cascade of rupture instabilities where large icequakes are triggered.Comment: 16 pages, 7 figure

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

Time forecast of a break-off event from a hanging glacier

A cold hanging glacier located on the south face of the Grandes Jorasses (Mont Blanc, Italy) broke off on the 23 and 29 September 2014 with a total estimated ice volume of 105000m3. Thanks to accurate surface displacement measurements taken up to the final break-off, this event was successfully predicted 10 days in advance, enabling local authorities to take the necessary safety measures. The break-off event also confirmed that surface displacements experienced a power law acceleration along with superimposed log-periodic oscillations prior to the final rupture. This paper describes the methods used to achieve a satisfactory time forecast in real time and demonstrates, using a retrospective analysis, their potential for the development of early-warning systems in real time.ISSN:1994-0416ISSN:1994-042

Water controls the seasonal rhythm of rock glacier flow

Rock glaciers are creeping periglacial landforms experiencing strong acceleration during recent atmospheric warming and raising concerns with regard to their future behaviour and stability. High resolution kinematic observations show strong seasonal and multi-annual variations in rock glacier creep, but the linking mechanisms to environmental forcing remain poorly understood and lack quantitative models. Here we investigate the interaction between rock glacier creep and climatic forcing - temperature and precipitation - by developing a novel conceptual and numerical modelling approach. The model is constrained and the results are compared with data from the Dirru Rock Glacier (Vallis - CH). We are able to reproduce the observed velocity variations both in magnitude and phase on seasonal and inter-annual time scales. We find that water from liquid precipitation and snow melt, rather than air temperature, is the main driver of variations in rock glacier creep. Our results imply that the influence of water on rock glacier creep is fundamental and must be considered when investigating the historic and future evolution of rock glaciers

Towards early warning of gravitational slope failure with co-detection of microseismic activity: the case of an active rock glacier

We developed a new strategy for disaster risk reduction for gravitational slope failure: we propose validating on a case study a simple method for real-time early warning of gravity-driven failures that considers and exploits both the heterogeneity of natural media and characteristics of acoustic emissions attenuation. This method capitalizes on co-detection of elastic waves emanating from micro-cracks by a network of multiple and spatially distributed sensors. Event co-detection is considered to be surrogate for large event size with more frequent co-detected events marking imminence of catastrophic failure. In this study we apply this general method to a steep active rock glacier, a natural heterogeneous material sharing all relevant properties of gravitational slope failure, and demonstrate the potential of this simple strategy for real world cases, i.e., at slope scale. This new strategy being theoretically valid for all types of failures, it constitutes a first step towards the development of a new early warning system for gravitational slope failure.ISSN:1561-8633ISSN:1684-998

Acoustic and microseismic characterization in steep bedrock permafrost on Matterhorn (CH)

Understanding of processes and factors influencing slope stability is essential for assessing the stability of potentially hazardous slopes. Passive monitoring of acoustic emissions and microseismology provides subsurface information on fracturing (timing and identification of the mechanism) and therefore complement surface displacement data. This study investigates for the first time acoustic and microseismic signals generated in steep, fractured bedrock permafrost, covering the broad frequency range of 1 − 105Hz. The analysis of artificial forcing experiments using a rebound hammer in a controlled setting led to two major findings: First, statistically insignificant cross correlation between signals indicates that waveforms change strongly with propagation distance. Second, a signficant amplification is found in the frequency band 33–67 Hz. This finding is strongly supported by evidence from artificial rockfall events and more importantly by naturally occurring fracture events identified in fracture displacement data. Thus, filtering this frequency band enables enhanced detection of microseismic events relevant for slope stability assessment. The analysis of 2-year time series in this frequency band further suggests that the detected energy rate baseline of all automatically triggered events using the STA/LTA algorithm is not sensitive to temperature forcing, an observation of primary importance for long-term data collection, analysis, and interpretation. The event detection in the established frequency band is not only improved but also not affected by the short- and long-term temperature changes in such a rapidly changing environment

Acoustic and Microseismic Characterization in Steep Bedrock Permafrost on Matterhorn (CH)

ISSN:0148-0227ISSN:2169-9003ISSN:2169-901