Viscoelastic Modeling of Nocturnal Thermal Fracturing in a Himalayan Debris‐Covered Glacier

Abstract

Recent observations suggest that the nocturnal thermal fracturing of ice occurs at relatively warm temperatures (above −15 °C) at a high‐altitude Himalayan glacier system unless the ice is shielded by a debris mantle. Here we estimate the stresses induced by diurnal temperature variations using viscous, elastic, and two viscoelastic models, and various thicknesses of the debris mantle. Only the elastic and visco‐elastic models are in agreement with the observations. The timing and amplitudes of the stresses in the upper 15 cm of the glacier are different among the models despite the ability of each approach to predict a diurnal increase in tension exceeding the critical threshold proposed for crevasse formation. For example, the elastic stress is several times larger than the viscous stress at the ice surface (650 vs. 250 kPa) and reaches its peak up to 5–6 hr later in the night. The time lag is in line with the seismic records, suggesting that the viscous model is not appropriate. Furthermore, a debris layer of ≥50 cm in thickness suppresses the diurnal fluctuations in thermal stress and therefore protects the ice from mechanical damage. We suggest that high‐amplitude diurnal cooling and weak ice properties due to weathering are essential factors that influence thermal fracturing in the Himalayan environment. The ongoing expansion of seismic networks into cryospheric regions, which will be capable of detecting local thermal‐contraction‐induced cracks, in combination with the fact that such cracks can erode and weaken the ice, and thereby serve as meltwater and heat channels, warrants further research to better understand these near‐surface processes and to monitor ice properties.

Ice Cliff Dynamics of Debris-Covered Trakarding Glacier in the Rolwaling Region, Nepal Himalaya

“…Debris-covered Trakarding Glacier (27.9°N, 86.5°E) is located in Rolwaling Valley in the eastern Nepal Himalaya (Figure 1A,B); the debris-free Trambau Glacier is situated above (Sunako et al, 2019), and has been disconnected from Trakarding Glacier since the 1970s. Previous studies have treated the two glaciers as the "Trakarding-Trambau Glacier system" (Podolskiy et al, 2018;Podolskiy et al, 2019;Sunako et al, 2019). The total area of the system is 31.7 km 2 (Nuimura et al, 2015), and spans elevations of 4,500-6,690 m above sea level (a.s.l.).…”

Hanging glacier monitoring with icequake repeaters and seismic coda wave interferometry: a case study of the Eiger hanging glacier

“…Such a time shift can suggest a time delay between the transfer of stresses generated in the back of the glacier to the front of the glacier that is frozen to its bed, and the unstable part. This behaviour could be potentially described using a viscoelastic model based on, e.g., a spring (elasticity)-dashpot (viscosity) Maxwellian model (e.g., Podolskiy et al, 2019). The lamella could be described as an additional dashpot introducing a time shift between the applied stress and the deformation at the glacier front.…”

Long term analysis of cryoseismic events and associated ground thermal stress in Adventdalen, Svalbard

“…Previous studies such as Mellon (1997), Maloof et al (2002), Schulson and Duval (2009) and Podolskiy et al (2019) have demonstrated that ice and frozen soil deform elastically on short timescales and viscously on long timescales. Thermal loading due to temperature changes acts as an external driving agent, and the resulting dynamical balance between the elastic and viscous response governs whether creep or fracture become dominant.…”