Nocturnal Thermal Fracturing of a Himalayan Debris‐Covered Glacier Revealed by Ambient Seismic Noise

Abstract

Scientifically valuable information can be learned by listening to the tiny vibrations emanating from a glacier with seismometers. However, this approach has never been employed to better understand glaciers protected from heat by a debris mantle, despite being common in the Himalayas, one of the most glacierized regions in the world. Here we installed a seismic network at a series of challenging high-altitude sites on a glacier in Nepal. Our results show that the diurnal air temperature modulates the glacial seismic noise. The exposed surface of the glacier experiences thermal contraction when the glacier cools, whereas the areas that are insulated with thick debris do not suffer such thermal stress. Thus, the unprotected ice surface bursts with seismicity every night due to cracking, which gradually damages and weathers the ice. This is the first time such processes have been observed at relatively warm temperatures and outside of the polar regions.Plain Language Summary It has been realized that much scientifically valuable information can be learned by listening to the tiny vibrations emanating from a glacier with sensitive sensors. However, due to their remoteness and the difficulties in accessing glacial environments, this approach has rarely been employed to better understand these important systems. For example, debris-covered glaciers, which are protected from heat by a debris mantle, remain to be studied despite being common in the Himalayas, one of the most glacierized regions in the world. Here we installed a seismic network at a series of challenging high-altitude sites on a glacier in Nepal. Our results show that the diurnal air temperature modulates the glacial seismic activity. A debris mantle dampens the diurnal amplitude of temperature and thus protects the ice from cyclic mechanical damage, whereas debris-free (exposed) ice experiences intensive near-surface fracturing early in the morning. This implies that the unprotected ice surface bursts with seismicity every night due to cracking, which gradually damages and weathers the ice. This is the first time such processes have been observed outside of the polar regions. These findings are in agreement with the personal experiences of climbers who felt and heard loud cracks on high-altitude glaciers at night.

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

“…A detailed explanation of the temporary observational network deployed across the glacier system can be found in Podolskiy et al (). A comprehensive description of the study area, highlighting the importance of glaciological monitoring in this rapidly changing region near Mount Everest, is also available in Podolskiy et al (, and references within). An overview map of the installed instruments is presented in Figure .…”

“…However, volumetric expansion may be spatially limited by the collective expansion of particles and therefore lead to the buildup of stresses at locked contacts that may break coseismically when some critical stress can no longer be opposed by friction. Such hypothesis stems from the observations showing minor pulses of seismicity during the local afternoon, most pronounced at the station C1 with the thickest debris mantle (Podolskiy et al, ). To our knowledge, indirect evidence of such behavior has already been investigated in granular materials.…”

“…The model output will be compared with a proxy for seismic activity, which is the continuous power spectral density of the relative displacements integrated over the 10‐ to 30‐Hz frequency range (after Podolskiy et al, ). For a visual comparison of stresses with the seismic noise, we show an average depth‐integrated stress, , within the upper centimeters of the layer under consideration, defined as where d is the starting depth of integration (i.e., d =0 corresponds to integration either from the debris surface or from the exposed ice surface), l is the layer thickness, and τ +/− is either positive or negative stress at a particular time step and depth.…”

“…The model output will be compared with a proxy for seismic activity, which is the continuous power spectral density of the relative displacements integrated over the 10-to 30-Hz frequency range (after Podolskiy et al, 2018b). For a visual comparison of stresses with the seismic noise, we show an average depth-integrated stress,̂, within the upper centimeters of the layer under consideration, defined aŝ…”

“…No focused analysis of the relationship between thermal stress and ice fracturing has been made on debris-covered glaciers to date, despite recent interest in the role of debris on the mass balance of glaciers and snow patches (e.g., Fujita & Sakai, 2014;Podolskiy et al, 2015). The main possible factors that lead to the processes observed by Podolskiy et al (2018b) are (1) the elastic response of the ice, (2) high diurnal temperature fluctuations, and (3) weak ice with high stress at heterogeneities.…”