Diurnal seismicity cycle linked to subsurface melting on an ice shelf

MacAyeal, Douglas R.; Banwell, Alison F.; Okal, Emile A.; Lin, Jinqiao; Willis, Ian; Goodsell, Becky; Macdonald, Grant J.

doi:10.1017/aog.2018.29

Cited by 28 publications

(42 citation statements)

References 71 publications

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“…At temperature of −5 • C, Mellon (1997) and Petrich et al (2015) approaches lead to Young's modulus of approximately 4.2 and 6.0 GPa, respectively, which is consistent with a constant value of 5 GPa used by MacAyeal et al (2019) for estimating diurnal thermal bending moment of an ice plate.…”

Section: Appendix: A: Empirical Temperature Modelsupporting

confidence: 78%

“…Sea ice thermal-contraction cracking at such relatively modest temperatures might be due to thermal bending, which intensifies the stress (Bazant, 1992). Similarly, the thermal bending of a frozen lid overlying slushy ice/water was associated with seismicity on an ice shelf in Antarctica (MacAyeal et al, 2019). To our knowledge, with the exception of a study in Tibet by Zhang et al (2019), the thermal contraction cracking of glacier ice has not been observed at temperate alpine glaciers (Neave & Savage, 1970) or in the ablation zone of the Greenland Ice Sheet, where dynamic stresses and hydrofracturing are dominant (Röösli et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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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.

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Section: Appendix: A: Empirical Temperature Modelsupporting

confidence: 78%

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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“…Jacking accompanied by seismicity is reported during rapid drainage events of supraglacial lakes (Das et al, 2008;Doyle et al, 2013). We also note that high occurrence rates of overlapping fracturing icequakes may result in sustained tremor-like fracturing events (Podolskiy et al, 2018;MacAyeal et al, 2019).…”

Section: Introductionmentioning

confidence: 61%

“…We expect that conduits tend to close due to ice creep as discharge decreases at the end of the drainage. However, we note that the contraction of conduits takes place on the order of days to weeks, especially for thin ice (less than 100 m) as encountered on Glacier de la Plaine Morte (Mathews, 1973). This suggests that our inferred closure rates of the relative hydraulic radius (Fig.…”

Section: Discussionmentioning

confidence: 76%

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Glaciohydraulic seismic tremors on an Alpine glacier

Lindner¹,

Walter²,

Laske

et al. 2020

The Cryosphere

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Abstract. Hydraulic processes impact viscous and brittle ice deformation. Water-driven fracturing as well as turbulent water flow within and beneath glaciers radiate seismic waves which provide insights into otherwise hard-to-access englacial and subglacial environments. In this study, we analyze glaciohydraulic tremors recorded by four seismic arrays installed in different parts of Glacier de la Plaine Morte, Switzerland. Data were recorded during the 2016 melt season including the sudden subglacial drainage of an ice-marginal lake. Together with our seismic data, discharge, lake level, and ice flow measurements provide constraints on glacier hydraulics. We find that the tremors are generated by subglacial water flow, in moulins, and by icequake bursts. The dominating process can vary on sub-kilometer and sub-daily scales. Consistent with field observations, continuous source tracking via matched-field processing suggests a gradual up-glacier progression of an efficient drainage system as the melt season progresses. The ice-marginal lake likely connects to this drainage system via hydrofracturing, which is indicated by sustained icequake signals emitted from the proximity of the lake basin and starting roughly 24 h prior to the lake drainage. To estimate the hydraulics associated with the drainage, we use tremor–discharge scaling relationships. Our analysis suggests a pressurization of the subglacial environment at the drainage onset, followed by an increase in the hydraulic radii of the conduits and a subsequent decrease in the subglacial water pressure as the capacity of the drainage system increases. The pressurization is in phase with the drop in the lake level, and its retrieved maximum coincides with ice uplift measured via GPS. Our results highlight the use of cryo-seismology for monitoring glacier hydraulics.

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Narrowband signals recorded near a moulin that are not moulin tremor: a cautionary short note

Carmichael¹

2019

Ann. Glaciol.

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Geophysicists that deploy seismic sensors in ablation zones of glaciers and ice sheets to record glaciogenic signatures can confront recording challenges caused by instrument melt-out or tilt. These challenges often require installing sensors in boreholes to delay melt-out, or securing sensors to structures that improve coupling. We show that some of these structures that were buried near a moulin at a snow-free site in the ablation zone of the Western Greenland Ice Sheet resonated as they became exposed, and caused their geophones to record temporally evolving, narrowband signals that mimic features of glaciogenic sources like moulin tremor. We quantify these artifacts with a mechanical model that shows instruments undergo structural resonance as they melt-out, at exposure rates that we predict from an ablation model (RACMO). These models reproduce general spectral features in our data, and enable us to estimate what instrument exposure reduces ice-to-sensor coupling enough to prevent icequake detection. Last, we use our resonance data to quantitatively measure how narrowband signals that originate from either artificial or glaciogenic sources will reduce the ability of certain waveform detectors (correlators) to capture transient seismic events, even if sensors remain coupled.

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Diurnal seismicity cycle linked to subsurface melting on an ice shelf

Cited by 28 publications

References 71 publications

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

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

Glaciohydraulic seismic tremors on an Alpine glacier

Narrowband signals recorded near a moulin that are not moulin tremor: a cautionary short note

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