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Distributed acoustic sensing (DAS) is a promising technology for providing dense (metre-scale) sampling of the seismic wavefield. However, harnessing this potential for earthquake detection with accurate phase picking and associated localization remains challenging. Single-channel algorithms are limited by individual channel noise, while machine learning and semblance methods are typically imited to specific geological settings, have no physically constrained phase association and/or require specific fibre geometries. Here, we present a method that seeks to detect seismicity for any geological setting, applicable for any fibre geometry, and combining both fibreoptic and conventional seismometer data to maximize the information used for detection and source localization. This method adapts a proven back-migration detection method to also include DAS observations, migrating energy from many receivers back in time to search for localized peaks in energy, corresponding to seismic sources. The strengths of this method are capitalizing on coherency over many channels to enhance detection sensitivity even in high-noise environments compared to single-channel algorithms, applicability to arbitrary fibre geometries, as well as built-in, physics-informed phase association and source localization. We explore the performance of the method using three geologically and geometrically diverse settings: a glacier, a volcanic eruption and a geothermal borehole. Our results evidence the effect of spatial-sampling extent and non-optimal fibreoptic geometries, accounting for P - and S -wave sensitivity, coupling effects and how the sensitivity of native fibreoptic strain measurements to shallow subsurface heterogeneities can affect detection. Finally, we attempt to also present a method-ambivalent overview of key challenges facing fibreoptic earthquake detection and possible avenues of future work to address them. [ABSTRACT FROM AUTHOR]

This paper presents the first results from the monitoring of local seismicity in Severnaya Zemlya from the end of 2016 to 2023 inclusive as recorded by a single permanent seismic station installed on Bolshevik Island. A total of 73 local seismic events have been identified with P and S phases. We considered the question whether these events could be classified (earthquakes or icequakes) by comparing waveforms and spectral-temporal analysis diagrams with regional earthquakes that have occurred in the archipelago area. The spatio-temporal distribution and the rate of migration for the events show that glacial events can arise by stress release in glaciers as shallow crustal earthquakes occur within ~30 km. It is shown that, when a seismograph network is difficult to deploy, even a single permanent seismic station can furnish useful information on glacial events and crustal earthquakes. [ABSTRACT FROM AUTHOR]

Seismology is increasingly used to infer the magnitude and direction of glacial ice flow. However, the effects of interstitial meltwater on seismic properties remain poorly constrained. Here, we extend previous studies on seismic anisotropy in temperate ices to consider the role of melt preferred orientation (MPO). We used the ELLE numerical toolbox to simulate microstructural shear deformation of temperate ice with variable MPO strength and orientation, and calculated the effective seismic properties of these numerical ice‐melt aggregates. Our models demonstrate that even 3.5% melt volume is sufficient to rotate fast directions by up to 90°, to increase Vp anisotropy by up to +110%, and to modify Vs anisotropy by −9 to +36%. These effects are especially prominent at strain rates ≥3.17 × 10−12 s−1. MPO may thus obscure the geophysical signatures of temperate ice flow in regions of rapid ice discharge, and is therefore pivotal for understanding ice mass loss. Plain Language Summary: Ice on Earth is pulled toward the sea by gravity, contributing to global mean sea level rise. To better understand the flow, or movement, of ice at the continent scale, geophysical surveys are increasingly being used to measure the microscopic alignment ("fabric") of ice crystals, since ice with a strong fabric flows more readily. However, in temperate regions close to the ice melting point, melt pockets may also become aligned, creating additional macroscopic geophysical signatures. Here, we use numerical simulations to examine the combined effects of ice crystal fabric and melt alignment on the geophysical (seismic) properties of ice containing small amounts of melt. We show that melt can change the seismic fast direction (related to the inferred flow direction) of ice by up to 90°, particularly as the volume of melt exceeds 3.5%, and that different melt orientations can either enhance or diminish the anisotropy‐based estimates of flow that some studies use to predict ice mass loss. These effects are especially prominent in faster‐flowing ice, highly relevant to ice mass loss in warming regions. Geophysical studies that do not account for melt orientation may therefore produce incorrect estimates of flow, leading to inaccuracies in future climate models. Key Points: We used simulations of deforming temperate ice to show that the alignment of interstitial meltwater changes bulk seismic propertiesThe amount of melt required to significantly modify seismic properties is at least 50% lower than previously reportedMelt should be considered when using seismic and radar anisotropy to interpret viscous ice deformation and enhancement [ABSTRACT FROM AUTHOR]

Glacier seismology is a valuable tool for investigating ice flow dynamics, but sufficient data acquisition in remote and exposed glaciated terrain remains challenging. For data acquisition on a highly crevassed and remote outlet glacier in Greenland we developed self-sufficient and easily deployable seismic stations, "SG-boxes". The SG-boxes contain their own power supply via solar panel, a three-component omni-directional geophone and a GNSS receiver. The SG-boxes can be deployed and retrieved from a hovering helicopter, allowing for deployment in difficult terrain. To assess their performance we conducted a field test comparing the SG-boxes to established on-ice geophone installations at Gornergletscher in Switzerland. Moreover, data from a first SG-box deployment in Greenland were analyzed. The SG-boxes exhibit consistently higher noise levels relative to colocated conventional geophones and a correlation between noise levels, wind and air temperature is found. Despite their noise susceptibility, the SG-boxes detected a total of 13,114 Gornergletscher icequakes over 10 days, which is 30% of the total number of icequakes detected by conventional geophone stations. Hence, even in sub-optimal weather conditions and without additional noise reduction measures, the SG-boxes can provide unique and valuable data from challenging glaciated terrain where no conventional seismic installations are possible.

Seismic instruments such as broadband seismometers and distributed acoustic sensors (DAS) have a demonstrated potential for wide‐scale and continuous in situ monitoring of near‐surface environmental and anthropogenic processes. DAS is attractive for development as a multi‐geophysical observatory due to the prevalence of existing fiber infrastructure in regions with environmental, cultural, or strategic significance. Here we present results from a multi‐seasonal acquisition of DAS data on a seafloor cable in the Beaufort Sea, Alaska. During a November 2021 data collect we captured the rapid transition of ambient noise characteristics from an "ice‐free" state to an "ice‐bound" state. A sea ice formation front was plainly visible on the DAS record and was observed to propagate 20 km seaward over a period of 8 hr. Satellite‐based instrumentation were unable to record this event due to cloud cover, low light conditions, and orbital frequency. Plain Language Summary: Distributed acoustic sensing (DAS) is an emerging technology that uses fiber optic cable to mimic a high resolution array of one dimensional seismometers. Distributed acoustic sensors is compatible with any single‐mode fiber optic cable, including seafloor telecommunications cables. Previous recordings of DAS data on seafloor cables have demonstrated that this technology is able to detect and track individual ocean waves, surface vessels, and whales. In this first‐ever acquisition of DAS data from a seafloor beneath a frozen ocean, we show that DAS is also capable of mapping sea ice coverage with unprecedented spatial resolution and can track sea ice formation and breakup up in near real time. Key Points: Seafloor distributed acoustic sensing can distinguish between sea ice and open water on the basis of endemic wave modesThis technology is capable of high resolution, near real time imaging of sea ice dynamics, such as rapid freezing of marginal iceThis can complement large scale, low resolution satellite‐based imaging in regions of high scientific or operational importance [ABSTRACT FROM AUTHOR]

It is critical to understand the dynamic conditions of Earth's cryosphere, yet the subglacial and englacial environments that control many aspects of ice behavior are inherently difficult to observe. The study of seismicity in glaciers and ice sheets has provided valuable insights about the cryosphere for decades, more recently aided by tools from machine learning. Here, we present an unsupervised machine‐learning approach to discovering and interpreting cryoseismic patterns using 5 weeks of seismic data recorded at Gornergletscher, Switzerland. Our algorithm utilizes non‐negative matrix factorization and hidden Markov modeling to reduce spectrograms into characteristic, low‐dimensional "fingerprints," which we reduce further using principal component analysis, then cluster using k‐means clustering. We investigate the timing, locations, and statistical properties of the clusters in relation to temperature, GPS and lake‐level measurements, and find that signals associated with lake flooding tend to occupy one cluster, whereas signals associated with afternoon and evening melt‐water flow reside in others. We suggest that the one cluster contains signals that include the true initiation of the flood's englacial and subglacial drainage components. This work demonstrates an unsupervised machine‐learning approach to exploring both continuous and event‐based glacial seismic data. Plain Language Summary: Earth's climate change poses extreme risks for the stability of glaciers and ice sheets. One such glacier, Gornergletscher in the Swiss Alps, has been monitored for decades, along with a lake on the glacier which from the 1950s until recently filled and drainedevery summer, releasing huge amounts of water under, through, and on top of the glacier. To study this glacier, which is hundreds of meters thick, and its lake, scientists employ "cryoseismology," the study of seismic excitations in the ice. In our study, we use unsupervised machine‐learning, a type of artificial intelligence that identifies patterns in large data sets, to organize 5 weeks of cryoseismic records into clusters composed of similar types of signals. Although the machine‐learning algorithm has no knowledge of glacial hydrology nor its evolution over time, the output places the signals associated with the beginning of lake flooding in one cluster, and signals associated with afternoon and evening melt‐water flow in others. We also find that "icequakes" are clustered based on extremely subtle, similar, vibrations. From these findings, we provide an improved chronology for the glacial lake flood, and a better understanding of the cryoseismic signals caused by water flowing underneath and through glaciers. Key Points: We use unsupervised machine‐learning and clustering to reveal time‐varying features in seismic records from Gornergletscher, Swiss AlpsWe find clusters of seismic noise that correspond to varying hydraulic conditions at the glacierWe propose an updated timeline for a glacial lake drainage and describe seismic signatures of subglacial meltwater flow [ABSTRACT FROM AUTHOR]

Dense seismic array monitoring combined with advanced processing can help retrieve and locate a variety of seismic sources with unprecedented resolution and spatial coverage. We present a methodology that goes beyond classical localization algorithms through gathering various types of sources (impulsive or continuous) using a single scheme based on a gradient‐descent optimization and evaluating different levels of phase coherence. We apply our methodology on an Alpine glacier and demonstrate that we can retrieve the dynamics of active crevasses with a metric resolution using sources associated with high phase coherence; the presence of diffracting materials (e.g., rocks) trapped in transverse crevasses using sources with moderate phase coherence; and the two‐dimensional time evolution of the subglacial hydrology system using sources with low phase coherence. Our study highlights the strength of using an appropriate and systematic seismological approach to image a wide range of subsurface structures and phenomena in settings with complex wavefields. Plain Language Summary: Over the past two decades, the growing use of dense seismic arrays has often overcome limitations of traditional observations methods and yielded new insights on the physics of subsurface process and properties. Yet scientific and computational challenges remain to be addressed for using the appropriate array‐processing approaches and automating the techniques on large volume of data and for complex wavefields. In this paper we address such challenges in the particular case of monitoring glaciers, which host numerous and diverse sets of seismic sources that produce signals ranging from impulsive to tremor‐like. We combine a physics‐based and a statistical approach to explore with a dense seismic array the spatial coherence of the seismic wavefield generated by such a diversity of sources. We show that even a small coherence in the phase signal remains rich in statistical information on concomitant and/or low amplitudes micro‐seismic sources. This allows us to localize seismic sources with a super‐resolution (meter to decameter) and identify emerging patterns associated with a wide range of glacier features and their dynamics, ranging from active crevasses, debris in transverse passive crevasses and subglacial water flow. Such methodological and conceptual advance may enable a more efficient and complete imaging of geophysical objects. Key Points: We present an innovative array‐processing approach to image glaciers structures at high resolution by locating seismic sourcesWe investigate a large range of spatial phase coherences, from very low up to very high, over narrow frequency bands and short time windowsWe image the spatial and temporal dynamics of sources originating from active and passive crevasses as well as from subglacial hydrology [ABSTRACT FROM AUTHOR]

Understanding the dynamic response of glaciers to climate change is vital for assessing water resources and hazards, and subglacial hydrology is a key player in glacier systems. Traditional observations of subglacial hydrology are spatially and temporally limited, but recent seismic deployments on and around glaciers show the potential for comprehensive observation of glacial hydrologic systems. We present results from a high‐density seismic deployment spanning the surface of Lemon Creek Glacier, Alaska. Our study coincided with a marginal lake drainage event, which served as a natural experiment for seismic detection of changes in subglacial hydrology. We observed glaciohydraulic tremor across the surface of the glacier that was generated by the subglacial hydrologic system. During the lake drainage, the relative changes in seismic tremor power and water flux are consistent with pressurization of the subglacial system of only the upper part of the glacier. This event was not accompanied by a significant increase in glacier velocity; either some threshold necessary for rapid basal motion was not attained, or, plausibly, the geometry of Lemon Creek Glacier inhibited speedup. This pressurization event would have likely gone undetected without seismic observations, demonstrating the power of cryoseismology in testing assumptions about and mapping the spatial extent of subglacial pressurization. Plain Language Summary: It is important to understand how glaciers are affected by climate change because glaciers contribute to fresh water resources, flood hazards, and sea levels. We want to understand how liquid water moves underneath solid glacier ice, because that water affects how glaciers move, melt, and crack. We put seismometers (the same sensors that detect earthquakes) on an Alaska glacier to detect tiny vibrations from the flowing water below. By observing the changing strength of the vibrations over time and comparing them to the amount of water that goes into and out of the glacier, we mapped where and when the water below the ice was under pressure. When a lake on top of the glacier drained into the water channels below the glacier, we used the seismic data to see that the water became pressurized under half of the glacier. We did not see other obvious signs of pressurization, such as the glacier speeding up dramatically, so we likely would not have known about the pressurization without the seismometers. To get the best understanding of glaciers, scientists need to combine different methods of observation, and this case is an example of why including seismic observations can be very helpful. Key Points: Subglacial water flow generates continuous seismic tremor observable in the power spectra recorded by on‐ice seismometersUsing a dense array of seismometers to detect glaciohydraulic tremor allows us to map subglacial pressurization in space and timeSeismic data indicate pressurization in the upper glacier, particularly near the margins, that was not perceptible in glacier velocity data [ABSTRACT FROM AUTHOR]

A subglacial groundwater system beneath Taylor Glacier, Antarctica, discharges hypersaline, iron‐rich brine episodically at the glacier surface to create Blood Falls. However, the triggering mechanism for these brine release events is not yet understood. Identifying which fracture processes are observed seismically can help us better characterize the hydrological system at Taylor Glacier, and more generally, provide us with a broader understanding of englacial hydrologic activity in cold glaciers. We document wintertime brine discharge using time‐lapse photography. Subfreezing air temperatures during the brine discharge indicate that surface melt‐induced hydrofracture is an unlikely trigger for brine release. Further, we analyze local seismic data to test a hypothesis that fracturing generates elevated surface wave energy preceding and/or coinciding with brine release events. Our results show no discernible elevated Rayleigh wave activity prior to or during Blood Falls brine release. Instead, we find a pattern of seismic events dominated by a seasonal signal, with more Rayleigh events occurring in the summer than the winter from the Blood Falls source area. We calculate that the volumetric opening of cracks that would generate Rayleigh waves at our detection limits are of similar size to myriad cracks in glacier ice, lake ice, and frozen sediment in the terminus area. We therefore propose that any fracturing coincident with brine release activity likely consists of a series of smaller opening events that are masked by other seismicity in the local environment. Plain Language Summary: Blood Falls is a reddish feature that forms at the terminus of Taylor Glacier in Antarctica when hypersaline, iron‐rich brine flows from sediment under the glacier up through the ice to emerge from cracks at the surface. We used data from a time‐lapse camera and nearby seismic sensors to document a brine release. Our images show a brine release event starting in May 2014 (austral winter), and fractures in the glacier surface observed following the event encouraged us to hypothesize that we would detect an increase in Rayleigh waves (a type of seismic wave that can be generated by surface crack opening). However, we do not observe an increase in Rayleigh wave activity prior to or during brine release. When we estimate the average size of the fractures that we detect, we find the size is similar to many types of cracks in the nearby environment (for instance, cracks in the lake ice). We conclude that any Rayleigh wave seismicity that occurred during the Blood Falls brine release must be from a series of crack openings smaller than our detection limit, and that other cracks opening in the nearby environment may mask any signal specific to the Blood Falls release. Key Points: Time‐lapse photos capture wintertime Blood Falls brine releaseBrine release occurs without evidence for enhanced Rayleigh wave seismicity near the release pointThe Blood Falls crack may open as a series of small fracture events, masked by local seismicity [ABSTRACT FROM AUTHOR]

Glacially- and magmatically-derived seismic events have been noted to heavily overlap in characteristics, thus there exists the potential for false-alarms or missed warnings at ice-covered volcanoes. Here we present the first study to specifically target icequakes at an ice-covered volcano in Southern Chile. Two months of broadband seismic data collected at Llaima volcano in 2015 were analyzed in order to quantify, characterize, and locate glacially-derived seismic events at one of the most active ice-covered volcanoes in the region. We find over 1,000 repeating seismic events across 11 families, the largest of which contains 397 events. Approximate locations and characteristics of the largest families lead us to conclude that these events were derived from persistent stick-slip motion along the ice-rock interface at the base of a glacier near the volcano summit. These results have implications for future seismic monitoring at Llaima volcano and other ice-covered active volcanoes in the region.

Subglacial water flowstrongly modulates glacier basalmotion, which itself strongly influences the contributions of glaciers and ice sheets to sea level rise. However, our understanding of when and where subglacial water flow enhances or impedes glacier flow is limited due to the paucity of direct observations of subglacial drainage characteristics. Here, we demonstrate that dense seismic array observations combined with an innovative systematic seismic source location technique allows the retrieval of a two-dimensional map of a subglacial drainage system, as well as its day-to-day temporal evolution. We observe with unprecedented detail when and where subglacial water flows through a cavity-like system that enhances glacier flow versus when and where water mainly flows through a channel-like system that impedes glacier flow. Most importantly, we are able to identify regions of high hydraulic connectivity within and across the cavity and channel systems, which have been identified as having a major impact on the long-term glacier response to climate warming. Applying a similar seismicmonitoring strategy in other glacier settings, including for ice sheets, may help to diagnose the susceptibility of their dynamics to increased meltwater input due to climate warming. [ABSTRACT FROM AUTHOR]

Icequakes, microseismic earthquakes at glaciers, offer insights into the dynamics of ice sheets. For the first time in the Antarctic, we explore the use of fiber optic cables as Distributed Acoustic Sensors (DAS) as a new approach for monitoring basal icequakes. We present the use of DAS for studying icequakes as a case study for the application of DAS to microseismic datasets in other geological settings. Fiber was deployed on the ice surface at Rutford Ice Stream in two different configurations. We compare the performance of DAS with a conventional geophone network for: microseismic detection and location; resolving source and noise spectra; source mechanism inversion; and measuring anisotropic shear‐wave splitting parameters. Both DAS array geometries detect fewer events than the geophone array. However, DAS is superior to geophones for recording the microseism signal, suggesting the applicability of DAS for ambient noise interferometry. We also present the first full‐waveform source mechanism inversions using DAS anywhere, successfully showing the horizontal stick‐slip nature of the icequakes. In addition, we develop an approach to use a 2D DAS array geometry as an effective multi‐component sensor capable of accurately characterizing shear‐wave splitting due to the anisotropic ice fabric. Although our observations originate from a glacial environment, the methodology and implications of this work are relevant for employing DAS in other microseismic environments. Plain Language Summary: Icequakes are like small earthquakes but are caused by the movement of ice rather than two plates sliding past one another. They allow us to investigate glacier processes. For the first time in the Antarctic, we use lasers fired down fiber optic cables to detect and analyze icequake signals. This technique is called Distributed Acoustic Sensing (DAS). These fiber optic cables were laid on the surface of Rutford Ice Stream, Antarctica, in two different shapes. We compare the performance of DAS to conventional geophones for icequake detection and location, investigating the frequency of the earthquake source, investigating the physics that generates the icequake, and the effect of the ice fabric on the travel of seismic waves through ice. For our experiment, DAS is not as good as conventional geophones for detecting icequakes. However, DAS is better than geophones for looking at the frequency of an icequake and the physics that causes an icequake. It also allows us to investigate ice fabric properties in a similar way to geophones. Although our results are for icequakes at a glacier, the methods we use and our findings are relevant for using DAS in many other environments where small earthquakes occur. Key Points: Distributed acoustic sensing can outperform geophones for source spectra and full‐waveform source mechanism inversion2D distributed acoustic sensing array geometries can be used as a multi‐component sensor capable of measuring shear‐wave splittingLarger surface distributed acoustic sensing deployments and/or hybrid networks are required for accurate microseismic detection/location [ABSTRACT FROM AUTHOR]

Sliding of glacial ice over its base is typically described by a frictionless or slowly deforming bed. This view is challenged by recent seismic observations of stick‐slip motion at the ice‐bed interface. We revisit a high‐frequency (20–35 Hz) harmonic tremor recorded on Gornergletscher, Switzerland. In contrast to previous interpretation in terms of glaciohydraulic tremor, we present evidence for superimposed stick‐slip episodes as tremor sources: we locate the tremor source with matched field processing polarity optimization, which allows for azimuthal polarity patterns associated with nonisotropic moment tensors and yields a tremor source clustering near the glacier bed. Our analysis confirms an S wave radiation pattern in agreement with a double‐couple source derived from ice sliding over bedrock and explains our tremor observations in terms of glacier stick‐slip motion. Adding to observations of stick‐slip tremor beneath polar ice streams, this first report on stick‐slip tremor beneath Alpine ice favors widespread seismogenic glacier sliding. Plain Language Summary: For many years, researchers have observed cryoseismic stick‐slip tremor exclusively in Antarctica. Stick‐slip tremor is due to small repeating slip events at the glacier bed as a glacier advances downstream. This type of tremor is a telltale sign of what is happening at the ice‐bed interface and indicates frictional sliding. Here, we present first evidence for stick‐slip tremor at an Alpine glacier—Gornergletscher, Switzerland. We identify indicators in the seismic signature and apply data processing techniques that reveal that the creeping glacier sole moves under the influence of gravity and irregularly rubs over a sticky area at the bed. Key Points: We revisit a harmonic tremor recorded by a seismic array on an Alpine glacier, which was previously interpreted as hydraulic tremorApplying matched field processing that accounts for nonisotropic radiation patterns suggests a tremor source at the ice‐bedrock interfaceA focal mechanism derived from ice slip over bedrock explains our results and suggests seismogenic stick‐slip motion at the glacier's base [ABSTRACT FROM AUTHOR]

Improving our understanding of glacial sliding is crucial for constraining basal drag in ice dynamics models. We use icequakes, sudden releases of seismic energy as the ice slides over the bed, to provide geophysical observations that can be used to aid understanding of the physics of glacial sliding and constrain ice dynamics models. These icequakes are located at the bed of an alpine glacier in Switzerland and the Rutford Ice Stream, West Antarctica, two extremes of glacial settings and spatial scales. We investigate a number of possible icequake source mechanisms by performing full waveform inversions to constrain the fundamental physics and stress release during an icequake stick‐slip event. Results show that double‐couple mechanisms best describe the source for the events from both glacial settings and the icequakes originate at or very near the ice‐bed interface. We also present an exploratory method for attempting to measure the till shear modulus, if indirect reflected icequake radiation is observed. The results of this study increase our understanding of how icequakes are associated with basal drag while also providing the foundation for a method of remotely measuring bed shear strength. Key Points: Investigate icequakes associated with glacial sliding at alpine and ice sheet spatial scalesDouble‐couple source mechanism near ice‐bed interface best describes the stick‐ slip icequakesIn certain circumstances, this can estimate bed shear modulus directly from icequake observations [ABSTRACT FROM AUTHOR]

Glacial earthquakes are slow earthquakes of magnitude M~5 associated with major calving events at near-grounded marine-terminating glaciers. These globally detectable earthquakes provide information on the grounding state of outlet glaciers and the timing of large calving events. Seismic source modeling of glacial earthquakes provides information on the size and orientation of forces associated with calving events. We compare force orientations estimated using a centroid-single-force technique with the calving-front orientations of the source glaciers at or near the time of earthquake occurrence. We consider earthquakes recorded at four glaciers in Greenland – Kangerdlugssuaq Glacier, Helheim Glacier, Kong Oscar Glacier, and Jakobshavn Isbræ – between 1999 and 2010. We find that the estimated earthquake force orientations accurately represent the orientation of the calving front at the time of the earthquake, and that seismogenic calving events are produced by a preferred section of the calving front, which may change with time. We also find that estimated earthquake locations vary in a manner consistent with changes in calving-front position, though with large scatter. We conclude that changes in glacial-earthquake source parameters reflect true changes in the geometry of the source glaciers, providing a means for identifying changes in glacier geometry and dynamics that complements traditional remote-sensing techniques.

Abstract: Determining the exact timing of ice wedge cracking in permafrost is challenging. Five miniature accelerometers were installed near the ground surface in the trough of a primary ice wedge within a network of low‐centered polygons in Adventdalen, Svalbard, to test whether these instruments could be used to detect dynamics of thermal contraction cracking. Data from 2003 to 2013 were analyzed to characterize cryoseismic signals in the ice wedge trough. High‐magnitude accelerations (from 5 g to at least 100 g) were typically registered in late winter, when the top of permafrost had cooled to about −10°C; these likely correspond to ice wedge cracking in permafrost. Tensile stresses calculated from temperatures measured in the ice wedge trough are near laboratory strengths reported for ice and mineral active layer sediments, supporting the interpretation that large accelerations are caused by thermal contraction cracking. Lower magnitude accelerations occurred throughout the freezing season, usually coinciding with rapid cooling at the ground surface. These small accelerations may be associated with (i) the initiation of small cracks in the active layer of the trough or (ii) the horizontal and vertical propagation of existing ice wedge cracks. The results of this investigation indicate that miniature accelerometers are an effective, inexpensive, and simple method to determine the timing of ice wedge cracking and rates of crack propagation along ice wedge troughs. [ABSTRACT FROM AUTHOR]

Hydrology is important for glacier dynamics, but it is difficult to monitor the subsurface drainage systems of glaciers by direct observations. Since meltwater drainage generates seismic signals, passive seismic analysis has the potential to be used to monitor these processes. To study continuous seismic radiation from the drainage, we analyze geophone data from six stations deployed at the Kaskawulsh Glacier in Yukon, Canada, during the summer of 2014 using ambient noise cross-correlation techniques. We locate the noise sources by backprojecting the amplitude of the cross correlation to the glacier surface. Most of the ambient noise sequences are found in two clusters, with each cluster located in the vicinity of a moulin identified at the surface. Stronger seismic radiation is observed during the day, consistent with expected variability in melt rates. We demonstrate that the sparse seismic network array with 2 km station separation has the ability to detect moulins within the array with a precision of 50 m. We confirm that seismic activity is correlated with air temperature, and thus, melt, on a diurnal timescale, and precipitation correlates with the activity at longer timescales. Our results highlight the potential of passive seismic observations for monitoring water flow into subglacial channels through moulins with an affordable number of seismic stations, but quantification of water flow rates still remains a challenge. The cross-correlation backprojection technique described here can also potentially be applied to any localized source of ambient noise such as ocean noise, tectonic tremor, and volcanic tremor. [ABSTRACT FROM AUTHOR]

Glacial earthquakes are slow earthquakes of magnitude M~5 associated with major calving events at near-grounded marine-terminating glaciers. These globally detectable earthquakes provide information on the grounding state of outlet glaciers and the timing of large calving events. Seismic source modeling of glacial earthquakes provides information on the size and orientation of forces associated with calving events. We compare force orientations estimated using a centroid-single-force technique with the calving-front orientations of the source glaciers at or near the time of earthquake occurrence. We consider earthquakes recorded at four glaciers in Greenland – Kangerdlugssuaq Glacier, Helheim Glacier, Kong Oscar Glacier, and Jakobshavn Isbræ – between 1999 and 2010. We find that the estimated earthquake force orientations accurately represent the orientation of the calving front at the time of the earthquake, and that seismogenic calving events are produced by a preferred section of the calving front, which may change with time. We also find that estimated earthquake locations vary in a manner consistent with changes in calving-front position, though with large scatter. We conclude that changes in glacial-earthquake source parameters reflect true changes in the geometry of the source glaciers, providing a means for identifying changes in glacier geometry and dynamics that complements traditional remote-sensing techniques. [ABSTRACT FROM AUTHOR]

Identifying the source mechanisms of low-frequency earthquakes at ice-covered volcanoes can be challenging due to overlapping characteristics of glacially and magmatically derived seismicity. Here we present an analysis of two months of seismic data from Llaima volcano, Chile, recorded by the permanent monitoring network in 2019. We find over 2000 repeating low-frequency events split across 82 families, the largest of which contains over 200 events. Estimated locations for the largest families indicate shallow sources directly beneath or near the edge of glaciers around the summit vent. These low-frequency earthquakes are part of an annual cycle in activity at the volcano that is strongly correlated with variations in atmospheric temperature, leading us to conclude that meltwater from ice and snow strongly affects the seismic source mechanisms related to glacier dynamics and shallow volcanic processes. The results presented here should inform future assessments of eruptive potential at Llaima volcano, as well as other ice-covered volcanoes in Chile and worldwide. • We investigate micro-seismic activity at Llaima volcano in early 2019. • We observe dozens of families of persistent repeating earthquakes. • Estimated locations suggest sources at shallow depths near or beneath glaciers. • Long-term activity suggests strong relationship with snow and ice meltwater. [ABSTRACT FROM AUTHOR]

The compacted firn layer skin of an ice sheet protects it from surface melting. Vibrations of the firn layer measured with seismometers Chaput et al. (2018, https://doi.org/10.1029/2018GL079665) can portray how the firn layer melts, absorbs water, and refreezes, thus giving a sense of how ice sheets are resisting being denuded of their firn layer skin by climate warming. Plain Language Summary: This highlight discusses, in plain language, the paper by Chaput et al. (2018). The main point is that seismology provides a way to continuously observe melting processes in firn. The importance of firn to an ice sheet's health is described in the commentary, and a simple schematic figure showing how the seismological method works is provided. Key Points: Firn layers protect ice sheetsSeismology provides a way to observe melting, meltwater percolation, and refreezing within firn layersFirm layers are found to vibrate, buzz from wind forcing; but when they fill with meltwater, the vibration stops [ABSTRACT FROM AUTHOR]