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Over the past four decades, the decline in the extent and thickness of Arctic sea ice has accelerated dramatically due to global warming. The rate of this decline is much faster than predicted by existing climate models. The evolution of the decline in the extent and average thickness of sea ice is subject to thermodynamic processes that are affected by important parameters, such as the mechanical properties and thickness of the ice. Although the physical processes explaining this decline are understood, sea ice and climate models do not clearly reproduce the observed variations. Therefore, continuous and accurate monitoring of these parameters is essential given the need to update our climate models.The use of seismic methods to study sea ice has long been neglected despite their potential for very accurate estimates of ice properties. However, thanks to rapid technological and methodological advances in the last decade, these approaches have recently been reconsidered.Part of this thesis aims to prove the concept of a methodology based on the measurement of guided seismic waves in ice to estimate its Young's modulus, Poisson's ratio, density and thickness in different directions using ambient seismic noise recorded by a dense array of sensors. To illustrate the potential of this method, we present the exploitation of continuous seismic data recorded by an autonomous array of 247 geophones deployed between March 1 and 24, 2019, on the fast ice of Lake Vallunden, Van Mijen Fjord, Norwegian Svalbard Archipelago.The method consists of extracting a noise correlation function and then inverting the dispersion curves of guided modes propagating in the sea ice. To calculate the daily noise correlation function, we show that selecting time windows where the dominant seismic source is aligned with the receivers significantly improves the signal-to-noise ratio. The dispersion curves of the three fundamental guided modes are inverted with MCMC sampling to derive the probability density function of the sea ice parameters. We obtain satisfactory results that are consistent with observations and measurements made in situ. Thus, we demon- strate that by using this method, it is possible to constrain the thickness, density, and elastic properties of sea ice both precisely and on a daily basis.Secondly, we have exploited particular events contained in the recorded seismic noise: icequakes. These are sudden, impulsive releases of energy from various ice mechanisms such as fracturing. There are more than a hundred of them per day in our records. They have the advantage of being isotropic around our seismic array. To isolate these specific signals from the continuous noise records, a waveform sorting method based on a deep scattering network was used. A careful manual selection of the time windows associated with the icequakes was then performed. We then performed the first steps prior to a tomographic inversion by measuring the time shifts in the arrival velocity of the wavefield within our array. We discuss the prospects of this work, which paves the way for imaging from the wavefield emitted by icequakes.; Durant ces quatre dernières décennies, le déclin de l’extension et l’épaisseur moyenne de la glace de mer Arctique s’est accéléré de manière spectaculaire face au réchauffement climatique. Ce déclin est beaucoup plus rapide que celui prédit par les modèles de climat existant. L’évolution de l’étendue de la glace de mer est soumise à des processus thermodynamiques qui sont affectés par des paramètres importants évoluant à un rythme accéléré, tels que les propriétés mécaniques et l’épaisseur de la glace. Bien que les processus physiques expliquant ce déclin soient compris, les modèles actuels de banquise et climatiques ne reproduisent pas clairement les variations observées. Par conséquent, une surveillance continue et précise de ces paramètres est essentielle compte tenu de la nécessité de mettre à jour nos modèles climatiques. L’utilisation de méthodes sismiques pour étudier la glace de mer a longtemps été délaissée malgré leur potentiel pour des estimations très précises des propriétés de la glace. Toutefois, grâce aux rapides progrès technologiques et méthodologiques de la dernière décennie, ces approches ont été récemment reconsidérées.Une partie de cette thèse vise à prouver l’efficacité d’une méthodologie basée sur la mesure des ondes sismiques guidées dans la glace pour estimer son module d’Young, son coefficient de Poisson, sa densité et son épaisseur dans différentes directions en utilisant le bruit sismique ambiant enregistré par un réseau dense de capteurs. Pour illustrer le potentiel de cette méthode, nous présentons l’exploitation des données sismiques continues enregistrées par un réseau autonome de 247 géophones déployés entre le 1er et le 24 mars 2019, sur la banquise côtière du lac Vallunden, dans le fjord de Van Mijen, dans l’archipel norvégien de Svalbard. La méthode consiste à extraire une fonction de corrélation de bruit puis à inverser les courbes de dispersion des modes guidés se propageant dans la glace de mer. Pour calculer la fonction de corrélation de bruit quotidienne, nous montrons que la sélection des fenêtres de temps où la source sismique dominante est alignée avec les récepteurs améliore significativement le rapport signal/bruit. Les courbes de dispersion des trois modes guidés fondamentaux sont inversées avec un échantillonnage de type Monte Carlo par chaînes de Markov pour déduire la fonction de densité de probabilité des paramètres de la glace de mer. Nous obtenons des résultats cohérents avec les observations et les mesures effectuées in situ. Ainsi, nous démontrons qu’en utilisant cette méthode, il est possible d’estimer l’épaisseur, la densité et les propriétés élastiques de la glace de mer avec précision et sur une base quotidienne.Ensuite, nous avons exploité des évènements particuliers contenus dans le bruit sismique enregistré : les tremblements de glace - icequakes en anglais. Il s’agit de libérations soudaines et impulsives d’énergie issues de divers mécanismes propres à la glace tels que la fracturation. On en compte plus d’une centaine par jour dans nos enregistrements. Ils ont l’avantage d’être isotropes autour de notre réseau sismique. Pour isoler ces signaux spécifiques des enregistrements continus debruit, une méthode de hiérarchisation des formes d’ondes basée sur un réseau de diffusion profond a été utilisée. Nous avons ensuité sélectionné manuellement et minutieusement les fenêtres temporelles associées aux icequakes. Finalement, les premières étapes antérieures à une inversion tomographique ont été réalisées en mesurant les décalages temporels d’arrivée du champ d’ondes au sein de notre réseau. Nous discutons des perspectives de ce travail qui ouvre la voie à l’imagerie à partir du champ d’ondes émis par les icequakes.

In press in JGR Earth Surface 2022/11/23; Deformation mechanisms of glaciers are highly sensitive to basal temperature; the motion of temperate glaciers is dominated by basal slip while cold-based glaciers deform mainly by internal creep. While basal slip is usually aseismic, unstable slip sometimes occurs and can be detected by seismometers. I have detected clusters of repeating low-frequency icequakes (LFIs) in the Mont-Blanc massif. Some properties of LFIs are similar to the high-frequency icequakes (HFIs) located at the base of Argenti\`ere glacier (Helmstetter et al., 2015). Both HFIs and LFIs occur as bursts of tens to several thousand events lasting for days or weeks, with typical inter-event times of several minutes during bursts. Unlike HFIs that have a broad spectra, LFIs have a characteristic frequency of about 5 Hz at all stations, suggesting a rupture length of about 100 m. Seismic amplitudes and seismic waveforms of LFIs progressively evolve with time within each cluster, suggesting changes in either rupture length or rupture velocity. Most LFIs are detected during snowfall episodes while HFIs are not correlated with snowfall episodes. In this study, I used all available seismic stations within or around the Mont-Blanc massif between 2017 and 2022. I found LFIs located all over the massif but mainly above 3000 m. Some clusters are clearly associated with cold basal ice (near Mont-Blanc summit) while others below 2700 m a.s.l. are likely located under temperate glaciers and two clusters could be associated with landslides. This observation of LFIs on cold glaciers is consistent with laboratory friction experiments suggesting that cold ice promotes unstable slip (McCarthy et al., 2017; Saltiel et al., 2021; Zoet et al., 2013, 2020).

Detecting and locating microearthquakes from continuous waveform records is the fundamental step in microseismic processing. Dense local networks and arrays have introduced the possibility to detect large numbers of far weaker events, but when viewed on seismic records from individual stations their waveforms are often obscured by noise. Furthermore, areas of interest for microseismic monitoring often feature extremely high event rates, highlighting the limitations of traditional techniques based on phase picking and association. In order to maximise the new insights gained, we require fully automated techniques which can exploit modern recordings to produce highly complete earthquake catalogues containing few artefacts. QuakeMigrate is a new modular, open-source Python package providing a framework to efficiently, automatically and robustly detect and locate microseismicity. The user inputs continuous seismic data, a velocity model or pre-calculated look-up table and list of station locations. Instead of reducing the raw waveforms to discrete time picks, they are transformed (by amplitude, frequency and/or polarisation analysis) to continuous functions representing the probability of a particular phase arrival through time. These ‘onset functions’ from stations across the network are then migrated according to a travel-time look-up table and stacked to perform a grid-search for coherent sources of energy in the subsurface. This enables detection of earthquakes at close to or below the signal-to-noise ratio at individual stations, and implicitly associates phase arrivals even at very small inter-event times. We demonstrate the flexibility and power of this approach with examples of basal icequakes detected at the Rutford Ice Stream, Antarctica, dike- and caldera-collapse induced seismicity at Bárðarbunga central volcano, Iceland, and the aftershock sequence from a M5 earthquake at Mt. Kinabalu, northern Borneo. The modular nature of the workflow and wide range of automatic plotting options makes parameter choice straightforward, and robust event location uncertainty statistics facilitate filtering to produce a robust catalogue. QuakeMigrate also outputs phase picks and local magnitude estimates, with an architecture designed to promote further community-driven extension in future.

In this thesis, I present three studies in environmental seismology. First, I present an analysis of seismic tremor generated from subglacial water flow during the rapid drainage of an ice-marginal supraglacial lake, collected by an on-ice nodal seismic array. I find that seismic tremor indicates a partial pressurization of the subglacial hydrologic system that was not accompanied by the expected change in glacier surface velocity, suggesting that factors like glacier geometry play a significant role in whether pressurization necessarily leads to velocity change. Using seismometers in this way allows remote observation of active subglacial hydrologic systems as they vary over space and time, a vital parameter for understanding how liquid water affects glacier motion, melting, fracture, and hazards. Second, I present observations of glacier surface crevasse development over space and time, as detected by a dense array of seismometers atop the glacier. I find that icequakes associated with surface crevassing have a magnitude distribution that is swarm-like, rather than aftershock-like, and that the spatiotemporal distribution of events indicates that crevasses regularly widen, deepen, reactivate, and trigger activity at nearby crevasses through cryoseismicity. Understanding surface crevassing activity is valuable for constraining the degree to which glacier surface velocity measurements represent ice flow as a whole, and for interpreting how glacier flow responds to changes in forcing over time. Third, I present an investigation of changes in anthropogenic urban seismic noise in Los Angeles associated with changes in community behavior. I find that changes in human activity from the scale of hours to the scale of months create distinguishable differences in ambient seismic noise power that correlate well with other measures of community behavior. Characterizing anthropogenic seismic noise is beneficial for accurately interpreting measurements of transient seismic wave data collected in urban areas toward goals such as hazard mapping.

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. ©2020. The Authors. ISSN:0148-0227 ISSN:2169-9003 ISSN:2169-9011

Owing to the implantation of the “Princess Elisabeth” polar base in East Antarctica, the Royal Observatory of Belgium conducted research in seismology by installing in February 2010 a permanent broadband seismic station on the bedrock near the base. Due to the poor coverage of permanent seismic stations in Antarctica and the small number of them built on the bedrock, the station (code name: ELIB) is an interesting new source of information for global seismicity studies. Since its installation, the station has also recorded numerous local and regional seismic events related to the interaction between the ice sheet flow and the bed­rock. To study this seismicity, we installed five additional temporary broadband seismic stati­ons separated by 25-30 km distance in January 2014. All those stations were operational from January to April 2014, which led to the identification of different spots of ice-related seismicity in a radius of 200 km around ELIB and to the analysis of the processes behind them. As many of the ice-related events located by the temporary broad-band seismic network were recorded by ELIB, it is now possible to identify similar events when only ELIB is working, providing a unique opportunity to follow the evolution of this ice seismicity in some target areas where it would be representative of the ice-sheet dynamic evolution., Paper presented at the meeting of the Section of Technical Sciences held on 27 April 2017. Text received on 13 March 2018 and submitted to peer review. Final version, approved by the reviewers, received on 26 April 2019.

Динамика сейсмического процесса арх. Шпицберген за январьоктябрь 2017 г. изучалась с помощью системы NSDL. Данные по пяти сейсмическим станциям были обработаны системой: обнаружено и слоцировано 35 199 событий. Для наглядного картографического отображения большого количества сейсмических событий применен новый подход. Обнаружены рои землетрясений как тектонической, так и ледниковой природы. Показано, что система NSDL является эффективным средством изучения региональной сейсмичности., The dynamics of seismic process of the Spitsbergen archipelago for JanuaryOctober 2017 was studied using the NSDL system. The data of five seismic stations were processed by the system, 35 199 events were detected and located. A new approach has been applied to visual mapping of the large number of seismic events. The swarms of earthquakes both of tectonic and glacial nature, have been discovered. It is shown that the NSDL system is an effective tool for studying regional seismicity., №3 (2019)

Here we present new measurements of an anisotropic ice fabric in a fast moving (377 ma−1) ice stream in West Antarctica. We use ∼6000 measurements of shear wave splitting observed in microseismic signals from the bed of Rutford Ice Stream, to show that in contrast to large-scale ice flow models, which assume that ice is isotropic, the ice in Rutford Ice Stream is dominated by a previously unobserved type of partial girdle fabric. This fabric has a strong directional contrast in mechanical properties, shearing 9.1 times more easily along the ice flow direction than across flow. This observed fabric is likely to be widespread and representative of fabrics in other ice streams and large glaciers, suggesting it is essential to consider anisotropy in data-driven models to correctly predict ice loss and future flow in these regions. We show how passive microseismic monitoring can be effectively used to provide these data.

In this paper, we analysed 3-component seismic signals recorded during 27 November 2016-10 January 2017 by two stations installed in Tethys Bay (Victoria Land, Antarctica), close to Mario Zucchelli Station. Due to the low noise levels, it was possible to identify three different kinds of signals: teleseismic earthquakes, microseisms, and icequakes. We focus on the latter two. A statistically significant relationship was found between microseism amplitude and both wind speed and sea swell. Thus, we suggest that the recorded microseism data are caused by waves at the shore close to the seismic stations rather than in the deep ocean during storms. In addition, we detected three icequakes, with dominant low frequencies (below 2 Hz), located in the David Glacier area with local magnitude of 2.4-2.6. These events were likely to have been generated at the rock-ice interface under the glacier. This work shows how seismic signals recorded in Antarctica provide insights on the interactions between the atmosphere-cryosphere-hydrosphere. Since climate patterns drive these interactions, investigations on Antarctic seismic signals could serve as a proxy indicator for estimating climate changes. © 2017 by Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.

The problem of direction of arrival (DoA) estimation of seismic plane waves impinging on an array of sensors is considered from a new deterministic perspective using tensor decomposition techniques. In addition to temporal and spatial sampling, further information is taken into account, based on the different propagation speed of body waves (P and S) through solid media. Performances are evaluated through simulated data in terms of the Cramer-Rao bounds and compared to other reference methods such as ESPRIT and MUSIC, in the presence of additive Gaussian circular noise. The proposed approach is then applied to real seismic data recorded at the Argentiere glacier, occurring at the interface between the ice mass and the underlying bedrock. MUSIC and ESPRIT rely on the estimation of the covariance matrix of received data, thus requiring a large number of time samples. Moreover, information about propagation speed diversity is not taken into account by existing models in array processing. The discovered advantage in terms of the average error in estimating the direction of arrival of body waves is noteworthy, especially for a low number of sensors, and in separating closely located sources. Additionally, an improvement of precision in processing real seismic data is observed. HighlightsA deterministic approach (tensor decomposition) for seismic array processing.Integration of the content of P and S waves for direction of arrival estimation.Evaluation of statistical performances through numerical simulations.Application to real seismic data recorded at Argentiere glacier (Mont Blanc).Comparison with existent narrowband methods such as ESPRIT and MUSIC.

Avalanching glacier instabilities are gravity-driven rupture phenomena that might cause major disasters, especially when they are at the origin of a chain of processes. Reliably forecasting such events combined with a timely evacuation of endangered inhabited areas often constitute the most efficient action. Recently, considerable efforts in monitoring, analyzing, and modeling such phenomena have led to significant advances in destabilization process understanding, improving early warning perspectives. The purpose of this paper is to review the recent progress in this domain. Three different types of instabilities can be identified depending on the thermal properties of the ice/bed interface. If cold (1), the maturation of the rupture is associated with a typical time evolution of surface velocities and passive seismic activity. A prediction of the final break off is possible using these precursory signs. For the two other types, water plays a key role in the development of the instability. If the ice/bed interface is partly temperate (2), the presence of meltwater may reduce the basal resistance, which promotes the instability. No clear and easily detectable precursory signs are known in this case, and the only way to infer any potential instability is to monitor the temporal evolution of the thermal regime. The last type of instability (3) concerns steep temperate glacier tongues switching for several days/weeks during the melting season into a so-called “active phase” followed in rare cases by a major break-off event. Although the prediction of such events is still far from being achievable, critical conditions promoting the final instability can be identified., Reviews of Geophysics, 53 (2), ISSN:8755-1209, ISSN:0096-1043, ISSN:1944-9208

A passive seismic study was carried out underneath Glacier d’Argentière, Mont Blanc, France, where an array of seismometers was installed in a subglacial access tunnel. The data show a very high emissivity from the glacier. Fracturing can be discriminated from serac falls using the signal characteristics. We apply seismic array methods to locate the sources of these signals, using a two-step grid search in the parameter space. Four clusters of activity are found close to the network, showing that this fracturing does not take place uniformly over the glacier, but rather in isolated small zones. We compute a local magnitude using regional earthquakes for calibration. The magnitudes follow a classical Gutenberg–Richter law in the rangeML= −3 to 0.15, showing that no characteristic size events dominate the process. We suggest that those spatial clusters of icequakes could reveal the heterogeneous nature of the friction at the base of the glacier, with patches of high frictional stresses locally generating intense fracturing within the ice mass.

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