11 research outputs found
Numerical Modeling of Iceberg Capsizing Responsible for Glacial Earthquakes
The capsizing of icebergs calved from marineâterminating glaciers generate horizontal forces on the glacier front, producing longâperiod seismic signals referred to as glacial earthquakes. These forces can be estimated by broadband seismic inversion, but their interpretation in terms of magnitude and waveform variability is not straightforward. We present a numerical model for fluid drag that can be used to study buoyancyâdriven iceberg capsize dynamics and the generated contact forces on a calving face using the finiteâelement approach. We investigate the sensitivity of the force to drag effects, iceberg geometry, calving style, and initial buoyancy. We show that there is no simple relationship between force amplitude and iceberg volume, and similar force magnitudes can be reached for different iceberg sizes. The force history and spectral content varies with the iceberg attributes. The iceberg aspect ratio primarily controls the capsize dynamics, the force shape, and force frequency, whereas the iceberg height has a stronger impact on the force magnitude. Iceberg hydrostatic imbalance generates contact forces with specific frequency peaks that explain the variability in glacial earthquake dominant frequency. For similar icebergs, topâout and bottomâout events have significantly different capsize dynamics leading to larger topâout forces especially for thin icebergs. For realistic iceberg dimensions, we find contactâforce magnitudes that range between 5.6 Ă 1011 and 2 Ă 1014 kg·m, consistent with seismic observations. This study provides a useful framework for interpreting glacial earthquake sources and estimating the ice mass loss from coupled analysis of seismic signals and modeling results
Complex force history of a calving-generated glacial earthquake derived from broadband seismic inversion
The force applied to the Earth by the calving of two icebergs at Jakobshavn Isbrae, Greenland, has been quantified. The source force history was recovered by inversion of regional broadband seismograms without any a priori constraint on the source time function, in contrast with previous studies. For periods 10-100 s, the three-component force can be obtained from distant stations alone and is proportional to the closest station seismograms. This inversion makes it possible to quantify changes of the source force direction and amplitude as a function of time and frequency. A detailed comparison with a video of the event was used to identify four forces associated with collision, then bottom-out and top-out rotation of the first and second icebergs, and ice mĂ©lange motion. Only the two iceberg rotations were identified in previous studies. All four processes are found here to contribute to the force amplitude and variability. Such a complete time-frequency force history provides unique dynamical constraints for mechanical calving models.ERC. Grant Number: ERC-CG-2013-PE10-617472 ANR. Grant Number: ANR-11-BS01-0016 LANDQUAKES, CNCSâUEFISCDI. Grant Number: PN-II-ID-PCE-2011-3-004
Monitoring Greenland ice sheet buoyancy-driven calving discharge using glacial earthquakes
Since the 2000s, Greenland ice sheet mass loss has been accelerating, followed by increasing numbers of glacial earthquakes (GEs) at near-grounded glaciers. GEs are caused by calving of km-scale icebergs which capsize against the terminus. Seismic record inversion allows a reconstruction of the history of GE sources which captures capsize dynamics through iceberg-to-terminus contact. When compared with a catalog of contact forces from an iceberg capsize model, seismic force history accurately computes calving volumes while the earthquake magnitude fails to uniquely characterize iceberg size, giving errors up to 1 km Âł . Calving determined from GEs recorded ateight glaciers in 1993â2013 accounts for up to 21% of the associated discharge and 6% of the Greenland mass loss. The proportion of discharge attributed to capsizing calving may be underestimated by at least 10% as numerous events could not be identified by standard seismic detections (Olsen and Nettles, 2018). While calving production tends to stabilize in East Greenland, Western glaciers have released more and larger icebergs since 2010 and have become major contributors to Greenland dynamic discharge. Production of GEs and calving behavior are controlled by glacier geometry with bigger icebergs being produced when the terminus advances in deepening water. We illustrate how GEs can help in partitioning and monitoring Greenland mass loss and characterizing capsize dynamics
Analysis and modelling of seismic waves generated by glacial earthquakes and iceberg calving
Les sĂ©ismes glaciaires ont des magnitudes Mw~5 et sont liĂ©s au vĂȘlage dâicebergs instables volumineux (km3). Dans le but de caractĂ©riser la force Ă lâorigine des signaux sismiques longue-pĂ©riode (10-100~s) mesurĂ©s lors de tels Ă©vĂšnements, nous dĂ©veloppons une mĂ©thode dâinversion de la fonction source et un modĂšle mĂ©canique numĂ©rique du phĂ©nomĂšne. GrĂące Ă lâanalyse dĂ©taillĂ©e des histoires de forces inversĂ©es, nous mettons en lumiĂšre lâexistence de plusieurs phĂ©nomĂšnes responsables de la gĂ©nĂ©ration du signal sismique, Ă savoir (1) une avalanche de glace dĂ©clenchĂ©e par le dĂ©tachement dâun iceberg, (2) le vĂȘlage et le basculement de lâiceberg qui applique une force horizontale normale au glacier, et (3) lâaccĂ©lĂ©ration de lâice-mĂ©lange dans le fjord. On montre que la magnitude des sĂ©ismes glaciaires ne peut ĂȘtre interprĂ©tĂ©e de maniĂšre simple car elle englobe plusieurs processus distincts. DâaprĂšs les rĂ©sultats de modĂ©lisation numĂ©rique, lâamplitude de la force gĂ©nĂ©rĂ©e par un iceberg tournant contre un terminus ne varie pas de maniĂšre linĂ©aire avec le volume de glace mais est largement dĂ©terminĂ©e par plusieurs facteurs qui influent sur la dynamique du mouvement. Ceci dĂ©montre que, lâestimation de la perte de glace lors des sĂ©ismes glaciaires ne semble pas Ă©vidente Ă partir de leur magnitude. La comparaison des forces inversĂ©es et modĂ©lisĂ©es montre que nous sommes capables de reproduire les signaux sismiques et dâaccĂ©der ainsi Ă la dynamique du phĂ©nomĂšne. Les informations contenues dans les variations temporelles de la force de vĂȘlage permettent notamment dâinverser chaque paramĂštre individuel du modĂšle et dâestimer ainsi les dimensions et le volume de glace dĂ©tachĂ©Glacial earthquakes have magnitudes Mw~5 and associated to the calving of large-scale (km3) unstable icebergs. In order to characterize the force at the origin of long-period seismic signals (10-100~s), we develop a source inversion scheme and a numerical modeling of iceberg capsize. Thanks to detailed analysis of the inverted force histories, we reveal the existence of several phenomena responsible for the seismic signal generation, being (1) an ice-avalanche triggered by the detachment of a first iceberg, (2) the calving and capsize of icebergs which apply a horizontal force normal to the glacial terminus, and (3) the ice-melange acceleration in the fjord. This shows that the interpretation to the event magnitudes is not straightforward as they represent the energy that is released by distinct mechanisms. With mechanical numerical modeling of the phenomenon, we show that the force amplitude does not linearly scales with the iceberg volume but also depends on various parameters that control capsize dynamics. This implies that the calving-induced mass loss cannot be estimated from the glacial earthquake magnitude only. Nevertheless, by comparing between seismic inverted forces and the modeling results, we are able to reproduce seismic signals and access the event dynamics. Informations that are contained in the force histories enable to invert each model parameter and thus estimate the iceberg dimensions and then volum
PersĂ©vĂ©rer ou ne pas persĂ©vĂ©rer ? Ătude qualitative de la persĂ©vĂ©rance selon une adaptation du modĂšle de Tinto chez des Ă©tudiants en premiĂšre bachelier
Le but de ce travail est dâaborder la question de la persĂ©vĂ©rance Ă partir dâun modĂšle universellement reconnu, Ă savoir celui de Tinto (1975). Plus encore, notre volontĂ© a Ă©tĂ© de le rendre encore plus pertinent, non seulement Ă partir de critiques adressĂ©es par dâautres chercheurs, mais aussi sur base de nos propres recherches. Nous avons ainsi affirmĂ© lâimportance du sentiment dâefficacitĂ© personnelle et de lâajustement Ă©motionnel, mais aussi ajoutĂ© deux concepts qui nous semblent essentiels Ă la comprĂ©hension du phĂ©nomĂšne de la persĂ©vĂ©rance : les Ă©vĂ©nements de vie et la valeur perçue de la tĂąche. Ces rĂ©sultats se basent sur quatorze entretiens rĂ©alisĂ©s avec des Ă©tudiants ayant fait au moins une premiĂšre annĂ©e en psychologie. Ceux-ci ont Ă©tĂ© menĂ©s de façon semi-directive et analysĂ©s de façon thĂ©matique. Ils ont permis Ă la fois de confronter la thĂ©orie au terrain, mais aussi de faire Ă©merger de nouvelles pistes de rĂ©flexion sur la persĂ©vĂ©rance.Master [120] en sciences psychologiques, UniversitĂ© catholique de Louvain, 201
Frequency-dependent noise sources in the North Atlantic Ocean
International audienceSecondary microseisms are the most energetic waves in the noise spectra between 3 and 10 s. They are generated by ocean wave interactions and are predominantly Rayleigh waves. We study the associated noise sources in the North Atlantic Ocean by coupling noise polarization analysis and source mapping using an ocean wave model that takes into account coastal reflections. From the Rayleigh wave polarization analysis, we retrieve the back azimuth to the noise sources in the time-frequency domain. Noise source modeling enables us to locate the associated generation areas at different times and frequencies. We analyze the distribution of secondary microseism sources in the North Atlantic Ocean using 20 broadband stations located in the Arctic and around the ocean. To model the noise sources we adjust empirically the ocean wave coastal reflection coefficient as a function of frequency. We find that coastal reflections must be taken into account for accurately modeling 7â10 s noise sources. These reflections can be neglected in the noise modeling for periods shorter than 7 s. We find a strong variability of back azimuths and source locations as a function of frequency. This variability is largely related to the local bathymetry. One direct cause of the time-dependent and frequency-dependent noise sources is the presence of sea-ice that affects the amplitude and polarization of microseisms at stations in the Arctic only at periods shorter than 4 s
Glacial hydraulic tremor on Rhonegletscher, Switzerland
Glacial hydraulic tremor (GHT) can be monitored to observe changes in location and distribution of water flow beneath glacial ice, allowing the spatiotemporal evolution of subglacial hydrology to be studied continuously and remotely. We use frequency-dependent polarization analysis (FDPA) to classify types of GHT and assess its spatio-temporal extent beneath Rhonegletscher, Switzerland, in a continuous seismic record through 2018 and 2019 at three ice-proximal bedrock-based seismometers. We determine the frequency bands composing the GHT and calculate back azimuth angles pointing to a previously known subglacial channel. We also inspect the relationship between GHT seismic power and water discharge from the glacier to compare daily and seasonal shifts with the observed GHT. We observed the seasonal shift from a distributed system to a channelized system, and our dataset allowed comparison of channel locations within and across seasons, with implications for sediment evacuation and bed erosion. The successful use of this method to assess GHT previously on Taku glacier (the methods of which this project follows) and now Rhonegletscher shows that existing ice-proximal passive seismic installations can be used to easily and continuously monitor subglacial hydraulic activity.ISSN:0022-1430ISSN:1727-565
On the Green's function emergence from interferometry of seismic wave fields generated in high-melt glaciers: implications for passive imaging and monitoring
International audienceAmbient noise seismology has revolutionized seismic characterization of the Earth's crust from local to global scales. The estimate of Green's function (GF) between two receivers, representing the impulse response of elastic media, can be reconstructed via cross-correlation of the ambient noise seismograms. A homogenized wave field illuminating the propagation medium in all directions is a prerequisite for obtaining an accurate GF. For seismic data recorded on glaciers, this condition imposes strong limitations on GF convergence because of minimal seismic scattering in homogeneous ice and limitations in network coverage. We address this difficulty by investigating three patterns of seismic wave fields: a favorable distribution of icequakes and noise sources recorded on a dense array of 98 sensors on Glacier d'ArgentiĂšre (France), a dominant noise source constituted by a moulin within a smaller seismic array on the Greenland Ice Sheet, and crevasse-generated scattering at Gornergletscher (Switzerland). In Glacier d'ArgentiĂšre, surface melt routing through englacial channels produces turbulent water flow, creating sustained ambient seismic sources and thus favorable conditions for GF estimates. Analysis of the cross-correlation functions reveals non-equally distributed noise sources outside and within the recording network. The dense sampling of sensors allows for spatial averaging and accurate GF estimates when stacked on lines of receivers. The averaged GFs contain high-frequency (>30âHz) direct and refracted P waves in addition to the fundamental mode of dispersive Rayleigh waves above 1âHz. From seismic velocity measurements, we invert bed properties and depth profiles and map seismic anisotropy, which is likely introduced by crevassing. In Greenland, we employ an advanced preprocessing scheme which includes match-field processing and eigenspectral equalization of the cross spectra to remove the moulin source signature and reduce the effect of inhomogeneous wave fields on the GFs. At Gornergletscher, cross-correlations of icequake coda waves show evidence for homogenized incident directions of the scattered wave field. Optimization of coda correlation windows via a Bayesian inversion based on the GF cross coherency and symmetry further promotes the GF estimate convergence. This study presents new processing schemes on suitable array geometries for passive seismic imaging and monitoring of glaciers and ice sheets
Modelling the source of glacial earthquakes
International audienceOne current concern in Climate Sciences is the estimation of the annual amount of ice lost by glaciers and the corresponding rate of sea level rise. Greenland ice sheet contribution is significant with about 30% to the global ice mass losses. Ice loss in Greenland is distributed approximately equally between loss in land by surface melting and loss at the front of marine-terminating glaciers that is modulated by dynamic processes. Dynamic mass loss includes both submarine melting and iceberg calving. The processes that control ablation at tidewater glacier termini, glacier retreat and calving are complex, setting the limits to the estimation of dynamic mass loss and the relation to glacier dynamics. It involves interactions between bedrock – glacier – icebergs – ice-mélange – water – atmosphere. Moreover, the capsize of cubic kilometer scale icebergs close to a glacier front can destabilize the glacier, generate tsunami waves, and induce mixing of the water column which can impact both the local fauna and flora.We aim to improve the physical understanding of the response of glacier front to the force of a capsizing iceberg against the terminus. For this, we use a mechanical model of iceberg capsize against the mobile glacier interacting with the solid earth through a frictional contact and we constrain it with measured surface displacements and seismic waves that are recorded at teleseismic distances. Our strategy is to construct a solid dynamics model, using a finite element solver, involving a deformable glacier, basal contact and friction, and simplified iceberg-water interactions. We fine-tune the parameters of these hydrodynamic effects on an iceberg capsizing in free ocean with the help of reference direct numerical simulations of fluid-structure interactions involving full resolution of Navier-Stokes equations. We simulate the response of a visco-elastic near-grounded glacier to the capsize of an iceberg close to the terminus. We assess the influence of the glacier geometry, the type of capsize, the ice properties and the basal friction on the glacier dynamic and the observed surface displacements. The surface displacements simulated with our model are then compared with measured displacements for well documented events. </p