Seismic Monitoring of Permafrost in Svalbard, Arctic Norway

We analyze data from passive and active seismic experiments conducted in the Adventdalen valley of Svalbard in the Norwegian Arctic. Our objective is to characterize the ambient wavefield of the region and to investigate permafrost dynamics through estimates of seismic velocity variations. We are motivated by a need for early geophysical detection of potentially hazardous changes to permafrost stability. We draw upon several data sources to constrain various aspects of seismic wave propagation in Adventdalen. We use f-k analysis of five years of continuous data from the Spitsbergen seismic array (SPITS) to demonstrate that ambient seismic noise on Svalbard consists of continuously present body waves and intermittent surface waves appearing at regular intervals. A change in wavefield direction accompanies the sudden onset of surface waves when the average temperature rises above the freezing point, suggesting a cryogenic origin. This hypothesis is supported further by our analysis of records from a temporary broadband network, which indicates that the background wavefield is dominated by icequakes. Synthetic Green's functions calculated from a 3D velocity model match well with empirical Green's functions constructed from the recorded ambient seismic noise. We use a shallow shear-wave velocity model, obtained from active seismic measurements, to estimate the maximum depth of Rayleigh wave sensitivity to changes in shear velocity to be in the 50-100 m range. We extract seasonal variations in seismic velocities from ambient noise cross-correlation functions computed over three years of SPITS data. We attribute relative velocity variations to changes in the ice content of the shallow (2-4 m depth) permafrost, which is sensitive to seasonal temperature changes. A linear decreasing trend in seismic velocity is observed over the years, most likely due to permafrost warming.Peer reviewe

Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift

International audienceIn this study, we explore the origin of lower crustal seismicity and the factors controlling rift propagation using seismological data recorded within the youngest part of the East African Rift System, the North Tanzanian Divergence (NTD). Most earthquakes below Lake Manyara occur at depth ranging between 20 and 40 km and have a swarm-like distribution. Focal mechanisms of 26 events indicate a combination of strike-slip and normal faulting involving Archaean basement structures and forming a relay zone. The derived local stress regime is transtensive and the minimum principal stress is oriented N110°E. Crustal seismic tomography reveals low-velocity anomalies below the rifted basins in the NTD, interpreted as localized thermomechanical perturbations promoting fluid release and subsequent seismicity in the lower crust. SKS splitting analysis in the NTD indicates seismic anisotropy beneath 17 stations most likely due to aligned magma lenses and/or dikes beneath the rift and to the lithospheric fabrics. Our results favor a strain pattern intermediate between purely mechanical and purely magmatic. We suggest that melt products arising from a large asthenospheric thermal anomaly enhance lithospheric weakening and facilitate faulting and creeping on critically oriented inherited structures of the Precambrian lower crust. Although the crust is unlikely weakened at a point comparable to other parts of the East African Rift System, this deep-seated thermomechanical process is efficient enough to allow slow rift propagation within the eastern Tanzanian cratonic edge

Seismotectonics of southeast France: from the Jura mountains to Corsica

The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics at the scale of the crust and by the joint action of different mechanisms of deformation. The distribution of the historical events is in fairly good agreement with the instrumental seismicity, but several earthquakes of $M >6$ are highlighted since the 14th century until the beginning of the 20th

Seismotectonics of southeast France: from the Jura mountains to Corsica

The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics at the scale of the crust and by the joint action of different mechanisms of deformation. The distribution of the historical events is in fairly good agreement with the instrumental seismicity, but several earthquakes of $M >6$ are highlighted since the 14th century until the beginning of the 20th

Relations entre déformation active, rhéologie et magmatisme dans un rift continental : Etude sismologique de la Divergence Nord-Tanzanienne, Rift Est-Africain

Rifting a continent requires sufficient tensional stresses to deform continental plates (far-field plate motions, asthenospheric upwelling, traction form asthenospheric convection) and is controlled by different factors as rheology and lithospheric inheritance. Magmatic (dyking) and tectonic (faulting, aseismic creep) processes are involved in this deformation and their relative importance is not well known and appears to change from one rift to another. The North Tanzanian Divergence (NTD), East African Rift, offers quite favorable conditions to better understand how these processes and factors interact : indeed, it represents the early stage of rifting and is characterized by a sharp change of volcanic and morphotectonic styles. From a local seismological network deployed for 6 months in the NTD (35 stations, SEISMO-TANZ'07), we have recorded earthquakes in order to study their triggering mechanisms, crustal strength, strain and stress fields, and seismic anisotropy. Earthquakes are essentially clustered in the NTD central branch, south of lakes Natron (south of Gelai volcanoe) and Manyara. Magmatic and tectonic processes are both involved in the deformation observed at Gelai : dyking, aseismic slip, faulting (culmination with a Mw 5.9) in the upper crust. The direction of these geological structures is oblique compared with the ~N-S orientation of the rift and parallel to the NE-SW inherited structures, where the rift is partially developping (Eyasi rift). The Manyara cluster is deep (~20-35 km) and associated with significant NE-SW strike-slip faulting. It illustrates the south/southwest rift propagation along the Natron-Manyara-Balangida central branch and the craton buried contact. The minimum principal stress in the area is WNW-ESE and the associated stress field is transtensive. Fluids may play a significant role in the triggering of this deep and long lasting seismic sequence. The influence of the structural inheritance on the magmatic and tectonic processes is also observed at lithospheric scale through the study of the seismic anisotropy. Three main points are underlined by our results : (1) inherited listhopheric structures (rheological contrast, crustal and upper mantle fabric) exert a major control on the location and early expression of the continental rifting; (2) the depth distribution of earthquakes well indicates the rheological properties of the crust (brittle-ductile transition); (3) even in the earliest stages of rifting, magmatic processes seem to play a strong role in the accommodation of extension, in addition to tectonic processes.Les rifts continentaux résultent de l'action de contraintes extensives dont la magnitude est suffisante pour déformer un continent (forces aux limites des plaques, mouvements asthénosphériques). Cette déformation, contrôlée notamment par la rhéologie ou encore l'héritage structural lithosphérique, se réalise par des processus magmatiques (“dyking”) et tectoniques (rupture sur faille et étirement ductile) dont l'importance relative est mal connue et variable d'un segment de rift à l'autre. Afin de mieux comprendre comment ces différents facteurs et processus interagissent, la Divergence Nord-Tanzanienne (DNT) apparaît comme une cible privilégiée du Rift Est-Africain : elle représente un stade précoce du rift et montre une transition abrupte dans le style morphotectonique et l'expression du volcanisme. Cette étude a consisté à déployer un réseau sismologique local dans la DNT et à exploiter les données issues de l'enregistrement continu de l'activité sismique pendant 6 mois (campagne SEISMO-TANZ 2007). Les signaux des séismes proches et lointains sont utilisés comme indicateurs de la sismogénèse, de la résistance crustale, des champs de déformation et de contraintes, et renseignent aussi sur la structure et la fabrique (anisotropie) lithosphérique. La sismicité est essentiellement localisée dans la branche centrale de la DNT, au sud des lacs Natron (sud du volcan Gelaï) et Manyara. La crise sismique observée à Gelaï illustre la co-existence de processus magmatiques et tectoniques, avec la mise en place d'un dyke et le comportement à la fois asismique (glissement lent) et sismique (séisme de magnitude Mw 5.9) de failles normales. Les structures géologiques mises en jeu dans cette crise sont orientées NE-SW, obliques à l'axe ~N-S du rift dans la zone. Cette direction est parallèle à la fabrique tectonique antérieure qui est ré-empruntée par le rift Cénozoïque (faille Eyasi). A Manyara, les séismes sont remarquablement profonds (~20-35 km) et révèlent un décrochement sénestre sur un plan NE-SW. Ils illustrent le développement du rift vers le sud/sud-ouest sur la branche centrale Natron-Manyara-Balangida, au contact du craton tanzanien en profondeur. La contrainte principale minimum calculée dans la zone est orientée WNW-ESE et le régime tectonique local associé est transtensif. Il est fort probable que des fluides soient associés au déclenchement de cette séquence sismique profonde et de longue durée. L'influence de l'héritage structural dans l'expression des processus magmatique et tectonique accommodant la déformation s'observe aussi à l'échelle lithosphérique, par le biais de l'anisotropie sismique. Nos résultats soulignent 3 points majeurs: (1) les structures lithosphériques héritées (contrastes rhéologiques, fabriques crustale et mantellique) exercent un contrôle majeur sur la localisation et l'expression précoce du rifting continental; (2) la distribution des séismes en profondeur apparaît être un bon révélateur des propriétés rhéologiques de la croûte (transition fragile-ductile); et (3) dès le stade du rift immature (où la croûte est peu étirée), les processus magmatiques semblent jouer un rôle prépondérant dans l'accommodation de la déformation, en étroite interaction avec les processus tectoniques

Relations entre déformation active, rhéologie et magmatisme dans un rift continental (étude sismologique de la divergence nord-tanzanienne, rift est-africain)

Cette étude vise à mieux comprendre le rôle respectif et l interaction entre les différents mécanismes qui contrôlent l initiation et le développement des rifts (magmatisme, faille, fabrique lithosphérique, rhéologie). Pour ce faire, le premier réseau local de 35 stations sismologiques a été déployé dans la Divergence Nord-Tanzanienne, la portion la plus jeune du Rift Est-Africain, pendant une période de six mois. Les signaux des séismes proches et lointains sont utilisés comme indicateurs de l histogénèse, de la résistance crustale, de champs de déformation et de contraintes, et renseignent aussi sur la structure et l anisotropie lithosphérique. Une crise sismique enregistrée au Nord de la Tanzanie (Gelai) illustre la co-existence de processus magmatiques et tectoniques, avec mise en place d un dyke et e comportement à la fois asismique (glissement lent) et sismique (séisme de magnitude Mw 5.9) de failles normales. La crise de Gelai indique le rôle majeur et auparavant Insoupçonné des processus asismiques dans l accommodation de la déformation. Des séismes crustaux remarquablement profonds ont été observés plus au sud, dans la région du Lac Manyara. La sismicité de Manyara est associée à des glissements en faille normale et en décrochement probablement déclenchés par des fluides en base de croûte. L ensemble de la sismicité enregistrée dans la DNT indique l influence de l héritage structural, ce qui s observe aussi à l échelle lithosphérique, par le biais de l anisotropie sismiqueThe objective of this study is to better understand the role and the interaction between the different mechanisms that control rift initiation and development (magmatism, fauting, lithospheric fabric, rheology). To this end, a local seismic network has been deployed for the first time in the youngest part of the East African Rift, the North Tanzanian Divergence, for six months. Seismic signal was analyzed to characterize earthquake triggering mechanisms, crustal strength, strain and stress field, and seismic anisotropy. A seismic crisis occurred in North Tanzania (Gelai) involving dyking, seismic and aseismic slip. The Gelai crisis highlights the potential major role of aseismic processes in strain accommodation. Deep crustal earthquakes were recorded in the Lake Manyara region, Manyara seismicity is associated with strike-slip and normal faulting most Iikely triggered by fluids at depth. The influence of structural inheritance on rifting is indicated at lithospheric scale by seismicity and seismic anisotropy.BREST-BU Droit-Sciences-Sports (290192103) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Crustal rheology and depth distribution of earthquakes: Insights from the central and southern East African Rift System

International audienceThe seismicity depth distribution in the central and southern East African Rift System (EARS) is investigated using available catalogs from local, regional and global networks. We select well-determined events and make a re-assessment of these catalogs, including a relocation of 40 events and, where necessary, a declustering. About 560 events are finally used for determining foci depth distribution within 6 areas of the EARS. Assuming that short-term deformation expressed by seismicity reflects the long-term mechanical properties of the lithosphere, we build yield strength envelopes from seismicity depth distribution. Using brittle and ductile laws, we predict the strength percentage spaced every 5 km (or sometimes 2 km) in the crust, for a given composition and a specific geotherm, and constrain it with the relative abundance of seismicity. Results of this modeling indicate significant local and regional variations of the thermo-mechanical properties of the lithosphere which are broadly consistent with previous studies based on independent modelings. In order to explain relatively deep earthquakes, a highly resistant, mafic lower crust is generally required. We also find evidence for changes in the strength magnitude and in the depth of the brittle-ductile transitions which are clearly correlated to tectonic provinces, characterized by contrasted thermal gradients and basement types. A clear N-S increase and deepening of the peak strength level is evidenced along the eastern branch of the EARS, following a consistent southward migration of rifting since ~ 8 Ma. We also detect the presence of a decoupling layer in the Kenya rift, which suggests persisting influences of the deep crustal structures (Archaean and Proterozoic) on the behavior of the extending crust. More generally, our results suggest that seismicity peaks and cut-off depths may provide good proxies for bracketing the brittle-ductile transitions within the continental crust

Combined Impacts of Climate Change and Water Withdrawals on the Water Balance at the Watershed Scale—The Case of the Allier Alluvial Hydrosystem (France)

The Allier River and its alluvial aquifer constitute a shallow but highly productive water resource due to their hydrodynamic properties. This hydrosystem provides almost all of the water requirements for domestic supply and irrigation. Recent dry summers (such as those in 2015, 2019, and 2022) and the lack of winter recharge have led managers to question the sustainability of this resource. We proposed the use of hydrological modelling with Gardenia with which the water balance can be determined at the watershed scale (7020 km2) and with which forecasting simulations can be performed for 2030–2070. Thus, this work was divided into (1) model calibration (2000–2020), (2) the determination of the main drivers of the water balance (2000–2020), (3) and river flow and groundwater level simulation (2030–2070). For the latter, Gardenia was used considering a “better case”, using the RCM Aladin63 in RCP2.6, and considering a “worst case”, using the RCM RegCM4-6 in RCP8.5. The calibration for 2000–2014 showed good reproducibility of river flows (NSE = 0.91) and groundwater levels (NSE = 0.85). The model showed that the major drivers in 2000–2020 were actual evapotranspiration and effective precipitation, which, respectively, represented 68% and 32% of mean annual precipitation. Water withdrawals did not significantly contribute to the water balance with the exception of those in very dry summers, such as those in 2003, 2005, 2015, and 2019. Climate appeared, therefore, as a prevalent factor of the Allier hydrosystem functioning compared to global withdrawals except for that during these dry years. Prospective simulations showed a decline in annual river flows and groundwater levels by a maximum of −15% and −0.08 m asl (“worst case”), respectively. These simulations showed that the Allier hydrosystem will be able to meet the water needs for various uses until 2070. In detail, it is likely that summer shortages will no longer be compensated by the Naussac Dam if the hydrosystem faces more than two years of drought. In this case, water-saving solutions will have to be found. This study is, thus, a good example of the application of hydrological modelling to address management issues in such a hydrosystem

Present‐day deformations of the Jura arc inferred by GPS surveying and earthquake focal mechanisms

International audienceThis study presents a compilation of more than 40 years of seismotectonic data, including 54 computed focal mechanisms, combined with 15 years of GPS data coming from a dense network of 35 permanent GPS stations within the Jura arc and its vicinity. These data are compared to previous available geomorphological, geophysical, and structural studies in order to discuss the 3D distribution of the deformation within the Jura arc. GPS data show coherent schemes in terms of velocities and allowed to discriminate between two provinces (NE of the belt and in its front/foreland). They also constrain a low but significant overall strain tensor with a NNW-SSE shortening of 2.16 nanostrain/year associated with an ENE-WSW extension of 0.44 nanostrain/year. The seismotectonic approach is based on a data set of 2,400 events and 54 focal mechanisms. Inversions of the focal mechanisms both globally and in homogeneous sectors highlight a general strike-slip deformation regime, with sigma1 oriented NW-SE and sigma3 oriented NE-SW. We discriminate two different sectors in terms of basement/cover (un)coupling: (1) potentially decoupled deformation between the basement and the sedimentary cover in the NE part; and (2) coupled deformation in the sedimentary cover and its basement in the Jura foreland