23 research outputs found
Active tectonics and seismic hazard in Central Asia
No full textIn this thesis, I investigate the behaviour of active faults near major cities in Central Asia, to better understand the seismic hazard they pose. First, I explore past and potential earthquakes on the Zailisky Range Front fault, which lies at the northern boundary of the Tian Shan mountains and runs beneath Almaty, Kazakhstan’s largest city. I present the first paleoseismic trench on the fault, which reveals two earthquakes of at least Mw ~6.6 – 6.7. The penultimate earthquake occurred at 9.5 +/- 0.8 ka and the most recent event between 8.4 – 2.6 ka. I use high resolution digital elevation data to map and measure fault scarps along the ~300 km of the range front. Scarp heights are consistent with ruptures of at least Mw ~6.5 – 6.7 for the most recent event or events. A tentative mapping of fault segmentation and the historical record of the 1887 Mw ~7.2 – 7.7 Verny earthquake indicate that the fault is capable of rupturing in larger events. I estimate Quaternary slip rates of ~0.1 – 0.4 mm/yr. A higher geodetic slip rate derived from published GNSS velocities (1.9 +/- 0.7 mm/yr) indicates deformation is accommodated by several parallel structures, some of which may be unmapped.
The second study region is the northern part of the Tajik basin close to Dushanbe, Tajikistan’s capital. I present ~130 m resolution surface velocity maps of the Tajik basin from a Multi-Temporal InSAR analysis of ~5 years of Sentinel-1 data. The rate maps show aseismic slip on several faults associated with an evaporite horizon. The right-lateral Ilyak fault is creeping at ~ 6.7 – 8.7 mm/yr in the east and ~ 4.2 – 4.5 mm/yr in the west. The rate decreases from east to west as slip transfers to the basin thrust sheets. There are no signals indicative of strain accumulation at depth across the Ilyak fault in the surface velocity maps, but moderate earthquakes in the basement suggest that deformation around a locked fault may be obscured by the sedimentary cover. Finally, using satellite-derived elevation models, I document geomorphic evidence for an active thrust fault within the Dushanbe Trough which may be capable of producing a Mw 7.0 – 7.2 earthquake based on the mapped length, highlighting the need for field studies to determine its seismic potential. The research presented in this thesis highlights the seismic hazard posed to major cities in Central Asia and the need for further research to better characterise the active faults
Active tectonics and palaeoseismicity of the Northern Tien Shan and Dzhungaria
No full textThis thesis focuses on the active tectonics and the palaeoseismicity around the Dzhungarian Basin. The study of surface ruptures is crucial to comprehending the earthquake occurrences of faults. I investigate geomorphic displacements along the boundary strike-slip Dzhungarian Fault using high-resolution drone and Pléiades satellite imagery. The results reveal possible single-event fault slip along the Dzhungarian Fault in the most recent earthquake. I suggest this earthquake is likely linked with a previously identified palaeo-earthquake rupture on the Lepsy Fault. With a joint rupture of the two faults, it could generate an earthquake with a magnitude up to Mw 8.4, which would be amongst the largest magnitude inferred for a continental earthquake. I further use Quaternary dating techniques and InSAR time-series analysis to determine the geological and geodetic slip rates of the Dzhungarian Fault. The results show that the northern Dzhungarian Fault has a long-term uplift rate of 0.6 ± 0.2 mm/yr, whilst the southern Dzhungarian Fault has geological and geodetic strike-slip rates consistent with a range of 2.1 – 4.7 mm/yr. I also re-investigate three historical earthquakes with magnitudes greater than Mw 7.0: the 1812 Nilke, the 1906 Manas and the 1944 Xinyuan Earthquakes in the Borohoro Shan. By re-analysing source parameters and integrating published data, seismological analysis results, and remote-sensing mapping, the study demonstrates the significance of both reverse and strike-slip faulting in the regional seismotectonics, which also indicates the deformation kinematics of the Borohoro Shan as being in a transpressional zone. I collate my results with those from the literature to propose updated earthquake scaling relationships of intra-continental earthquakes. Finally, this study suggests that the Dzhungarian Basin and its surrounding tectonic units are rotating anticlockwise to accommodate both the N-S crustal shortening and the left-lateral shearing within a large-scale zone from the SW Tien Shan to the Altay Mountains
Mesures géodésiques et modélisation de la convergence oblique au travers de failles transformantes. Application au bord Nord du Plateau Tibétain et à la Californie du Sud
No full textI focus on three major oblique transform faults in Tibet and in Southern California, in order to better measure and quantify the present-day strain accumulation on these structures. Interferometric synthetic Aperture Radar (InSAR) has the potential to map and localize precisely the deformation over wide areas and thus constrain the deep geometry of these structures. However, its application in natural environments in hindered by strong decorrelation of the radar phase due to vegetation, relief, and freeze and thaw cycles, but also due to variable tropospheric phase delays across topographic feature and long-wavelength residual orbital ramps. Here, I develop methodologies to circumvent these limitations and separate tectonic from other parasite signals. In Tibet, I process data from the Envisat satellite archives, at the boundary of the Tibetan plateau, in two seismic gaps, which appear interesting to study the partitioning of the convergence: the Haiyuan Fault system in northeastern Tibet and the left-lateral Altyn Tagh Fault, in northwestern Tibet. A specific focus on the permafrost related deformation signal allows us to: (1) correctly unwrap interferograms from north to south, (2) quantify the temporal behavior of the freeze/thaw cycles, and (3) isolate bedrock pixels that are not affected by the permafrost signal for further tectonic analysis. I show that the seasonal subsidence depends greatly on the geological land unit and that lower elevations are thawing faster than higher elevations. I analyze the atmospheric signal across the high plateau margin and estimate proxy for the uncertainty on atmospheric corrections. I observe a strike-slip deformation of around 11-15 mm/yr across the Altyn Tagh fault, a clear line of concentrated strike-slip deformation of around 3 mm/yr within the Tarim basin, trending parallel to the Altyn Tagh Fault trace, as well as thrust signal uplifting terraces at a rate of 1 mm/yr. This work also shows a strain accumulation around the west extension of the south trace of the Kunlun Fault, redefining the block boundaries in northwestern Tibet. In parallel this data acquisition, I develop Monte Carlo inversion tools in order to explore the various geometries in agreement with observations and estimate the compatibility of actual surface displacements with long-term slip partitioning models. I thus show a uniform convergence rate of 8.5-11.5 mm/yr with a N81-98E across the Haiyuan fault system and quantify the partitioning along the various structures. I also apply my approach in Southern California, across the « Big Bend » of the San Andreas Fault, where, in analogy with structural geological models, I use conservation of motion to help constraining the geometry and the kinematics of blind thrust faults. I show the compatibility of surface displacements with a large-scale décollement and quantify a loading rate of 2.5 mm/yr along the major thrust structure developing under Los Angeles.Je me focalise sur trois grands systèmes de failles transformantes obliques au Tibet et en Californie du Sud, et ce, afin de mieux comprendre et quantifier les relations entre les différentes structures qui les définissent. L'interférométrie radar à Synthèse d'Ouverture (InSAR) dispose du potentiel pour cartographier et localiser précisément la déformation sur des zones étendues et ainsi contraindre la géométrie des structures profondes. Cependant son utilisation en milieu naturel se trouve fortement entravée par la décorrelation due à la végétation, au relief, et aux cycles de gel et dégel, mais aussi par les délais troposphériques et les rampes orbitales résiduelles. J'ai développé des méthodes pour palier ces limitations. Au Tibet, j'ai ainsi traité les archives du satellite Envisat au niveau de deux zones de lacune sismique, à la bordure Nord du plateau, se présentant comme des zones intéressantes pour étudier le partitionnement de la convergence: le système de faille de Haiyuan au north-est Tibet et la faille sénestre de l'Altyn Tagh, au nord-ouest du plateau. Une attention spécifique sur les déformations liées au pergélisol m'a permis de (1) retrouver la continuité du signal sur de grandes zones, (2) de quantifier le comportement temporel des cycles de gel et dégel des sédiments recouvrant le pergélisol, (3) d'isoler les zones stables des sédiments se déformant. Je montre que les déformations saisonnières sont fortement dépendantes des unités géomorphologiques et que la fonte du pergélisol est plus important à faible qu'à haute altitude. J'analyse aussi le signal saisonnier au travers la marche topographique et je définie un proxy pour les incertitudes de la correction atmosphérique. J'observe un gradient de déformation au travers la faille de l'Altyn Tagh de l'ordre de 11-15 mm/an et un alignement claire de la déformation dans le Tarim, parallèle à la faille de l'Altyn Tagh, ainsi que des soulèvements de l'ordre de 1 mm/an associés à des chevauchements. Ce travail montre aussi un gradient de déformation associé à la terminaison ouest de la faille du Kunlun, re-définissant ainsi la géométrie des blocs tectoniques dans cette région. Parallèlement à cette acquisition de données, je développe des outils d'inversion basés sur des algorithmes de Monte Carlo afin d'explorer l'ensemble des géométries en accord avec les observations et d'estimer la compatibilité de la déformation actuelle avec des modèles tectoniques long-termes. Je montre ainsi une convergence uniforme de 8.5-11.5 mm/an et d'orientation N81-98E à travers le système de faille d'Haiyuan et quantifie son partitionnement le long des différentes structures. Par ailleurs, j'applique mon approche en Californie du Sud, au niveau du « Big Bend » de la faille de San Andreas où, en analogie avec des modèles structuraux géologiques, j'utilise des lois de conservations du mouvement pour contraindre la géométrie des chevauchements aveugles. Je montre la compatibilité du champs de déformation actuel avec un décollement grande échelle et quantifie une accumulation de contrainte de 2.5 mm/an le long de la structure majeure sous Los Angeles
Geodetic measurements and modeling of oblique convergence across transform faults. Application to the Northern Tibetan Plateau and to Southern California
No full textJe me focalise sur trois grands systèmes de failles transformantes obliques au Tibet et en Californie du Sud, et ce, afin de mieux comprendre et quantifier les relations entre les différentes structures qui les définissent. L'interférométrie radar à Synthèse d'Ouverture (InSAR) dispose du potentiel pour cartographier et localiser précisément la déformation sur des zones étendues et ainsi contraindre la géométrie des structures profondes. Cependant son utilisation en milieu naturel se trouve fortement entravée par la décorrelation due à la végétation, au relief, et aux cycles de gel et dégel, mais aussi par les délais troposphériques et les rampes orbitales résiduelles. J'ai développé des méthodes pour palier ces limitations. Au Tibet, j'ai ainsi traité les archives du satellite Envisat au niveau de deux zones de lacune sismique, à la bordure Nord du plateau, se présentant comme des zones intéressantes pour étudier le partitionnement de la convergence: le système de faille de Haiyuan au north-est Tibet et la faille sénestre de l'Altyn Tagh, au nord-ouest du plateau. Une attention spécifique sur les déformations liées au pergélisol m'a permis de (1) retrouver la continuité du signal sur de grandes zones, (2) de quantifier le comportement temporel des cycles de gel et dégel des sédiments recouvrant le pergélisol, (3) d'isoler les zones stables des sédiments se déformant. Je montre que les déformations saisonnières sont fortement dépendantes des unités géomorphologiques et que la fonte du pergélisol est plus important à faible qu'à haute altitude. J'analyse aussi le signal saisonnier au travers la marche topographique et je définie un proxy pour les incertitudes de la correction atmosphérique. J'observe un gradient de déformation au travers la faille de l'Altyn Tagh de l'ordre de 11-15 mm/an et un alignement claire de la déformation dans le Tarim, parallèle à la faille de l'Altyn Tagh, ainsi que des soulèvements de l'ordre de 1 mm/an associés à des chevauchements. Ce travail montre aussi un gradient de déformation associé à la terminaison ouest de la faille du Kunlun, re-définissant ainsi la géométrie des blocs tectoniques dans cette région. Parallèlement à cette acquisition de données, je développe des outils d'inversion basés sur des algorithmes de Monte Carlo afin d'explorer l'ensemble des géométries en accord avec les observations et d'estimer la compatibilité de la déformation actuelle avec des modèles tectoniques long-termes. Je montre ainsi une convergence uniforme de 8.5-11.5 mm/an et d'orientation N81-98E à travers le système de faille d'Haiyuan et quantifie son partitionnement le long des différentes structures. Par ailleurs, j'applique mon approche en Californie du Sud, au niveau du « Big Bend » de la faille de San Andreas où, en analogie avec des modèles structuraux géologiques, j'utilise des lois de conservations du mouvement pour contraindre la géométrie des chevauchements aveugles. Je montre la compatibilité du champs de déformation actuel avec un décollement grande échelle et quantifie une accumulation de contrainte de 2.5 mm/an le long de la structure majeure sous Los Angeles.I focus on three major oblique transform faults in Tibet and in Southern California, in order to better measure and quantify the present-day strain accumulation on these structures. Interferometric synthetic Aperture Radar (InSAR) has the potential to map and localize precisely the deformation over wide areas and thus constrain the deep geometry of these structures. However, its application in natural environments in hindered by strong decorrelation of the radar phase due to vegetation, relief, and freeze and thaw cycles, but also due to variable tropospheric phase delays across topographic feature and long-wavelength residual orbital ramps. Here, I develop methodologies to circumvent these limitations and separate tectonic from other parasite signals. In Tibet, I process data from the Envisat satellite archives, at the boundary of the Tibetan plateau, in two seismic gaps, which appear interesting to study the partitioning of the convergence: the Haiyuan Fault system in northeastern Tibet and the left-lateral Altyn Tagh Fault, in northwestern Tibet. A specific focus on the permafrost related deformation signal allows us to: (1) correctly unwrap interferograms from north to south, (2) quantify the temporal behavior of the freeze/thaw cycles, and (3) isolate bedrock pixels that are not affected by the permafrost signal for further tectonic analysis. I show that the seasonal subsidence depends greatly on the geological land unit and that lower elevations are thawing faster than higher elevations. I analyze the atmospheric signal across the high plateau margin and estimate proxy for the uncertainty on atmospheric corrections. I observe a strike-slip deformation of around 11-15 mm/yr across the Altyn Tagh fault, a clear line of concentrated strike-slip deformation of around 3 mm/yr within the Tarim basin, trending parallel to the Altyn Tagh Fault trace, as well as thrust signal uplifting terraces at a rate of 1 mm/yr. This work also shows a strain accumulation around the west extension of the south trace of the Kunlun Fault, redefining the block boundaries in northwestern Tibet. In parallel this data acquisition, I develop Monte Carlo inversion tools in order to explore the various geometries in agreement with observations and estimate the compatibility of actual surface displacements with long-term slip partitioning models. I thus show a uniform convergence rate of 8.5-11.5 mm/yr with a N81-98E across the Haiyuan fault system and quantify the partitioning along the various structures. I also apply my approach in Southern California, across the « Big Bend » of the San Andreas Fault, where, in analogy with structural geological models, I use conservation of motion to help constraining the geometry and the kinematics of blind thrust faults. I show the compatibility of surface displacements with a large-scale décollement and quantify a loading rate of 2.5 mm/yr along the major thrust structure developing under Los Angeles
Interseismic and postseismic shallow creep of the North Qaidam Thrust faults detected with a multitemporal InSAR analysis
No full textUnderstanding the mechanisms by which earthquake cycles produce folding and accommodate shortening is essential to quantify the seismic potential of active faults and integrate aseismic slip within our understanding of the physical mechanisms of the long-term deformation. However, measuring such small deformation signals in mountainous areas is challenging with current space-geodesy techniques, due to the low rates of motion relative to the amplitude of the noise. Here we successfully carry out a multitemporal Interferometric Synthetic Aperture Radar analysis over the North Qaidam fold-thrust system in NE Tibet, where eight Mw> 5.2 earthquakes occurred between 2003 and 2009. We report various cases of aseismic slip uplifting the thickened crust at short wavelengths. We provide a rare example of a steep, shallow, 13-km-long and 6-km-wide afterslip signal that coincides spatially with an anticline and that continues into 2011 in response to a Mw 6.3 event in 2003. We suggest that a buried seismic slip during the 2003 earthquake has triggered both plastic an-elastic folding and aseismic slip on the shallow thrusts. We produce a first-order two-dimensional model of the postseismic surface displacements due to the 2003 earthquake and highlight a segmented slip on three fault patches that steepen approaching the surface. This study emphasizes the fundamental role of shallow aseismic slip in the long-term and permanent deformation of thrusts and folds and the potential of Interferometric Synthetic Aperture Radar for detecting and characterizing the spatiotemporal behavior of aseismic slip over large mountainous regions
Post-earthquake fold growth imaged in the Qaidam Basin, China, with interferometric synthetic aperture radar
No full textQuestions regarding the development of folds and their interactions with the seismic faults within thrust systems remain unanswered. However, estimating fault slip and earthquake hazards using surface observations and kinematic models of folding requires an understanding of how the shortening is accommodated during the different phases of the earthquake cycle. Here, we construct 16-years of interferometric synthetic aperture radar time series across the North Qaidam thrust system (NE Tibet), where three Mw 6.3 earthquakes occurred along basement faults underlying shortened folded sediments. The analysis reveals spatio-temporal changes of post-earthquake surface displacement rates and patterns, which continue more than 10 years after the seismic events. The decomposition of the Sentinel-1 ascending and descending line of sight velocities into vertical and shortening post-earthquake components indicates that long-term transient uplift and shortening is in agreement with the deformation that might be expected from kinematic models of folding. Long-term uplift coincides spatially with young anticlines observed in the geomorphology, with steep gradients in the forelimbs, gentle gradients in the back-limbs, an absence of subsidence in the footwalls, and higher gradients along interpreted bedding planes. Long-term shortening is also different from the surface displacements expected for typical time-varying creep on a narrow dislocation interface and shows rates higher than the average convergence across the whole region. These findings provide evidence for anelastic fold buckling during the post-earthquake phase and highlight the contribution of distributed aseismic deformation to the growth of topography
Ice loss in the Northeastern Tibetan Plateau permafrost as seen by 16 yr of ESA SAR missions
No full textInSAR time series of surface deformation from 16 yr of Envisat (2003-2011) and Sentinel-1 (2014-2019) ESA satellite radar measurements have been constructed to characterise spatial and temporal dynamics of ground deformation over an 80,000 km2 area in the permafrost of the northeastern Tibetan Plateau. The regional deformation maps encompass various types of periglacial landforms and show that seasonal thaw effects are controlled by the sediment type and local topography. High seasonal ground movements are concentrated on shallow slopes and poor-drainage areas in unconsolidated, frost-susceptible and fine-grained sediments within glacier outwash plains, braided stream plains, alluvial deposits or floodplains. Fast subsidence due to thaw settlement takes place during June/July while frost heave is intense during December/January when two-sided freezing of pore water under pressure causes prolonged ice segregation near the permafrost table. The analysis reveals pervasive subsidence of the ground of up to ∼2 cm/yr, and increasing by a factor of 2 to 5 from 2003 to today, in high-relief and well-drained areas. The findings suggest that seasonal thaw increasingly affects ice-rich layers at the permafrost table, as well as high-rates of widespread mass movements of non-consolidated sediments, the latter amplified by an increase of effects from frost heave/thaw settlement
Downslope solifluction movements and permafrost degradation in the northeastern Qinghai-Tibetan Plateau revealed by InSAR
No full textInternational audienceInSAR measurements reveal widespread and high-amplitude permafrost-related solifluction movements across the Qinghai-Tibetan Plateau (QTP). This raises questions about the mechanisms driving these slope processes and, more specifically, the relationships between cyclic soil expansion during freezing and irreversible downslope motions during thawing. In this study, we develop and apply a methodology to map solifluction motions using multi-temporal Interferometric Synthetic Aperture Radar (InSAR) data and characterize their amplitudes and spatial distributions at a regional scale. This method decomposes line-of-sight InSAR time series into pluri-annual velocities and seasonally reversible surface displacements that are then inverted into motions parallel and normal to the local steepest slope. Pluri-annual downslope and subsidence velocities in the northeastern QTP are analyzed and cross-correlated with slope-normal seasonal cycles and the slope across unconsolidated sediments and indurated bedrock colluvium separately. Our analysis indicates significant downslope velocities ( 4 mm/yr), with varying sensitivities to slope. Overall, downslope velocities exceed those expected from seasonal frost creep alone and are likely triggered by basal soil shearing during the thawing of the ice-rich layers. Downslope velocities are particularly high for indurated bedrock colluvium, which exhibit low-amplitude slope-normal seasonal cycles compared to unconsolidated sediments. We explain these differences by variations in regolith layer thicknesses, which impact the downslope deformation depths. Measurements of subsidence and amplified seasonal cycles through time indicate ongoing permafrost degradation and ice loss within the QTP, directly impacting slope stability, primarily by intensifying seasonal basal solifluction processes
Optical Geodesy and the Measurement of Ground Deformation by Image Correlation
No full textInternational audienceOptical geodesy is a relatively new technique which allows deformation of the Earth's surface to be retrieved from the correlation of remotely sensed optical satellite or aerial images. In this chapter, we summarize the main aspects of optical geodesy, with a particular focus on using correlation methods for measuring ground deformations due to earthquakes. We discuss the various datasets which can be used, and common problems associated with using them for geodetic purposes. We touch upon bundle adjustment, which allows the fusion of many small images to produce seamless mosaics, which can them be used for correlation. Correlation methods are also discussed, with a focus on the two main approaches: zero-mean normalized cross correlation in the spatial domain, and phase correlation in the frequency domain. We discuss various methods of denoising used to minimize outliers, or unwanted artifacts and signals in the correlation maps. We finish with a brief discussion of future directions in optical geodesy, such as generating 3D displacement fields from stereo imagery, processing optical time-series, and potential methodological improvements to correlator performance.The recent boom within the optical satellite industry (Figure 1) has resulted in greatly improved access to higher quality, and increased resolution data for scientists. Thanks to coupling with technical and computational developments in the field of image processing, computer vision, and photogrammetry, the field of optical geodesy has been developing quickly. This chapter builds upon the earlier detailed summary of Avouac and Leprince (2015), as well as more recent summaries by Pierrot Deseilligny and Rupnik (2022) and Marchandon, et al. (2022), which cover recent technical developments and how they impact and facilitate the measurement of ground deformation using optical image correlation of modern and historic aerial and satellite images. We focus largely on the practical aspects of optical geodesy (rather than theoretical), with the aim of providing an accessible entry point for the non-specialist. We first summarize the common data types used in optical geodesy, and how they are processed (preprocessing, correlation, and post-processing) to obtain measurements of ground deformation in 1D, 2D, or 3D. We then discuss how the results can be interpreted for scientific analysis, highlighting key advantages and limitations of the technique. Finally, we discuss future avenues of research, largely methodological, such as big-data processing and time-series analysis, which will allow a variety of new research problems to be addressed in the coming years
A repetitive high voltage nanoseconds pulse generator ::first prototype design and test results
No full textIn this paper a repetitive high voltage pulse generator is presented. The nominal output voltage of the high voltage pulses is 10kV. The main purpose is to create a pulsed electrical field to be used in scientific applications such as medicine, biology or food processing. In these cases, the pulses width should be as short as 100 ns with rising and falling times as shorter as possible. Moreover, for these applications, the pulsed electrical field is used in continuous processing, meaning that it needs to be repetitive and repeatable. In this case, the repetition rate is specified at 1 kHz
