GPR measurements to assess the Emeelt active fault's characteristics in a highly smooth topographic context, Mongolia

International audienceTo estimate the seismic hazard, the geometry (dip, length and orientation) and the dynamics (type of displacements and amplitude) of the faults in the area of interest need to be understood. In this paper, in addition to geomorphologic observations, we present the results of two ground penetrating radar (GPR) campaigns conducted in 2010 and 2011 along the Emeelt fault in the vicinity of Ulaanbaatar, capital of Mongolia, located in an intracontinental region with low deformation rate that induces long recurrence time between large earthquakes. As the geomorphology induced by the fault activity has been highly smoothed by erosion processes since the last event, the fault location and geometry is difficult to determine precisely. However, by using GPR first, a non-destructive and fast investigation, the fault and the sedimentary deposits near the surface can be characterized and the results can be used for the choice of trench location. GPR was performed with a 50 MHz antenna over 2-D lines and with a 500 MHz antenna for pseudo-3-D surveys. The 500 MHz GPR profiles show a good consistency with the trench observations, dug next to the pseudo-3-D surveys. The 3-D 500 MHz GPR imaging of a palaeochannel crossed by the fault allowed us to estimate its lateral displacement to be about 2 m. This is consistent with a right lateral strike-slip displacement induced by an earthquake around magnitude 7 or several around magnitude 6. The 2-D 50 MHz profiles, recorded perpendicular to the fault, show a strong reflection dipping to the NE, which corresponds to the fault plane. Those profiles provided complementary information on the fault such as its location at shallow depth, its dip angle (from 23 • to 35 •) and define its lateral extension. Central Asia is known for its high level of seismic hazards, especially Mongolia, which has been one of the most seismically active intracontinental regions in the world with four large earthquakes (magnitude around 8) along its active faults in the western part of the country during the last century (Khilko et al. 1985). The deformation in Mongolia is located between compressive structures related to the collision and penetration of the Indian plate into the Eurasian plate and extensive structures in the north of the country related with the Baykal rift (Tapponnier & Molnar 1979; Baljinnyam et al. 1993; Schlupp 1996; Bayasgalan & Jackson 1999). The seismic activity observed in the vicinity of Ulaanbaatar (UB), capital of Mongolia, is relatively low compared to the activity observed in western Mongolia. Nevertheless, since 2005, the seismic activity around UB not only has increased, but is also organized (see Fig. 1) at the west of UB along two perpendicular directions, which determine two active faults: Emeelt fault, discovered in 2008 (NNW-SSE direction, 25-km-long minimum and situated about 10 km W of UB) and Hustai fault (WSW–ENE direction, 80 km long, with its NE tip at less than 20 km west of UB); their length and morphology indicate that they can produce earthquakes of magnitude 6.5–7.5 (Schlupp et al. 2012). Most of the Mongolian population (1.2 million over 3 million) is concentrated at UB, which is the main political and economical centre of the country. Hence, the study of seismic hazard and the estimation of the probability of future destructive earthquakes are of primary importance for the country (Dugarmaa et al. 2006). Since the last large earthquake, the faults geomorphology has been highly smoothed by erosional processes and the exact location of the fault plane surface rupture is thus hidden within a several metre wide strip. The GPR method has been proven to give good and useful results to characterize faults by identifying offsets of radar reflections (Malik et al. 2007; Christie et al. 2009; Yalçiner et al. 2013) an

The 2003 Chuya sequence (North Altay range): tectonic context and seismological study.

Thèse préparée dans le laboratoire Géosciences Azur (UMR 6526 à Sophia Antipolis) et au Centre de Recherche d'Astronomie et Géophysique (Oulan-Bator, Mongolie)The present work is focussed on the seismological study of the September 27th 2003 (M = 7.3) large earthquake in Altay and try to include the results on a tectonical study of the Altay range (Mongolia/Russia).In the first chapter is presented tectonic and geodynamic aspect of Altay range. A review of existing works on the morphological and geological context is completed by an interpretation of satellite images. We consider four active fault zones from their continuity. On images we interpret several offsets of river streams. The distribution of offsets is similar along the same active zone, but are different from one fault to the other. We propose a partitioning of movements inside the Altay block and discuss the compatibility of the observations with a global rotation of the Altay. In the second chapter is discussed the seismic catalogue of Mongolia produced by the RCAG and presented different methods to analyse it, in particular the clustering to reduce the impact of the aftershocks. We show distributions of b-values in the different zones and their relation with the abnormal occurrences of large events on the territory of Mongolia. Next, we focus more on Altay region and its observed seismic activity.The third chapter is focused on the geological context of the Chuya earthquake. We discuss the available field observations and compared the measured displacement values to the results obtained with a SPOT image correlation made on the southern part of the ruptured zone. Surfaces rupture observed on the field and deduced from spot correlation are geometrically very similar, but amplitude of the horizontal displacement are different.In the fourth chapter we analyze in detail location of the seismic sequence. Relocation of the first period (main shock and stronger aftershocks) is done with regional data. Next, we discuss the location procedure and show the aftershocks distribution obtained for the period covered by a temporary network. Main activity of aftershock occurred on southern margin of Chuya and Kurai depressions. The cluster is 90 km long in a NW-SE direction, superposed to the surface breaks in its central part, and extend to a depth of 20km. In the fifth chapter is presented a model of the rupture for the mainshock and two large aftershocks obtained using body wave inversion. The 3 analysed events are modeled as complex events with combination of two or more subevents with different focal mechanisms and different rupture velocities. The source duration, and rupture velocities allow to reconstruct from the epicentral locations a rupture which is in agreement with the surface observation but the history of the sequence can be related to the segmentation of the fault and the presence of different tectonic units.Le travail présenté est consacré à l'étude sismologique du séisme du 27 septembre 2003 qui a eu lieu en Altay (Mongolie, Russie) et à l'interprétation des résultats dans le cadre d'une étude tectonique de la chaîne de l'Altay.Dans le premier chapitre nous avons collecté les informations morphologiques, géologiques et tectoniques dans la littérature existante. Nous en avons déduit un schéma simple de déformation de la chaîne avec 4 failles NW-SE majeures. Nous avons complété ces informations avec une interprétation d'images satellites sur lesquelles sont mesurés des déplacements horizontaux. Les décalages semblent de même amplitude sur le même faille, mais variables d'une faille à l'autre. Nous proposons un partionnement du mouvement à l'intérieur du bloc Altay et discutons de la compatibilité avec une rotation anti-horaire du bloc.Dans le second chapitre nous analysons le catalogue de sismicité de Mongolie produit par le RCAG. Nous montrons qu'une grande partie des événements peuvent être considérés comme des répliques des grands séismes qui ont eu lieu au siècle dernier. Nous analysons la distribution de la b-value dans différentes régions en relation avec l'occurrence de grands séismes. L'accent est mis sur la région de l'Altay.Dans le 3ième chapitre nous introduisons le contexte géologique du séisme de Chuya (Ms = 7.3) et les observations de qui ont pu être faites en surface du déplacement horizontal produit par le séisme. Les mesures sur les traces de surface sont comparées à des mesures plus globales obtenues sur l'auto-corrélation d'image SPOT. La géométrie très semblable contraint très bien la position du séisme.Dans le 4ième chapitre est analysée la position des séismes majeurs de la séquence sismique à partir de données sismologiques de réseaux permanents. Une meilleure image de la distribution des répliques est produite par l'interprétation des données d'un réseau temporaire déployé sur le zone épicentrale. Cet essaim de séisme montre une activité sur 90 km de long dans la direction NW-SE le long du bord sud des dépressions de Chuya et Kurai et jusqu'à des profondeurs de 20 km.Dans le 5ième chapitre est présenté un modèle de rupture des 3 événements majeurs de la séquence obtenu par inversion des ondes de volume P et SH. Ces 3 séismes présentent une rupture complexe avec plusieurs sous événements de mécanisme différent et des vitesses de rupture assez variables. Les valeurs de durée de source et de vitesse de rupture sont en bon accord avec les observations de surface.L'ensemble de ces résultats permet de proposer un schéma global de la séquence cohérent en accord avec la morphologie et la tectonique dans cette partie de la chaîne

La séquence sismique de Chuya (Altay Nord) (contexte tectonique et étude sismologique)

La Mongolie a été le siège le siècle dernier de plusieurs très grands séismes sur de grandes failles décrochantes intra-continentales. La sismicité de ce territoire est bien suivie depuis maintenant plus de 40 ans par un réseau sismologique en évolution permanente. Elle est difficile à analyser car elle correspond en partie aux répliques des grands événements. L'Altay, à l'ouest du pays, est la région la plus sismique. L'analyse des photos satellites LANDSAT permet de mettre en évidence de grands décrochements dextres de direction NW-SE à l'intérieur même du bloc sur lesquels des déplacements horizontaux de 2 m à 4 m sont observés depuis le Pleistocène. On est donc amené à proposer un partitionnement de la déformation à travers le bloc. Le séisme de Chuya de Septembre 2003 de magnitude 7.3, est le plus gros événement enregistré par les réseaux sismologiques modernes. Il se situe au NW de la chaîne sur une faille qui borde au sud les basins de Chuya et Kurai. Il a été suivi par 2 grosses répliques. La modélisation des ondes de volume de ces 3 événements principaux montrent la complexité de la rupture, liée à une segmentation de la faille. La majorité du moment sismique relaché se fait sous forme de mouvement décrochant, mais certaines parties correspondent à des mécanismes compressifs. La confrontation des longueurs de faille déduites de cette modélisation, des traces de surface observées, du déplacement mesuré sur une corrélation de photo SPOT et de la distribution des répliques permet de proposer un modèle temporel pour cette séquence cohérent avec la tectonique du bloc et les contraintes régionales. Les caractéristqiues principales sont comparées à celles obtenues pour d'autres séismes mondiaux similaires.On Mongolian territory occurred in the last century many very large earthquakes on large intracontinental strike-slip faults. Since 40 years Mongolian seismicity is well followed with a seismic network in constant evolution. This seismicity is not easy to analysed as a large part of the events must be considered as aftershocks of the large events which occurred at the beginning of the XX century. Altay range, NW of the country, is one of the most seismicaly active region in Mongolia. Analyse of LANDSAT satellite images allows us to observe horizontal displacements in the range of 2 to 4 meters along NW-SE long strike-slip faults inside the bloc. We propose a partionning of the deformation as the main characteristics of the tectonics of the Altay bloc. The Chuya event (magnitude 7.3) occurred on September 2003 at the NW edge of on of these faults which lies on the SW border of Chuya and Kurai basins. It was the first event worldwide recorded on modern seismological stations. It was followed by 2 large aftershocks. The body waves modeling show that the rupture was complex, related to the field observation of fault segmentation. Most part of the seismic moment was released in strike slip motion as mainly observed on the faults, but some compressional movment was also observed. Segment lengths, surface ruptures, horizontal displacement measured on a SOPT correlation image and aftershock distribution allow to propose a coherent model of the sequence in agreement with tregional tectonics and stress field and the charcateristics of which are compared to results on similar events in the world.NICE-BU Sciences (060882101) / SudocSudocFranceF

La séquence sismique de Chuya (Altay Nord) (contexte tectonique et étude sismologique)

La Mongolie a été le siège le siècle dernier de plusieurs très grands séismes sur de grandes failles décrochantes intra-continentales. La sismicité de ce territoire est bien suivie depuis maintenant plus de 40 ans par un réseau sismologique en évolution permanente. Elle est difficile à analyser car elle correspond en partie aux répliques des grands événements. L'Altay, à l'ouest du pays, est la région la plus sismique. L'analyse des photos satellites LANDSAT permet de mettre en évidence de grands décrochements dextres de direction NW-SE à l'intérieur même du bloc sur lesquels des déplacements horizontaux de 2 m à 4 m sont observés depuis le Pleistocène. On est donc amené à proposer un partitionnement de la déformation à travers le bloc. Le séisme de Chuya de Septembre 2003 de magnitude 7.3, est le plus gros événement enregistré par les réseaux sismologiques modernes. Il se situe au NW de la chaîne sur une faille qui borde au sud les basins de Chuya et Kurai. Il a été suivi par 2 grosses répliques. La modélisation des ondes de volume de ces 3 événements principaux montrent la complexité de la rupture, liée à une segmentation de la faille. La majorité du moment sismique relaché se fait sous forme de mouvement décrochant, mais certaines parties correspondent à des mécanismes compressifs. La confrontation des longueurs de faille déduites de cette modélisation, des traces de surface observées, du déplacement mesuré sur une corrélation de photo SPOT et de la distribution des répliques permet de proposer un modèle temporel pour cette séquence cohérent avec la tectonique du bloc et les contraintes régionales. Les caractéristqiues principales sont comparées à celles obtenues pour d'autres séismes mondiaux similaires.On Mongolian territory occurred in the last century many very large earthquakes on large intracontinental strike-slip faults. Since 40 years Mongolian seismicity is well followed with a seismic network in constant evolution. This seismicity is not easy to analysed as a large part of the events must be considered as aftershocks of the large events which occurred at the beginning of the XX century. Altay range, NW of the country, is one of the most seismicaly active region in Mongolia. Analyse of LANDSAT satellite images allows us to observe horizontal displacements in the range of 2 to 4 meters along NW-SE long strike-slip faults inside the bloc. We propose a partionning of the deformation as the main characteristics of the tectonics of the Altay bloc. The Chuya event (magnitude 7.3) occurred on September 2003 at the NW edge of on of these faults which lies on the SW border of Chuya and Kurai basins. It was the first event worldwide recorded on modern seismological stations. It was followed by 2 large aftershocks. The body waves modeling show that the rupture was complex, related to the field observation of fault segmentation. Most part of the seismic moment was released in strike slip motion as mainly observed on the faults, but some compressional movment was also observed. Segment lengths, surface ruptures, horizontal displacement measured on a SOPT correlation image and aftershock distribution allow to propose a coherent model of the sequence in agreement with tregional tectonics and stress field and the charcateristics of which are compared to results on similar events in the world.NICE-BU Sciences (060882101) / SudocSudocFranceF

Tectonic Geomorphology and Paleoseismology of the Sharkhai fault: a new source of seismic hazard for Ulaanbaatar (Mongolia)

International audienceWe present first constraints from tectonic geomorphology and paleoseismology along the newly discovered Sharkhai fault near the capital city of Mongolia. Detailed observations from high-resolution Pleiades satellite images and field investigations allowed us to map the fault in detail, describe its geometry and segmentation, characterize its kinematics, and document its recent activity and seismic behavior (cumulative displacements and paleoseismicity). The Sharkhai fault displays a surface length of ∼ 40 km with a slightly arcuate geometry, and a strike ranging from N42 to N72∘. It affects numerous drainages that show left-lateral cumulative displacements reaching 94 m. Paleoseismic investigations document faulting and depositional/erosional events for the last ∼ 3000 years and reveal that the most recent event occurred between 775 and 1778 CE and the penultimate earthquake occurred between 1605 and 835 BCE. The resulting time interval of 2496 ± 887 years is the first constraint for the Sharkhai fault for large earthquakes. On the basis of our mapping of the surface rupture and the resulting segmentation analysis, we propose two possible scenarios for large earthquakes with likely magnitudes of 6.7 ± 0.2 or 7.1 ± 0.7. Furthermore, we apply scaling laws to infer coseismic slip values and derive preliminary estimates of long-term slip rates. Finally, these data help build a comprehensive model of active faults in that region and should be considered in the seismic hazard assessment for the city of Ulaanbaatar

La faille de Sharkhai : une nouvelle source de l'aléa sismique pour Oulan-Bator (Mongolie), approche morphologique et paléosismologique

International audienceOn présente les résultats des investigations morphotectoniques et de paléosismologiques réalisées le long de la faille de Sharkhai découverte près d'Oulan Bator. Des observations détaillées, à partir d'images satellites haute résolution et des travaux de terrain, nous ont permis de cartographier la faille, de caractériser sa cinématique, et de documenter son activité récente. Sharkhai, d'environ 40km de long, est une faille senestre. Les investigations paléosismiques révèlent que l'avant dernier séisme s'est produit entre 1515 ± 90 et 945 ± 110 avant JC et que le dernier séisme est plus récent que 860 ± 85 après JC. L'intervalle de temps entre ces deux évènements, la première contrainte temporelle, est ainsi de 2080±470 ans. Nous proposons deux scénarios possibles pour les grands séismes associés à des magnitudes probables comprises entre 6,4±0,2 et 7,1±0,2. En appliquant des lois d'échelle on obtient des taux de glissement à long terme entre 0,2± 0,2 et 1,0 ± 0,5 mm/an. Ces observations et résultats originaux doivent être pris en compte pour l'évaluation de l'aléa sismique pour la ville d'Oulan-Bator

The Ar-Hötöl surface rupture along the Khovd fault (Mongolian Altay)

International audienceWe present a 1:200,000 scale map of the Ar-Hötöl surface rupture along the Khovd fault(Mongolian Altay), presumed to be the expression of the 1761 CE Mw ∼ 7.8 Great Mongolearthquake. The detailed mapping combines airborne and terrestrial imaging andtopographic techniques (Sentinel-2, Pleiades, TanDEM-X, UAV and TLS) to quantify right-lateral and vertical offsets ranging from ∼ 1 m to ∼ 4 km over a length of 238 km. Thesmaller offsets document the deformation associated with the last surface-rupturingearthquake that affects several Bronze to Iron Age burial mounds. Their analysis yields arobust segmentation model comprising 6 segments of 20 to 51 km in length, a maximumco-seismic slip value of 4.8 m ± 0.5 m located near the center of the rupture. Ourobservations precise the varying kinematics along strike, bring new evidence of repeatedfaulting and confirm a Mw of 7.8 ± 0.3

Gravity waves produced by the total solar eclipse of 1 August 2008

International audienceGravity waves are a major component of atmospheric small scale dynamics because of their ability to transport energy and momentum over considerable distances and of their interactions with the mean circulation or other waves. They produce pressure variations which can be detected at the ground by microbarographs. The solar intensity reduction which occurs in the atmosphere during solar eclipses is known to act as a temporary source of large scale gravity waves. Despite decades of research, observational evidence for a characteristic bow-wave response of the atmosphere to eclipse passages remains elusive. A new versatile numerical model (Marty, J. and Dalaudier, F.: Linear spectral numerical model for internal gravity wave propagation. J. Atmos. Sci. (in press)) is presented and applied to the cooling of the atmosphere during a solar eclipse. Calculated solutions appear to be in good agreement with ground pressure fluctuations recorded during the total solar eclipse of 1 August 2008. To the knowledge of the authors, this is the first time that such a result is presented. A three-dimensional linear spectral numerical model is used to propagate internal gravity wave fluctuations in a stably stratified atmosphere. The model is developed to get first-order estimations of gravity wave fluctuations produced by identified sources. It is based on the solutions of the linearized fundamental fluid equations and uses the fully-compressible dispersion relation for inertia-gravity waves. The spectral implementation excludes situations involving spatial variations of buoyancy frequency or background wind. However density stratification variations are taken into account in the calculation of fluctuation amplitudes. In addition to gravity wave packet free propagation, the model handles both impulsive and continuous sources. It can account for spatial and temporal variations of the sources allowing to cover a broad range of physical situations. It is applied to the case of solar eclipses, which are known to produce large-scale bow waves on the Earth's surface. The asymptotic response to a Gaussian thermal forcing travelling at constant velocity as well as the transient response to the 4 December 2002 eclipse are presented. They show good agreement with previous numerical simulations. The model is then applied to the case of the 1 August 2008 solar eclipse. Ground pressure variations produced by the response to the solar intensity reduction in both stratosphere and troposphere are calculated. These synthetic signals are then compared to pressure variations recorded by IMS (International Monitoring System) infrasound stations and a temporary network specifically set up in Western Mongolia for this occasion. The pressure fluctuations produced by the 1 August 2008 solar eclipse are in a frequency band highly disturbed by atmospheric tides. Pressure variations produced by atmospheric tides and synoptic disturbances are thus characterized and removed from the signal. A low frequency wave starting just after the passage of the eclipse is finally brought to light on all stations. Its frequency and amplitude are close to the one calculated with our model, which strongly suggest that this signal was produced by the total solar eclipse

Gravity waves produced by the total solar eclipse of 1 August 2008

International audienceGravity waves are a major component of atmospheric small scale dynamics because of their ability to transport energy and momentum over considerable distances and of their interactions with the mean circulation or other waves. They produce pressure variations which can be detected at the ground by microbarographs. The solar intensity reduction which occurs in the atmosphere during solar eclipses is known to act as a temporary source of large scale gravity waves. Despite decades of research, observational evidence for a characteristic bow-wave response of the atmosphere to eclipse passages remains elusive. A new versatile numerical model (Marty, J. and Dalaudier, F.: Linear spectral numerical model for internal gravity wave propagation. J. Atmos. Sci. (in press)) is presented and applied to the cooling of the atmosphere during a solar eclipse. Calculated solutions appear to be in good agreement with ground pressure fluctuations recorded during the total solar eclipse of 1 August 2008. To the knowledge of the authors, this is the first time that such a result is presented. A three-dimensional linear spectral numerical model is used to propagate internal gravity wave fluctuations in a stably stratified atmosphere. The model is developed to get first-order estimations of gravity wave fluctuations produced by identified sources. It is based on the solutions of the linearized fundamental fluid equations and uses the fully-compressible dispersion relation for inertia-gravity waves. The spectral implementation excludes situations involving spatial variations of buoyancy frequency or background wind. However density stratification variations are taken into account in the calculation of fluctuation amplitudes. In addition to gravity wave packet free propagation, the model handles both impulsive and continuous sources. It can account for spatial and temporal variations of the sources allowing to cover a broad range of physical situations. It is applied to the case of solar eclipses, which are known to produce large-scale bow waves on the Earth's surface. The asymptotic response to a Gaussian thermal forcing travelling at constant velocity as well as the transient response to the 4 December 2002 eclipse are presented. They show good agreement with previous numerical simulations. The model is then applied to the case of the 1 August 2008 solar eclipse. Ground pressure variations produced by the response to the solar intensity reduction in both stratosphere and troposphere are calculated. These synthetic signals are then compared to pressure variations recorded by IMS (International Monitoring System) infrasound stations and a temporary network specifically set up in Western Mongolia for this occasion. The pressure fluctuations produced by the 1 August 2008 solar eclipse are in a frequency band highly disturbed by atmospheric tides. Pressure variations produced by atmospheric tides and synoptic disturbances are thus characterized and removed from the signal. A low frequency wave starting just after the passage of the eclipse is finally brought to light on all stations. Its frequency and amplitude are close to the one calculated with our model, which strongly suggest that this signal was produced by the total solar eclipse

Ground-Penetrating Radar Imaging of Near-Surface Deformation along the Songino Active Fault in the Vicinity of Ulaanbaatar, Mongolia

International audienceThe seismic activity observed in the vicinity of Ulaanbaatar (UB) capital city has been increased since 2005. Several active faults have been identified in the UB area. Most of the Mongolian population is concentrated around UB (1.5 million), which is the main political and economic center of the country. Hence, the study of seismic hazard is of first importance for the country. In this paper, we present the GPR results obtained on the Songino fault which is situated at 20 km west-southwest of UB at the northeast tip of Khustai fault. The combination of the morphotectonic, GPR and paleoseismological investigations brings essential information for seismic hazards assessments. The 2D GPR profiles are measured by using 250 and 500 MHz antennae and the topography using a differential GPS. An appropriate processing of the GPR data, including the topographic migration, allows us to bring out indirect characteristics of these faults. The objective is to identify near-surface geometry and coseismic deformation along the mapped fault. The 250 MHz GPR images of the Songino fault show the evolution of the sub-surface deformation mode induced by the arched geometry of the Songino fault. We observe a clear compressive structure at its NW section, strike slip at its central section and extensive structure in its SE part