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Bhutan-Himalayan district is located along the plate collision zone between Indian and Eurasian plates, which is one of the most seismically active region in the world. However, there is no seismic monitoring system established in the Kingdom of Bhutan, whose territory is in the center of the Bhutan-Himalayan region. In this project, we are establishing the first national permanent seismic monitoring network in the Kingdom of Bhutan. We started establishing permanent seismic monitoring network of minimum requirements which is composed of six (6) observation stations in Bhutan with short period high sensitivity seismometers, strong motion seismometers, and broad-band seismometers. Obtained data are transmitted to the central processing computers in the DGM (Department of Geology and Mines, Ministry of Economic Affairs) office in Thimphu. This seismic network is definitely utilized for not only for seismic disaster mitigation of the country but also for studying the seismotectonics in the Bhutan-Himalayan region which is not precisely revealed due to the lack of observation data in the past.

Recent studies based on surface observations from Sarpang area in southern-central Bhutan have estimated the Holocene slip rate of 20.8+/-8.8 mm/year. This value is based on a mean vertical uplift rate of 8.8+/-2.1 mm/year and assuming a constant frontal thrust dip angle of 25°+/-5° extrapolated from structural measurements. Since geometry of the fault is a key parameter for discerning the slip rate and its associated seismic hazard assessment, we employed near-surface geophysical approach to accurately constrain the Topographic Frontal Thrust (TFT) geometry at shallow depth. Based on proven effectiveness of near-surface geophysical techniques for studying active faults, we adopted gravity, seismic and electrical resistivity tomography.We deployed geophysical profiles at three key sites along the southern frontal areas of the Bhutan Himalayas. The first study area is in Sarpang, a small town located in southern-central Bhutan where we performed all three geophysical methods adopted. The second site is located in Phuentsholing in the south-western Bhutan, where we performed gravity and electrical resistivity survey. The third site is located between Sarpang and Phuentsholing, in the sub-district of Lhamoizingkha under Dagana district.A stochastic inversion approach was adopted to perform analysis of geophysical data collected from the above sites expect for Lhamoizingkha area. Unlike commonly used approaches based on search for the simplest model, the main advantages of this approach include its ability (1) to assess the fault geometry because no smoothing is applied, (2) to provide a measurement of the uncertainties on the obtained dip angle and (3) to allow trade-off analysis between geometric and either electrical resistivity, velocity or density properties.The stochastic inversion results from Sarpang site show a TFT that is characterized by a flat and listric-ramp geometry with a north dipping dip angle of ca 20°-30° at the upper depth of 0-5 m, steeply dipping angle of 70° in the middle 5-40 m depth and flattening with a dip angle of 20° at deeper depths. These new results allow us to estimate a minimum overthrusting slip rate of 10+/-2 mm/year on the TFT, which is about 60% of the far-field GPS convergence rate of ca 17 mm/year. Based on these constraints we propose that, in Sarpang site, significant deformation partitioning on different faults including the TFT, the Main Boundary Thrust (MBT) and the Frontal Back Thrust (FBT) cannot be ruled out. More importantly, assuming a constant slip rate, the dip angle variations constrained from the present study, corresponds to variations in the uplift rate with distance from the TFT. This, therefore, emphasizes the drawbacks in assuming constant dip angle measured from surface observations and uplift rate estimates based on terrace dating only at the front, which may significantly bias the slip rate estimation.Unlike in Sarpang, the TFT corresponds to the Main Frontal Thrust (MFT) in Phuentsholing. At this site a preliminary study suggests a MFT characterized by a flat and listric-ramp geometry. With additional terrace dating information, slip rate for the Phuentsholing area will be performed in a near future. Overall based on the stochastic inversion results, we propose a MFT geometry similar to that observed in Sarpang but with possible lateral variations in terms of deformation partitioning. In Lhamoizingkha area, the exact location of the MFT is not known. Our preliminary results suggest a complex fault trace and indicate that the MFT is located further north of the current resistivity line deployed in this area. Similar to Phuentsholing site (but contrary to Sarpang), we observed that the MFT is the most frontal structure and therefore most of the convergence in the area could be accommodated by the MFT, which is also in agreement with GPS observations.; Des études récentes menées dans la région de Sarpang au sud du centre du Bhoutan estiment un taux de glissement Holocène de 20,8 +/- 8,8 mm/an sur le chevauchement frontal himalayen (TFT). Cette valeur est basée sur un taux de surrection moyen mesuré de 8,8 +/- 2,1 mm/an et en supposant pour ce chevauchement un pendage constant de 25° +/- 5°. La géométrie des failles est un paramètre clé dans l’estimation de la vitesse de glissement et donc dans l’évaluation de l’aléa sismique. Dans le cadre de ce travail, nous avons utilisé une approche géophysique de proche surface afin d’estimer précisément la géométrie de ce chevauchement.Nous avons déployé des profils géophysiques dans trois sites clés le long de la frontière sud du Bhoutan. La première zone d'étude se trouve à Sarpang, une petite ville située au centre du Bhoutan où nous avons effectué des mesures gravimétriques, sismiques et électriques. Le deuxième site est situé à Phuentsholing dans le sud-ouest du Bhoutan, où nous avons effectué des mesures gravimétriques et de résistivité électrique. Le troisième site est situé entre Sarpang et Phuentsholing, à Lhamoizingkha dans le district de Dagana.Excepté pour la région de Lhamoizingkha, une approche d'inversion stochastique a été adoptée pour analyser des données géophysiques collectées. Contrairement aux approches couramment utilisées basées sur la recherche du modèle le plus simple, les principaux avantages de cette approche sont sa capacité (1) à mieux estimer la géométrie des zones de discontinuité car aucun lissage n'est appliqué, (2) à fournir une mesure des incertitudes sur le pendage obtenu et (3) à permettre une analyse des relations possibles entre les propriétés géométriques et celles du milieu (résistivité électrique, vitesse ou densité).Les résultats d'inversion stochastique du site de Sarpang montrent un TFT qui se caractérise par une géométrie en plat-rampe-plat avec un pendage vers le nord d'environ 20°-30° dans la partie la plus superficielle (profondeur < 5 m), un pendage fort de 70° entre 5 m et 40 m de profondeur et un l'aplatissement avec un pendage de 20° au-delà de 40 m. Ces nouveaux résultats nous permettent d'estimer un taux minimal de glissement de 10 +/- 2 mm/an sur le TFT, soit environ 60% des 17 mm/an associés au taux de convergence GPS moyen obtenu en champ lointain. Sur la base de ces contraintes, il apparait donc qu’on ne puisse pas exclure la possibilité que la déformation soit distribuée sur plusieurs failles, comprenant le TFT, mais également d’autres chevauchements comme le MBT (au nord) ou le FBT (au sud). De plus, en supposant un taux de glissement constant, les variations de pendage obtenues induisent des variations du taux de surrection en fonction de la distance au TFT. Cela souligne les faiblesses des hypothèses couramment faites pour estimer les taux de glissement Holocène sur les failles sismogènes : (1) pendage constant estimé uniquement à partir des observations de surface et (2) estimations du taux de surrection en supposant une surrection identique pour une terrasse fluviale donnée.Contrairement à Sarpang, à Phuentsholing le TFT correspond au chevauchement frontal himalayen (MFT). Sur ce site, l’étude préliminaire que nous avons menée suggère un MFT ayant une géométrie de faille listrique. Des mesures de datations doivent maintenant être effectuées pour estimer le taux de glissement sur le MFT dans cette zone. Dans la région de Lhamoizingkha, l'emplacement exact du MFT n'est pas connu. Nos résultats préliminaires suggèrent une géométrie complexe de la trace de la faille en surface et indiquent que le MFT est situé plus au nord de la ligne de résistivité déployée dans cette zone. À l'instar du site de Phuentsholing (mais contrairement à Sarpang), nous avons observé que le MFT était la structure la plus frontale et que l’essentiel de la convergence dans cette zone pouvait être accommodé par le MFT, comme semble le suggérer les observations GPS.

International audience; The seismic behavior of the Himalayan arc between central Nepal and Arunachal Pradesh remains poorly understood due to the lack of observations concerning the timing and size of past major and great earthquakes in Bhutan. We present here the first paleoseismic study along the Himalayan topographic front conducted at two sites in southern central Bhutan. Paleoseismological excavations and related OxCal modeling reveal that Bhutan experienced at least two great earthquakes in the last millennium: one between the seventeenth and eighteenth century and one during medieval times, producing a total cumulative vertical offset greater than 10 m. Along with previous studies that reported similar medieval events in Central Nepal, Sikkim, and Assam, our investigations support the occurrence of either (i) a series of great earthquakes between A.D. 1025 and A.D. 1520 or (ii) a single giant earthquake between A.D. 1090 and A.D. 1145. In the latter case, the surface rupture may have reached a total length of ~800 km and could be associated with an earthquake of magnitude Mw = 8.7–9.1.

Lateral variations along the Himalayan arc are suggested by an increasing number of studies and carry important information about the orogen’s segmentation. Here we compile the hitherto most complete land gravity dataset in the region which enables the currently highest resolution plausible analysis. To study lateral variations in collisional structure we compute arc-parallel gravity anomalies (APaGA) by subtracting the average arc-perpendicular profile from our dataset; we compute likewise for topography (APaTA). We find no direct correlation between APaGA, APaTA and background seismicity, as suggested in oceanic subduction context. In the Himalayas APaTA mainly reflect relief and erosional effects, whereas APaGA reflect the deep structure of the orogen with clear lateral boundaries. Four segments are outlined and have disparate flexural geometry: NE India, Bhutan, Nepal & India until Dehradun, and NW India. The segment boundaries in the India plate are related to inherited structures, and the boundaries of the Shillong block are highlighted by seismic activity. We find that large earthquakes of the past millennium do not propagate across the segment boundaries defined by APaGA, therefore these seem to set limits for potential rupture of megathrust earthquakes., Scientific Reports, 6, ISSN:2045-2322

Bhutan-Himalayan district is located along the plate collision zone between Indian and Eurasian plates, which is one of the most seismically active region in the world. However, there is no seismic monitoring system established in the Kingdom of Bhutan, whose territory is in the center of the Bhutan-Himalayan region. In this project, we are establishing the first national permanent seismic monitoring network in the Kingdom of Bhutan. We started establishing permanent seismic monitoring network of minimum requirements which is composed of six (6) observation stations in Bhutan with short period high sensitivity seismometers, strong motion seismometers, and broad-band seismometers. Obtained data are transmitted to the central processing computers in the DGM (Department of Geology and Mines, Ministry of Economic Affairs) office in Thimphu. This seismic network is definitely utilized for not only for seismic disaster mitigation of the country but also for studying the seismotectonics in the Bhutan-Himalayan region which is not precisely revealed due to the lack of observation data in the past.

International audience; We investigate flexural geometry and rheology of the India plate beneath the eastern Himalaya from a new gravity data set acquired in Bhutan. Compared to the well‒studied Nepal Himalaya, the obtained Bouguer anomaly profiles across the range show shorter wavelength flexure of the lithosphere with a narrower and shallower foreland basin. This new data set and seismic Moho constraints are interpreted together in terms of lithospheric flexure using a 2‒D thermomechanical model. Our results suggest that the strongest layer of the continental lithosphere beneath Bhutan is the upper mantle, as it is beneath Nepal. The observed west‒to‒east decrease in flexural wavelength is associated with weakening mantle rheology. The simulations show that this decrease can be related to ductile mantle behavior: either hydrated wet dunite or dry and hot olivine rheology. Both models display decoupled lithospheric layers leading to an eastward decrease of flexural rigidity from ∼1024 to ∼5*1022 N m in Nepal and Bhutan, respectively.