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International audience; The 11-11-2019 Le Teil earthquake (Mw4.9), located in the Rhône river valley occurred along the La Rouvière fault (LRF) within the NE termination of the Cévennes faults system (CFS). This very shallow moderate magnitude and reverse-faulting event inverted an Oligocene normal fault which was not assessed to be potentially active, causing surface rupture and strong ground shaking. Its morphology shows no evidence of cumulative reverse faulting during the Quaternary. These observations lead to the question whether the fault was reactivated for the first time since the Oligocene during the Teil earthquake, or if it had broken the surface before, during the Quaternary period, but could not be detected in the morphology.

The Himalayan arc is one of the most of active intra-continental orogen in the world. Geological explorations of the Himalayas since the late 19th century have emphasized a 2500-km-long roughly cylindrical structure, with striking continuity of main units and thrust faults. However recent geophysical and geological investigations have revealed lateral variations (present-day convergence rates, gravimetry, seismicity, morphology, exhumation rates, and crustal structures).Over the las centuries, several major earthquakes (Mw > 7.5) have struck the arc and the dramatic effects of the Gorkha earthquake sequence in 2015 point to the important societal dimension. However, both the maximum size and the occurence probability of such earthquakes are still debated. While the kingdom of Bhutan is a key area in the understanding of the whole Himaayan arc, the country is still poorly documented. Long considered as a 350-km-long seismic gap, a single historical account reports a devastating earthquake in AD 1713.The first part of this thesis focuses on the quantification of vertical deformation along a N-S transect, from the southern-most thrust (The Main Frontal Thrust) to the High Himalaya. In the past three years we have carried out three fieldwork campains during which we sampled different deformation features such as (1) alluvial terraces along the front in order to quantify and the spatial and temporal variation of the Holocene uplift, (2) watershed basins in order to quantify the short term denudation rate (< 20 ky) derived from cosmonuclides 10Be and (3) alluvial terraces hinterland within the Higher Himalaya to quantify the incision rate at longer time scale (< 50 ky).The second part focuses on the several paleoseismic study along the west and central bhutanese front. Six sites were investigated during three fieldwork campains. Detritic charcoals sampling and modelisation have lead to highlight theoccurence of at least five surface-rupturing earthquakes during the last 2700 years. These results demonstrate that the present-day low seismicity rate observed in Bhutan is not representative of the seismic activity at longer time scale. At regional scale, they also take part of a broader discussion on the probability of occurence of a magnitude 9 earthquake along the Himalayan arc.; La chaîne Himalayenne est l’un des exemples les plus spectaculaires de déformation active à la surface de notre planète. Bien que les structures lithologiques et tectoniques semblent montrer une cylindricité le long des 2500 km de l’arc Himalayen, de nombreuses études menées principalement au Népal ont permis de mettre en évidence des variations latérales (GPS, gravimétrie, sismicité, morphologie, taux d’exhumation ou structure profonde).Au cours des derniers siècles, de nombreux séismes majeurs (Mw > 7.5) ont affecté l’Himalaya et le lourd bilan humain du séisme de Gorkha en 2015 indique un aspect sociétal important. Cependant, la taille maximale et la probabilité d’occurrence de ces méga-séismes sont toujours matière à débat. Or, bien qu’étant une zone charnière le long de l’arc, le royaume du Bhoutan reste pas ou peu documenté. Longtemps considéré comme une zone asismique de 350 km de long, un seul écrit historique rapporte l’occurrence d’un séisme dévastateur en 1713.L’objectif de ce travail de thèse est de quantifier la déformation, à différentes échelles spatiale et temporelle, de l’Himalaya du Bhoutan à travers des études morphotectonique et paléosismologique. Un premier volet vise à quantifier les mouvements verticaux le long d’un profil N-S allant de la structure la plus frontale (le Main Frontal Thrust) à la haute chaîne. Trois campagnes de terrain ont été réalisées permettant l’échantillonnage de (1) de terrasses alluviales le long du front permettant de quantifier et d’étudier les variations du soulèvement frontal Holocène, (2) de bassins versants pour l’étude de la dénudation court-terme (< 20 ka) dérivée des cosmonucléides (10Be) et (3) de terrasses alluviales dans le Moyen-Pays pour quantifier la dénudation à une échelle de temps un peu plus longue (< 50 ka).Le second volet de cette étude porte sur différentes études paléosismologiques le long du front ouest et centre Bhoutanais. Trois campagnes de terrain ont permis d’investiguer six sites différents. La datation et la modélisation de charbons détritiques a permis de mettre en évidence l’occurrence de cinq séismes majeurs durant les derniers 2700 ans. Ces résultats montrent que le Bhoutan ne peut pas être considéré comme une zone asismique. A une échelle régionale, l’occurrence de séismes majeurs au Bhoutan participe au débat d’un séisme de magnitude 9 le long de l’arc Himalayen.

International audience; The region of Bhutan is thought to be the only segment of the Himalayas not having experienced a major earthquake over the past half millennium. A proposed explanation for this apparent seismic gap is partial accommodation of the India-Asia convergence further south across the Shillong Plateau, yet the seismic behavior of the Himalayan megathrust in Bhutan is unknown. Here we present historical documents from the region reporting on an earthquake in 1714 A.D. and geological evidence of surface rupture to constrain the latest large event in this area. We compute various earthquake scenarios using empirical scaling relationships relating magnitude with intensity, source location and rupture geometry. Our results constrain the 1714 A.D. earthquake to have ruptured the megathrust in Bhutan, most likely during a M7.5–8.5 event. This finding reclassifies the apparent seismic gap to a former information gap and implies that the entire Himalayan arc has a high level of earthquake potential.

The region of Bhutan is thought to be the only segment of the Himalayas not having experienced a major earthquake over the past half millennium. A proposed explanation for this apparent seismic gap is partial accommodation of the India‐Asia convergence further south across the Shillong Plateau, yet the seismic behavior of the Himalayan megathrust in Bhutan is unknown. Here we present historical documents from the region reporting on an earthquake in 1714 A.D. and geological evidence of surface rupture to constrain the latest large event in this area. We compute various earthquake scenarios using empirical scaling relationships relating magnitude with intensity, source location and rupture geometry. Our results constrain the 1714 A.D. earthquake to have ruptured the megathrust in Bhutan, most likely during a M7.5–8.5 event. This finding reclassifies the apparent seismic gap to a former information gap and implies that the entire Himalayan arc has a high level of earthquake potential., Geophysical Research Letters, 43 (20), ISSN:0094-8276, ISSN:1944-8007

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.

International audience; The Main Himalayan Thrust (MHT) is the source of great earthquakes that have been documented along the range. Its geometry is a key parameter that influences accommodation of tectonic loading and earthquake magnitudes along the Himalayan Arc. Although seismic images are available for both the western and the central part of the range, this geometry remains poorly constrained for the Bhutanese Himalayas. Here we address this issue using a 10Be cosmogenic nuclides denudation transect across western Bhutan. We observe a wide low denudation rate domain between 50 km and 110 km from the front followed by a strong northward increase. Using a joint inversion of denudation rates, GPS data, and Holocene uplift rates, we interpret this pattern as a consequence of a flat-ramp transition along the MHT. Compared to central Nepal and Sikkim, this location of the ramp suggests a wider décollement, with implications for greater seismogenic potential of the MHT in western Bhutan.