19 research outputs found
Joint approach combining damage and paleoseismology observations constrains the 1714 A.D. Bhutan earthquake at magnitude 8±0.5
International audienceThe 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 Here we present historical documents from the region reporting on an earthquake in 1714âA.D. and geological evidence of surface rupture to constrain the latest large event in this area.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 1714âA.D. earthquake to have ruptured the megathrust in Bhutan, most likely during a M7.5â8.5 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
DĂ©formation HolocĂšne de lâHimalaya du Bhoutan. Apport de la gĂ©omorphologie et de la palĂ©osismologie.
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 ( 7.5) ont affectĂ© lâHimalaya et le lourd bilan humain du sĂ©isme de Gorkha en 2015 indique un aspect sociĂ©tal important. Cependant, la taille maximale et la probabilitĂ© dâoccurrence de ces mĂ©ga-sĂ©ismes sont toujours matiĂšre Ă dĂ©bat. Or, bien quâĂ©tant une zone charniĂšre le long de lâarc, le royaume du Bhoutan reste pas ou peu documentĂ©. Longtemps considĂ©rĂ© comme une zone asismique de 350 km de long, un seul Ă©crit historique rapporte lâoccurrence dâun sĂ©isme dĂ©vastateur en 1713.Lâobjectif de ce travail de thĂšse est de quantifier la dĂ©formation, Ă diffĂ©rentes Ă©chelles spatiale et temporelle, de lâHimalaya du Bhoutan Ă travers des Ă©tudes morphotectonique et palĂ©osismologique. Un premier volet vise Ă quantifier les mouvements verticaux le long dâun profil N-S allant de la structure la plus frontale (le Main Frontal Thrust) Ă la haute chaĂźne. Trois campagnes de terrain ont Ă©tĂ© rĂ©alisĂ©es permettant lâĂ©chantillonnage de (1) de terrasses alluviales le long du front permettant de quantifier et dâĂ©tudier les variations du soulĂšvement frontal HolocĂšne, (2) de bassins versants pour lâĂ©tude de la dĂ©nudation court-terme (< 20 ka) dĂ©rivĂ©e des cosmonuclĂ©ides (10Be) et (3) de terrasses alluviales dans le Moyen-Pays pour quantifier la dĂ©nudation Ă une Ă©chelle de temps un peu plus longue (< 50 ka).Le second volet de cette Ă©tude porte sur diffĂ©rentes Ă©tudes palĂ©osismologiques le long du front ouest et centre Bhoutanais. Trois campagnes de terrain ont permis dâinvestiguer six sites diffĂ©rents. La datation et la modĂ©lisation de charbons dĂ©tritiques a permis de mettre en Ă©vidence lâoccurrence de cinq sĂ©ismes majeurs durant les derniers 2700 ans. Ces rĂ©sultats montrent que le Bhoutan ne peut pas ĂȘtre considĂ©rĂ© comme une zone asismique. A une Ă©chelle rĂ©gionale, lâoccurrence de sĂ©ismes majeurs au Bhoutan participe au dĂ©bat dâun sĂ©isme de magnitude 9 le long de lâarc Himalayen
Holocene deformation in the Bhutan Himalaya from geomorphic and paleoseismologic study
La chaĂźne Himalayenne est l'un des exemples les plus spectaculaires de dĂ©formation active Ă la surface de notre planĂšte. Au cours des derniers siĂšcles, de nombreux sĂ©ismes majeurs (Mw â» 7.5) ont affectĂ© cette zone et le lourd bilan humain du sĂ©isme de Gorkha en 2015 a une nouvelle fois rappelĂ© lâimportance de parvenir Ă mieux estimer lâalĂ©a sismique de cette rĂ©gion. Bien que les structures lithologiques et tectoniques semblent montrer au premier ordre une cylindricitĂ© le long des 2500 km de l'arc Himalayen, de nombreuses Ă©tudes, menĂ©es principalement au NĂ©pal, ont permis de mettre en Ă©vidence des variations latĂ©rales structurales, thermochronologiques, morphologiques, gravimĂ©triques, sismologiques ou gĂ©odĂ©siques. Le rĂŽle de ces variations latĂ©rales sur la segmentation sismique reste cependant mal contraint. La taille maximale et la probabilitĂ© dâoccurrence de ces mĂ©ga-sĂ©ismes sont donc toujours matiĂšre Ă dĂ©bat.Ainsi, le comportement sismique de lâHimalaya du Bhoutan reste Ă©nigmatique. Pour certains les faibles taux de sismicitĂ© observĂ©s actuellement font de ce royaume une zone asismique de 350 km de long. Pour dâautres, Ă lâinstar de lâHimalaya du NĂ©pal, la faible sismicitĂ© observĂ©e est associĂ© Ă une forte accumulation de contrainte susceptible de gĂ©nĂ©rer des sĂ©ismes majeurs. L'objectif de ce travail de thĂšse est dâamĂ©liorer notre connaissance de cette rĂ©gion en quantifiant la dĂ©formation Ă diffĂ©rentes Ă©chelles spatiales et temporelles via des Ă©tudes morphotectoniques et palĂ©osismologiques.La premiĂšre partie de cette thĂšse vise Ă quantifier les mouvements verticaux Ă lâouest Bhutan et le long du Main Frontal Thrust, structure la plus frontale situĂ©e au sud Bhoutan. Trois campagnes de terrain ont Ă©tĂ© rĂ©alisĂ©es permettant l'Ă©chantillonnage (1) de terrasses alluviales le long du front afin de quantifier et d'Ă©tudier les variations du soulĂšvement HolocĂšne, (2) de bassins versants pour l'Ă©tude de la dĂ©nudation court-terme (âș 20 ka) dĂ©rivĂ©e des cosmonuclĂ©ides (10Be) et (3) de terrasses alluviales dans le Moyen-Pays pour quantifier lâincision HolocĂšne. Les vitesses verticales obtenues au front sont comparables Ă celle proposĂ©es le long du reste de lâarc himalayen, suggĂ©rant une cinĂ©matique relativement simple. Par contre, nos rĂ©sultats indiquent une variation de la gĂ©omĂ©trie du chevauchement himalayen (Main Himalayan Thrust) entre lâest NĂ©pal et lâouest Bhoutan. La seconde partie porte sur plusieurs Ă©tudes palĂ©osismologiques le long du front ouest et centre Bhoutanais. Six sites diffĂ©rents ont Ă©tĂ© Ă©tudiĂ©s au cours de trois campagnes de terrain. La datation et la modĂ©lisation de charbons dĂ©tritiques a permis de mettre en Ă©vidence l'occurrence dâau moins cinq sĂ©ismes majeurs durant les derniers 2700 ans, faisant du Bhoutan une zone sismiquement aussi active que le NĂ©pal. A une Ă©chelle rĂ©gionale, cette Ă©tude apporte donc de nouvelles contraintes et contribue au dĂ©bat sur la possibilitĂ© dâoccurrence d'un sĂ©isme de magnitude 9 le long de l'arc Himalayen.The Himalayan arc is one of the most active intra-continental mountain belts in the world. Over the last centuries, several major earthquakes (Mw â» 7.5) have struck this arc. The dramatic effects of the Gorkha earthquake sequence in 2015 pointed once again the crucial need to improve seismic hazard assessment of this area. 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. The relationship between these variations and earthquakes segmentation along the arc remains poorly constraint. The maximum size and the occurrence probability of such earthquakes are still a matter of debate.For instance, the seismic behavior of Bhutan remains enigmatic. The present-day low seismicity rate observed in this area can reflect two opposite fault behaviors: an aseismic creeping zone or a zone of stress accumulation for future great earthquakes as the others parts of Himalayas. The main objective of this thesis is to bring new constraints on the deformation of the Bhutan Himalayas, at different space- and time-scales, through morphotectonic and paleoseismological approaches.The first part of this thesis focuses on the vertical deformation assessment along a N-S transect in western Bhutan and along the Main Frontal Thrust, which is the southern-most thrust in southern Bhutan. In the past three years, we have carried out three fieldwork campaigns to sample (1) frontal terraces to assess Holocene uplift rates, (2) watershed basins to quantify short term denudation rates (âș 20 ky) derived from cosmonuclides 10Be and (3) hinterland alluvial terraces to quantify the Holocene incision rate. Frontal Holocene uplift rates obtained in Bhutan are consistent with those obtained in the others parts of Himalayas. Furthermore, our results reveal a variation in the geometry of the Main Himalayan Thrust between eastern Nepal and western Bhutan. The second part focuses on several paleoseismic studies along the west and central bhutanese Himalayan front. Different sites were investigated during three fieldwork campaigns. Detritic charcoals sampling and modeling suggest the occurrence 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 occurrence of a magnitude 9 earthquake along the Himalayan arc
A 2600-year-long paleoseismic record for the Himalayan Main Frontal Thrust (western Bhutan)
International audienc
First paleoseismic evidence for great surface-rupturing earthquakes in the Bhutan Himalayas
International audienceThe 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 10âm. 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 ~800âkm and could be associated with an earthquake of magnitude Mwâ=â8.7â9.1
Quelle place pour lâimagerie sismique dans la caractĂ©risation des failles en domaine intraplaque ?
LocalisĂ©s Ă distance des grandes limites de plaques lithosphĂ©riques, les domaines intraplaques prĂ©sentent de faibles vitesses de dĂ©formation gĂ©ologique, se traduisant par des niveaux de sismicitĂ© faibles Ă modĂ©rĂ©s, avec des temps de retour longs entre deux sĂ©ismes significatifs. Dans ces rĂ©gions, les processus de surface sont plus rapides que les processus tectoniques et tendent Ă effacer les Ă©ventuelles traces de rupture laissĂ©es en surface par des sĂ©ismes majeurs du passĂ©. Lâestimation de lâactivitĂ© des failles dans de tels contextes nĂ©cessite la mise en Ćuvre dâune approche pluridisciplinaire combinant gĂ©ophysique, gĂ©ologie et palĂ©osismologie. Nous prĂ©sentons ici lâapproche dĂ©veloppĂ©e par EDF, dans le cadre de ses Ă©tudes dâalĂ©a sismique, pour amĂ©liorer la connaissance des failles en France mĂ©tropolitaine, en se focalisant sur lâutilisation de la sismique rĂ©flexion (imagerie) Ă diffĂ©rentes Ă©chelles. Les lignes sismiques de Haute RĂ©solution (HR ; i.e. sismique profonde de type pĂ©troliĂšre) permettent dâestimer la localisation et la gĂ©omĂ©trie des failles en profondeur, ainsi que leur histoire cinĂ©matique. La sismique ultra haute rĂ©solution en ondes S (UHRS) donne quant Ă elle une information sur la localisation des failles en trĂšs proche surface et permet de localiser dâĂ©ventuelles tranchĂ©es palĂ©osismologiques visant Ă analyser lâactivitĂ© des failles
Evidence of interseismic coupling variations along the Bhutan Himalayan arc from new GPS data
Although the first-order pattern of present-day deformation is relatively well resolved across the Himalayas, irregular data coverage limits detailed analyses of spatial variations of interseismic coupling. We provide the first GPS velocity field for the Bhutan Himalaya. Combined with published data, these observations show strong east-west variations in coupling between central and eastern Bhutan. In contrast with previous estimations of first-order uniform interseismic coupling along the Himalayan arc, we identify significant lateral variations: In western and central Bhutan, the fully coupled segment is 135-155km wide with an abrupt downdip transition, whereas in eastern Bhutan the fully coupled segment is 100-120km wide and is limited updip and downdip by partially creeping segments. This is the first observation of decoupling on the upper ramp along the Himalayan arc, with important implications for large earthquake surface rupture and seismic hazard
Joint approach combining damage and paleoseismology observations constrains the 1714âA.D. Bhutan earthquake at magnitude 8â±â0.5
Paleoseismological investigations of the La RouviĂšre fault, unexpected source of the 11-11-2019, Mw 4.9 Le Teil surface rupturing earthquake (CĂ©vennes fault system, France)
International audienceThe 11-11-2019 Le Teil earthquake (Mw4.9), located in the RhĂŽne river valley occurred along the La RouviĂšre fault (LRF) within the NE termination of the CĂ©vennes 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