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Girault F.,CNRS Paris Institute of Global Physics | Perrier F.,CNRS Paris Institute of Global Physics | Crockett R.,University of Northampton | Bhattarai M.,National Seismological Center | And 7 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2014

The Syabru-Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30-62°C) water springs and cold (<35°C) carbon dioxide (CO2) degassing areas. From 2007 to 2011, five gas zones (GZ1-GZ5) were studied, with more than 1600 CO2 and 850 radon flux measurements, with complementary self-potential data, thermal infrared imaging, and effective radium concentration of soils. Measurement uncertainties were evaluated in the field. CO2 and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m-2 d-1 and 38.5 ± 8.0 Bq m-2 s-1, with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t d-1 and 140 ± 30 MBq d-1, respectively. Soil-gas radon concentration is 40 × 103 Bq m-3 in GZ1-GZ2 and 70 × 103 Bq m-3 in GZ3-GZ4. Strong relationships between CO2 and radon fluxes in all gas zones (correlation coefficient R = 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO 2. CO2 carbon isotopic ratios (δ13C from -1.7 ± 0.1 to -0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/RA < 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO2 discharge located in a seismically active region, where we can test methodological issues and our understanding of transport properties and fluid circulations in the subsurface. ©2014. American Geophysical Union. All Rights Reserved.

Hossler T.,CEA DAM Ile-de-France | Hossler T.,National Seismological Center | Bollinger L.,CEA DAM Ile-de-France | Sapkota S.N.,National Seismological Center | And 3 more authors.
Earth and Planetary Science Letters | Year: 2016

The chronology of the seismic ruptures along the active faults of Western Nepal remains almost unconstrained despite their high seismogenic potential. We present here a slip history of one of these structures, a 120 km-long reactivated segment of the Main Boundary Thrust named the Surkhet-Gorahi fault. This slip history is based on geomorphologic and neotectonic mapping of active faults deduced from the analysis of a high resolution total station digital elevation model and 15 detrital charcoals radiocarbon ages constraining the age of deposition or abandonment of 4 alluvial terraces of the Bheri river in Botechaur. Our results show that the last two earthquakes occurred on this fault after 1860 and 640 BP, respectively, and accommodated slip greater than 8 m each, a value corresponding to the incremental vertical offset of the terraces. Such events released a significant part of the slip deficit accumulated on the Main Himalayan thrust fault. However, given the geometry of this fault system as well as the date of occurrence of the last events, the ruptures could be associated with major earthquakes also rupturing the Main Frontal Thrust, such as the great 1505 earthquake. © 2015 Elsevier B.V.

Berthet T.,Montpellier University | Hetenyi G.,ETH Zurich | Cattin R.,Montpellier University | Sapkota S.N.,National Seismological Center | And 6 more authors.
Geophysical Journal International | Year: 2013

The current understanding of the Himalayan lithosphere stems mostly from cross-sections through the range at the longitude of the Kathmandu Basin. In this paper we laterally extend the analyses of structures and rheology along the Nepal Himalayas between the Pokhara valley and the Arun river.We take advantage of available information and a new data set including gravity measurements and a receiver function profile. It appears that the geometry of theMoho inferred from seismological profiles and long-wavelength gravity anomalies does not exhibit major East-West variations within the 350-km-wide study area. Using thermomechanical modelling, we show that the northward deepening of the Moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong India Plate. This result suggests a gradual mechanical decoupling between the crust and the mantle, leading to a northward decrease of the effective elastic thickness of the Indian lithosphere from ~75 km to ~25 km beneath the Ganga Basin and the Tibetan Plateau, respectively. Our results also confirm(partially) eclogitized lower Indian crust beneath southern Tibet. At shorter wavelengths, the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east-west variations of the thickness of subsurface geological structures such as the Ganga Basin and the Tethyan Sedimentary Sequence. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Bollinger L.,CEA DAM Ile-de-France | Sapkota S.N.,National Seismological Center | Tapponnier P.,Nanyang Technological University | Klinger Y.,CNRS Paris Institute of Global Physics | And 6 more authors.
Journal of Geophysical Research B: Solid Earth | Year: 2014

The return times of large Himalayan earthquakes are poorly constrained. Despite historical devastation of cities along the mountain range, definitive links between events and specific segments of the Main Frontal Thrust (MFT) are not established, and paleoseismological records have not documented the occurrence of several similar events at the same location. In east central Nepal, however, recently discovered primary surface ruptures of that megathrust in the A.D. 1255 and 1934 earthquakes are associated with flights of tectonically uplifted terraces. We present here a refined, longer slip history of the MFT's two overlapping strands (Patu and Bardibas Thrusts) in that region, based on updated geomorphic/neotectonic mapping of active faulting, two 1.3 km long shallow seismic profiles, and logging of two river-cut cliffs, three paleoseismological trenches, and several pits, with constraints from 74 detrital charcoals and 14 cosmogenic nuclide ages. The amount of hanging wall uplift on the Patu thrust since 3650 ± 450 years requires three more events than the two aforementioned. The uplift rate (8.5 ± 1.5 mm/yr), thrust dip (25° ± 5°N), and apparent characteristic behavior imply 12-17.5 m of slip per event. On the Bardibas thrust, discrete pulses of colluvial deposition resulting from the coseismic growth of a flexural fold scarp suggest the occurrence of six or seven paleo-earthquakes in the last 4500 ± 50 years. The coeval rupture of both strands during great Himalayan earthquakes implies that in eastern Nepal, the late Holocene return times of such earthquakes probably ranged between 750 ± 140 and 870 ± 350 years. © 2014. American Geophysical Union. All Rights Reserved.

Ader T.,California Institute of Technology | Ader T.,Ecole Normale Superieure de Paris | Avouac J.-P.,California Institute of Technology | Liu-Zeng J.,CAS Institute of Tibetan Plateau Research | And 11 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = -1.34°±3.31°, latitude = 51.4°±0.3° with an angular velocity of Ω =0.5029±0.0072°/Myr. The pattern of coupling on the MHT is computed on a fault dipping 10° to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100km down dip, corresponding to a depth of 15 to 20km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350°C. The convergence between India and South Tibet proceeds at a rate of 17.8±0.5 mm/yr in central and eastern Nepal and 20.5±1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6±0.4×10 19 Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500years, including 14 M W≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9×1019 Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT. Copyright 2012 by the American Geophysical Union.

Grandin R.,CNRS ENS Geology Laboratory | Doin M.-P.,CNRS ENS Geology Laboratory | Bollinger L.,CEA DAM Ile-de-France | Pinel-Puyssegur B.,CEA DAM Ile-de-France | And 3 more authors.
Geology | Year: 2012

The rise and support of the ~5000 m topographic scarp at the front of Indian-Eurasian collision in the Himalaya involves long-term uplift above a mid-crustal ramp within the Main Himalayan Thrust (MHT) system. Locking of the shallower portion of the fl at-ramp-fl at during the interseismic period also produces transient uplift above the transition zone. However, spatial and temporal relationships between permanent and transient vertical deformation in the Himalaya are poorly constrained, leading to an unresolved causal relationship between the two. Here, we use interferometric synthetic aperture radar (InSAR) to measure interseismic uplift on a transect crossing the whole Himalaya in central Nepal. The uplift velocity of 7 mm/yr at the front of the Annapurna mountain range is explained by an 18-21 mm/yr slip rate on the deep shallow-dipping portion of the MHT, with full locking of the mid-crustal ramp underlying the High Himalaya. The transient uplift peak observed by InSAR matches spatially with the long-term uplift peak deduced from the study of trans-Himalayan river incision, although models of the seismic cycle involving thrusting over a ramp of fi xed geometry predict an ~20 km separation between the two peaks. We argue that this coincidence indicates that today's mid-crustal ramp in central Nepal is located southward with respect to its average long-term location, suggesting that mountain growth proceeds by frontward migration of the ramp driven by underplating of material from the Indian plate under the Himalaya. © 2012 Geological Society of America.

Girault F.,University Paris Diderot | Perrier F.,University Paris Diderot | Gajurel A.P.,Tribhuvan University | Bhattarai M.,National Seismological Center | And 4 more authors.
Geochimica et Cosmochimica Acta | Year: 2012

Effective radium concentration (ECRa) of 622 rock samples from 6 different sites in the Nepal Himalayas was measured in the laboratory using radon accumulation experiments. These sites, located from Lower Dolpo in Western Nepal to Eastern Nepal, are divided into 9 transects which cut across the Main Central Thrust zone (MCT zone) separating low-grade metamorphic Lesser Himalayan Sequence (LHS) units to the south and higher-grade metamorphic Greater Himalayan Sequence (GHS) units to the north. This boundary remains difficult to define and is the subject of numerous debates. ECRa values range from 0.03±0.03 to 251.6±4.0Bqkg-1, and appear to be representative of the formation and clearly related to the local lithology. For example, for the Upper Trisuli and Langtang Valleys site in Central Nepal, the most studied place with 350 available ECRa values, LHS rocks are characterized by a mean value of 5.3±1.3Bqkg-1 while GHS rocks of Formations I and II show significantly lower values with a mean value of 0.69±0.11Bqkg-1, thus leading to a LHS/GHS ECRa ratio of 7.8±2.2. This behavior was systematically confirmed by other transects (ratio of 7.9±2.2 in all other sites), with a threshold ECRa value, separating LHS from GHS, of 0.8Bqkg-1, thus bringing forward a novel method to characterize, within the MCT shear zone, which rocks belong to the GHS and LHS units. In addition, Ulleri augen gneiss, belonging to LHS rocks, occurred in several transects and were characterized by high ECRa values (17.9±4.3Bqkg-1), easy to distinguish from the GHS gneisses, characterized by low ECRa values at the bottom of the GHS, thus providing a further argument to locate the MCT. The measurement of ECRa data, thus, provides a cost-effective method which can be compared with neodymium isotopic anomalies or estimates of the peak metamorphic temperature. This study, therefore, shows that the measurement of ECRa provides additional information to discriminate different geological formations, and can be particularly useful in areas where geology mapping is not straightforward or still remains controversial. © 2012 Elsevier Ltd.

Girault F.,CNRS Paris Institute of Global Physics | Poitou C.,CNRS Paris Institute of Global Physics | Perrier F.,CNRS Paris Institute of Global Physics | Koirala B.P.,National Seismological Center | Bhattarai M.,National Seismological Center
Natural Hazards and Earth System Sciences | Year: 2011

Low-field magnetic susceptibility χm and effective radium concentration ECRa, obtained from radon emanation, have been measured in the laboratory with 129 soil samples from Nepal. Samples along horizontal profiles in slope debris or terrace scarps showed rather homogeneous values of both χm and ECRa. One sample set, collected vertically on a lateritic terrace scarp, had homogeneous values of ECRa while χm increased by a factor of 1 to 10 for residual soils and topsoils. However, for a set of samples collected on three imbricated river terraces, values of ECRa, homogeneous over a given terrace, displayed a gradual increase from younger to older terraces. By contrast, χm showed more homogeneous mean values over the three terraces, with a larger dispersion, however, for the younger one. Similarly, Kathmandu sediments exhibited a large increase in ECRa from sand to clay layers, while χm increased moderately. The combination of χm and ECRa, thus, provides a novel tool to characterize quantitatively various soil groups and may be of interest to distinguish modes of alteration or deposition histories. © 2012 Author(s).

Bhattarai M.,National Seismological Center | Adhikari L.B.,National Seismological Center | Gautam U.P.,National Seismological Center | Laurendeau A.,French Atomic Energy Commission | And 6 more authors.
Seismological Research Letters | Year: 2015

Central Nepal was struck on 25 April 2015 by the Mw 7.8 (ML 7.6) Gorkha earthquake, which initiated about 80 km northwest of Kathmandu and ruptured toward the east along a 140-km-long west-northwest-east-southeast fault segment. Kathmandu basin, located about halfway along the ruptured segment a few kilometers from the southernmost fault tip, was strongly affected. In this article, we present a preliminary analysis of the acceleration-time histories of the 25 April 2015 Mw 7.8 mainshock and the 12 May 2015 Mw 7.3 (ML 6.8) aftershock recorded at the Department of Mines and Geology office building in central Kathmandu valley. We analyze their frequency content using Stockwell et al. (1996) time-frequency decomposition and a polarization analysis (Pinnegar, 2006). We then compute their strong-motion parameters and finally compare their spectral accelerations with the Boore and Atkinson (2008) ground-motion prediction equation.

Richon P.,CEA DAM Ile-de-France | Richon P.,CNRS Paris Institute of Global Physics | Richon P.,University Paris Diderot | Perrier F.,CNRS Paris Institute of Global Physics | And 6 more authors.
Journal of Environmental Radioactivity | Year: 2011

Temporal variation of radon-222 concentration was studied at the Syabru-Bensi hot springs, located on the Main Central Thrust zone in Central Nepal. This site is characterized by several carbon dioxide discharges having maximum fluxes larger than 10 kg m-2 d-1. Radon concentration was monitored with autonomous Barasol™ probes between January 2008 and November 2009 in two small natural cavities with high CO2 concentration and at six locations in the soil: four points having a high flux, and two background reference points. At the reference points, dominated by radon diffusion, radon concentration was stable from January to May, with mean values of 22 ± 6.9 and 37 ± 5.5 kBq m-3, but was affected by a large increase, of about a factor of 2 and 1.6, respectively, during the monsoon season from June to September. At the points dominated by CO2 advection, by contrast, radon concentration showed higher mean values 39.0 ± 2.6 to 78 ± 1.4 kBq m-3, remarkably stable throughout the year with small long-term variation, including a possible modulation of period around 6 months. A significant difference between the diffusion dominated reference points and the advection-dominated points also emerged when studying the diurnal S1 and semi-diurnal S2 periodic components. At the advection-dominated points, radon concentration did not exhibit S1 or S2 components. At the reference points, however, the S2 component, associated with barometric tide, could be identified during the dry season, but only when the probe was installed at shallow depth. The S1 component, associated with thermal and possibly barometric diurnal forcing, was systematically observed, especially during monsoon season. The remarkable short-term and long-term temporal stability of the radon concentration at the advection-dominated points, which suggests a strong pressure source at depth, may be an important asset to detect possible temporal variations associated with the seismic cycle. © 2010 Elsevier Ltd.

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