Mukunda Bhattarai

Widespread ground motion distribution caused by rupture directivity during the 2015 Gorkha, Nepal earthquake

The ground motion and damage caused by the 2015 Gorkha, Nepal earthquake can be characterized by their widespread distributions to the east. Evidence from strong ground motions, regional acceleration duration, and teleseismic waveforms indicate that rupture directivity contributed significantly to these distributions. This phenomenon has been thought to occur only if a strike-slip or dip-slip rupture propagates to a site in the along-strike or updip direction, respectively. However, even though the earthquake was a dip-slip faulting event and its source fault strike was nearly eastward, evidence for rupture directivity is found in the eastward direction. Here, we explore the reasons for this apparent inconsistency by performing a joint source inversion of seismic and geodetic datasets, and conducting ground motion simulations. The results indicate that the earthquake occurred on the underthrusting Indian lithosphere, with a low dip angle, and that the fault rupture propagated in the along-strike direction at a velocity just slightly below the S-wave velocity. This low dip angle and fast rupture velocity produced rupture directivity in the along-strike direction, which caused widespread ground motion distribution and significant damage extending far eastwards, from central Nepal to Mount Everest.

The Syabru-Bensi hydrothermal system in central Nepal: 1. Characterization of carbon dioxide and radon fluxes

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 CO2. 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.

Large-scale organization of carbon dioxide discharge in the Nepal Himalayas

Gaseous carbon dioxide (CO 2) and radon-222 release from the ground was investigated along the Main Central Thrust zone in the Nepal Himalayas. From 2200 CO 2 and 900 radon-222 flux measurements near 13 hot springs from western to central Nepal, we obtained total CO 2 and radon discharges varying from 10 A3 to 1.6 mol s A1 and 20 to 1600 Bq s A1 , respectively. We observed a coherent organization at spatial scales of ≈ 10 km in a given region: low CO 2 and radon discharges around Pokhara (midwestern Nepal) and in the Bhote Kosi Valley (east Nepal); low CO 2 but large radon discharges in Lower Dolpo (west Nepal); and large CO 2 and radon discharges in the upper Trisuli Valley (central Nepal). A 110 km long CO 2-producing segment, with high carbon isotopic ratios, suggesting metamorphic decarbonation, is thus evidenced from 84.5°E to 85.5°E. This spatial organization could be controlled by geological heterogeneity or large Himalayan earthquakes.

Joint inversion of teleseismic, geodetic, and near-field waveform datasets for rupture process of the 2015 Gorkha, Nepal, earthquake

The 2015 Gorkha earthquake and its aftershocks caused severe damage mostly in Nepal, while countries around the Himalayan region were warned for decades about large Himalayan earthquakes and the seismic vulnerability of these countries. However, the magnitude of the Gorkha earthquake was smaller than those of historical earthquakes in Nepal, and the most severe damage occurred in the north and northeast of Kathmandu. We explore reasons for these unexpected features by performing a joint source inversion of teleseismic, geodetic, and near-field waveform datasets to investigate the rupture process. Results indicate that the source fault was limited to the northern part of central Nepal and did not reach the Main Frontal Thrust. The zone of large slip was located in the north of Kathmandu, and the fault rupture propagated eastward with an almost constant velocity. Changes in the Coulomb failure function (ΔCFF) due to the Gorkha earthquake were computed, indicating that southern and western regions neighboring the source fault are potential source regions for future earthquakes related to the Gorkha earthquake. These two regions may correspond to the historical earthquakes of 1866 and 1344. Possible future earthquakes in the regions are predicted, and the warning for Himalayan seismic hazards remains high even after the Gorkha earthquake.

Radon and carbon dioxide around remote Himalayan thermal springs

Abstract Radon-222 and carbon dioxide (CO2) emissions were studied around four remote Nepalese thermal springs near the Main Central Thrust: Timure and Chilime in the upper Trisuli Valley, central Nepal; and Sulighad and Tarakot in Lower Dolpo, western Nepal. A total of 279 radon fluxes and 670 CO2 fluxes were measured on the ground, complemented by radon concentration measurements in soil and water, and assisted by thermal infrared imaging. In Lower Dolpo, mean radon fluxes ranging from 270×10−3 to 450×10−3 Bq m−2 s−1, radon concentration in water greater than 100 Bq l−1, low mean CO2 fluxes (18–32 g m−2 day−1), and integrated radon and CO2 discharges of 70–180 Bq s−1 and (2.3–3.8)×10−3 mol s−1, respectively, suggest shallow-water-dominated transport with simultaneous radon and CO2 degassing from the hydrothermal water. In the upper Trisuli Valley, mean radon fluxes ranging from 140×10−3 to 570×10−3 Bq m−2 s−1, larger mean CO2 fluxes that range from 430 to 2930 g m−2 day−1, radon concentration in water of less than 6 Bq l−1, and integrated radon and CO2 discharges of 290–840 Bq s−1 and (390–830)×10−3 mol s−1, respectively, indicate fast gas-dominated transport of deep metamorphic-origin CO2 charged in radon along a fault network. Radon can thus give precious information on the gas transport properties of the shallow continental crust.

Persistent CO2 emissions and hydrothermal unrest following the 2015 earthquake in Nepal

Fluid–earthquake interplay, as evidenced by aftershock distributions or earthquake-induced effects on near-surface aquifers, has suggested that earthquakes dynamically affect permeability of the Earth’s crust. The connection between the mid-crust and the surface was further supported by instances of carbon dioxide (CO2) emissions associated with seismic activity, so far only observed in magmatic context. Here we report spectacular non-volcanic CO2 emissions and hydrothermal disturbances at the front of the Nepal Himalayas following the deadly 25 April 2015 Gorkha earthquake (moment magnitude Mw = 7.8). The data show unambiguously the appearance, after the earthquake, sometimes with a delay of several months, of CO2 emissions at several sites separated by > 10 kilometres, associated with persistent changes in hydrothermal discharges, including a complete cessation. These observations reveal that Himalayan hydrothermal systems are sensitive to co- and post- seismic deformation, leading to non-stationary release of metamorphic CO2 from active orogens. Possible pre-seismic effects need further confirmation.Earthquakes rarely affect hydrothermal systems in non-magmatic context. Here the authors report outbursts of CO2 and hydrothermal disturbances triggered by the 2015 Nepal earthquake, revealing high sensitivity of Himalayan hydrothermal systems to co-, post- and possibly pre- seismic deformation.

Anomalous Complex Electrical Conductivity of a Graphitic Black Schist From the Himalayas of Central Nepal

We analyzed in the laboratory the frequency-dependent, complex-valued, electrical conductivity of a graphitic black schist and an augen gneiss, both collected in the Main Central Thrust shear zone in the Himalayas of central Nepal, which was heavily affected by the deadly Mw7.8 Gorkha earthquake in 2015. We focused on anisotropy and salinity dependence of both cores and crushed material as well as the impact of CO2 on conductivity. This black schist possesses an extraordinarily high polarizability and a highly frequency-dependent conductivity. Its anisotropy is very pronounced. The investigations can relate the main polarization feature to disseminated, aligned plates of graphite. By contrast, the augen gneiss shows low polarizability and a moderately anisotropic conductivity dominated by the pore-filling brine. We further demonstrate that neglecting the complex and frequency-dependent nature of conductivity can lead to serious misinterpretation of magnetotelluric data during inversion if highly polarizable rocks are present. Plain Language Summary We investigated the electrical properties of a graphitic black schist and an augen gneiss, both collected in a shear zone in the Himalayas of central Nepal, which was heavily affected by the deadly Ghorka earthquake in 2015 (moment magnitude Mw7.8). We focused on electrical resistivity, polarization, anisotropy, and the influence of pore water salinity. Both cores and crushed material were analyzed, which allows for a more detailed understanding of the mechanisms of electric conduction in such rocks. The black schist shows a strongly frequency-dependent resistivity, which is associated with an extraordinarily high polarization. Its anisotropy is very pronounced. Scanning electron microscope images confirm that this behavior is due to disseminated, aligned plates of graphite. The augen gneiss on the other hand shows a regular electrical resistivity, which is dominated by the pore-filling brine. Besides the new insights in the mechanisms of electric conduction in these unusual, highly metamorphic rocks, our investigations bear relevance for large-scale geophysical surveys aiming at revealing the internal structure of the Himalayas and understanding the occurrence of large earthquakes in the area. We demonstrate that neglecting the unusual electrical properties of the black schist during interpretation of influenced data can lead to serious misinterpretation.