Pre- and post-seismic deformation related to the 2015, M_w 7.8 Gorkha earthquake, Nepal

We analyze time series from continuously recording GPS stations in Nepal spanning the pre- and post-seismic period associated to the M_w7.8 Gorkha earthquake which ruptured the Main Himalayan Thrust (MHT) fault on April 25th, 2015. The records show strong seasonal variations due to surface hydrology. After corrections for these variations, the time series covering the pre- and post-seismic periods do not show any detectable transient pre-seismic displacement. By contrast, a transient post-seismic signal is clear. The observed signal shows southward displacements consistent with afterslip on the MHT. Using additional data from stations deployed after the mainshock, we invert the time series for the spatio-temporal evolution of slip on the MHT. This modelling indicates afterslip dominantly downdip of the mainshock rupture. Two other regions show significant afterslip: a more minor zone updip of the rupture, and a region between the mainshock and the largest aftershock ruptures. Afterslip in the first ~ 7 months after the mainshock released a moment of [12.8 ± 0.5] × 10^(19) Nm which represents 17.8 ± 0.8% of the co-seismic moment. The moment released by aftershocks over that period of time is estimated to 2.98 × 10^(19) Nm. Geodetically observed post-seismic deformation after co-seismic offset correction was thus 76.7 ± 1.0% aseismic. The logarithmic time evolution of afterslip is consistent with rate-strengthening frictional sliding. According to this theory, and assuming a long-term loading velocity modulated on the basis of the coupling map of the region and the long term slip rate of 20.2 ± 1.1 mm/yr, afterslip should release about 34.0 ± 1.4% of the co-seismic moment after full relaxation of post-seismic deformation. Afterslip contributed to loading the shallower portion of the MHT which did not rupture in 2015 and stayed locked afterwards. The risk for further large earthquakes in Nepal remains high both updip of the rupture area of the Gorkha earthquake and West of Kathmandu where the MHT has remained locked and where no earthquake larger than M_w7.5 has occurred since 1505

Pre- and post-seismic deformation related to the 2015, M_w 7.8 Gorkha earthquake, Nepal

We analyze time series from continuously recording GPS stations in Nepal spanning the pre- and post-seismic period associated to the M_w7.8 Gorkha earthquake which ruptured the Main Himalayan Thrust (MHT) fault on April 25th, 2015. The records show strong seasonal variations due to surface hydrology. After corrections for these variations, the time series covering the pre- and post-seismic periods do not show any detectable transient pre-seismic displacement. By contrast, a transient post-seismic signal is clear. The observed signal shows southward displacements consistent with afterslip on the MHT. Using additional data from stations deployed after the mainshock, we invert the time series for the spatio-temporal evolution of slip on the MHT. This modelling indicates afterslip dominantly downdip of the mainshock rupture. Two other regions show significant afterslip: a more minor zone updip of the rupture, and a region between the mainshock and the largest aftershock ruptures. Afterslip in the first ~ 7 months after the mainshock released a moment of [12.8 ± 0.5] × 10^(19) Nm which represents 17.8 ± 0.8% of the co-seismic moment. The moment released by aftershocks over that period of time is estimated to 2.98 × 10^(19) Nm. Geodetically observed post-seismic deformation after co-seismic offset correction was thus 76.7 ± 1.0% aseismic. The logarithmic time evolution of afterslip is consistent with rate-strengthening frictional sliding. According to this theory, and assuming a long-term loading velocity modulated on the basis of the coupling map of the region and the long term slip rate of 20.2 ± 1.1 mm/yr, afterslip should release about 34.0 ± 1.4% of the co-seismic moment after full relaxation of post-seismic deformation. Afterslip contributed to loading the shallower portion of the MHT which did not rupture in 2015 and stayed locked afterwards. The risk for further large earthquakes in Nepal remains high both updip of the rupture area of the Gorkha earthquake and West of Kathmandu where the MHT has remained locked and where no earthquake larger than M_w7.5 has occurred since 1505

Sismicité associée au séisme de Gorkha du 25 avril 2015 au Népal : au coeur du cycle sismique en Himalaya

Le séisme du 25 avril 2015, avec une magnitude de moment M w 7.9, est le plus fort séisme et le plus meurtrier depuis le séisme de Bihar-Népal de1934 (M w 8.1). Ce séisme a rompu un segment de 150 km par 70 km du Grand Chevauchement Himalayen (MHT), avec un glissement moyen de 4 m. La rupture, initiée 80 à l’ouest de Katmandou, s’est propagée presque unilatéralement vers l’est. Elle se termina à environ 60 km à l’est de la capitale, à l’extrémité d’un segment de faille qui se rompit 17 jours plus tard pendant le séisme, plus petit, de Kodari (M w 7.2). Des dévastations majeures se produisirent au dessus de la zone de rupture, avec plus de 9000 victimes. Les deux chocs principaux furent suivis d’une intense activité de répliques, avec plus de 14 000 séismes de magnitude locale M L supérieure à 2.5, et 6 séismes avec M L supérieure à 6. Le séisme de Gorkha est le premier grand séisme Himalayen au Népal enregistré précisément grâce à un réseau permanent de 21 stations installées depuis 1994, avec 12 stations du réseau sismologique national (NSC) au Népal Central. Cette thèse est consacrée à l’analyse des répliques enregistrées par le réseau du NSC jusqu’au 25 avril 2020. Les répliques diminuent avec le temps conformément à une loi d’Omori modifiée. L’activité reste, en 2020, au dessus du niveau antérieur. Les zones de densité maximale des répliques correspondent aux zones de densité maximale régulièrement activées entre 1994 et 2015, et aux zones de radiation de haute fréquence pendant la rupture, ce qui suggère que des aspérités du MHT concentrent les relâchements de contrainte en régime inter-, co- comme post-sismique. Cependant, en plus de l’activité de ces zones d’activité permanente, deux essaims de sismicité sont apparus, provoquant des séismes largement ressentis par la population dans la vallée de Katmandou. Dans tous les cas, une relocalisation des épicentres par double différence confirme le fait que l’activité sismique est concentrée sur le MHT. En plus de ces répliques, un intense essaim de sismicité est apparu en août 2017, dans la région de l’Himalchuli, 30 km environ au nord-ouest de l’épicentre de Gorkha, avec plus de 6500 séismes de M L inférieure à 3.7 en moins de quatre mois. Cet essaim s’est réactivé en 2018, mais avec une intensité 10 fois plus faible. Il s’agit de l’essaim sismique le plus intense jamais enregistré en Himalaya. Une relocalisation avec l’aide de stations auxiliaires proches n’indique pas un lien avec des structures liées au MHT, mais plutôt suggère un mécanisme combinant un effondrement gravitaire lié à l’orogenèse, la déformation post-sismique et des réajustements saisonniers de contraintes. Une relation entre ces activités sismiques nouvelles et l’initiation d ’un prochain grand séisme dans l’Ouest Népal n’est pas établie, et demandera une analyse plus détaillée combinant des campagnes géologiques, géodésiques et géophysiques ; c’est le défi de la prochaine décennie.The April 25, 2015 Mw 7.9 earthquake was the largest and most deadly earthquake in Nepal since the 1934 Mw 8.1 Bihar Nepal earthquake. The earthquake ruptured a 150 km by 70 km segment of the Main Himalayan Thrust (MHT), with an average slip of 4 m. The rupture, initiated about 80 km West of Kathmandu, propagated almost unilaterally towards the east. It ended about 60 km east of the capital city, at the end of a fault segment that ruptured 17 days later during the smaller Mw 7.2 Kodari earthquake. Widespread devastation occurred above the rupture zone, causing nearly 9000 victims. Intense aftershock activity followed the two mainshocks, with more than 14,000 earthquakes with local magnitude ML larger than 2.5, and 6 earthquakes with ML larger than 6. The Gorkha earthquake sequence is the first large earthquake crisis in Nepal recorded precisely thanks to a permanent network of 21 seismological stations installed in 1994, including 12 stations of the National Seismological Center (NSC) in central Nepal. This thesis presents an analysis of these aftershocks recorded by the NSC network until April 25, 2020. The aftershocks decay with time according to a modified Omori law. The activity remains, in 2020, above the prior activity. The aftershocks are mainly located around the ruptured fault segments. The zones of highest aftershock density correspond to those already regularly activated during the interseismic period and to zones of high frequency radiation during rupture, suggesting that geometrical asperities of the MHT concentrate stress release during inter-, co-and post-seismic regimes. However, in addition to these permanently seismic active zones, two clusters of seismicity were initiated, causing widely felt local earthquakes in the Kathmandu valley. In all cases, double difference relocation of the epicenters confirms the association between the aftershocks and theMHT. In addition to the aftershocks in the rupture zone, an intense earthquake swarm has appeared in August 2017, in the Himalchuli region, about 30 km North-West of the Gorkha epicentre, with more than 6500 events with ML smaller than 3.7 in less than 4 months. The swarm reactivated in 2018, butwith 10 times less earthquakes. This is the most intense earthquake swarm ever observed in the Himalayas. Relocation using auxiliary proximal stations did not suggest association with structures linked to the MHT but, rather, interplay between orogenic collapse, post-seismic relaxation of the Gorkha earthquake and seasonal stress adjustments. The relationship between the new seismic active zones and a possible larger earthquake in West Nepal remains an open question, and further analysis combining geological, geodetic and geophysical surveys will be an important challenge in the coming years

Automatic analysis of the Gorkha earthquake aftershock sequence: evidences of structurally segmented seismicity

International audienceWe present the first 3 months of aftershock activity following the 2015 April 25 Gorkha earthquake M w 7.8 recorded on the Nepalese Seismic network. We deployed an automatic procedure composed of three main stages: (1) coarse determination of the P and S onsets; (2) phase association to declare events and (3) iterative addition and refinement of onsets using the Kurtosis characteristic function. In total 9188 events could be located in the Kathmandu region with the majority having small location errors (<4.5, 9 and 10 km in the X-, Y-and Z-directions, respectively). Additionally, we propose a new attenuation law to estimate local magnitudes in the region. This new seismic catalogue reveals a detailed insight into the Gorkha aftershock sequence and its relation to the main shock rupture models and tectonic structures in the region. Most aftershocks fall within the Main Himalayan Thrust (MHT) shear zone or in its hangingwall. Significant temporal and lateral variations of aftershocks location are observed among them: (1) three distinct stages, highlighting subsequent jump-offs at the easternmost termination, (2) the existence of a seismic gap north of Kathmandu which matches with a low slip zone in the rupture area of the main shock, (3) the confinement of seismic activity in the trace of the May 12 M w 7.3 earthquake within the MHT and its hangingwall through a 30 × 30 km 2 region and (4) a shallow westward-dipping structure east of the Kathmandu klippe. These new observations with the inferred tectonic structures at depth suggest a tectonic control of part of the aftershock activity by the lateral breaks along the MHT and by the geometry of the duplex above the thrust

Orogenic Collapse and Stress Adjustments Revealed by an Intense Seismic Swarm Following the 2015 Gorkha Earthquake in Nepal

International audienceThe April 25, 2015 M w 7.9 Gorkha earthquake in Nepal was characterized by a peak slip of several meters and persisting aftershocks. We report here that, in addition, a dense seismic swarm initiated abruptly in August 2017 at the western edge of the afterslip region, below the high Himalchuli-Manaslu range culminating at 8156 m, a region seismically inactive during the past 35 years. Over 6500 events were recorded by the Nepal National Seismological Network with local magnitude ranging between 1.8 and 3.7 until November 2017. This swarm was reactivated between April and July 2018, with about 10 times less events than in 2017, and in 2019 with only sporadic events. The relocation of swarm earthquakes using proximal temporary stations ascertains a shallow depth of hypocenters between the surface and 20 km depth in the High Himalayan Crystalline slab. This swarm reveals an intriguing localized interplay between orogenic collapse and stress adjustments, involving possibly CO 2 -rich fluid migration, more likely post-seismic slip and seasonal enhancements

Teleseismic depth estimation of the 2015 Gorkha−Nepal aftershocks

International audienceThe depth of 61 aftershocks of the 2015 April 25 Gorkha, Nepal earthquake, that occurred within the first 20 d following the main shock, is constrained using time delays between teleseismic P phases and depth phases (pP and sP). The detection and identification of these phases are automatically processed using the cepstral method developed by Letort et al., and are validated with computed radiation patterns from the most probable focal mechanisms. The events are found to be relatively shallow (13.1 ± 3.9 km). Because depth estimations could potentially be biased by the method, velocity model or selected data, we also evaluate the depth resolution of the events from local catalogues by extracting 138 events with assumed well-constrained depth estimations. Comparison between the teleseismic depths and the depths from local and regional catalogues helps decrease epistemic uncertainties, and shows that the seismicity is clustered in a narrow band between 10 and 15 km depth. Given the geometry and depth of the major tectonic structures, most aftershocks are probably located in the immediate vicinity of the Main Himalayan Thrust (MHT) shear zone. The mid-crustal ramp of the flat/ramp MHT system is not resolved indicating that its height is moderate (less than 5−10 km) in the trace of the sections that ruptured on April 25. However, the seismicity depth range widens and deepens through an adjacent section to the east, a region that failed on 2015 May 12 during an M w 7.3 earthquake. This deeper seismicity could reflect a step-down of the basal detachment of the MHT, a lateral structural variation which probably acted as a barrier to the dynamic rupture propagation

Imaging the Moho and the Main Himalayan Thrust in Western Nepal With Receiver Functions

International audienceThe crustal structure of Western Nepal is studied for the first time by performing receiver function analysis on teleseismic waveforms recorded at 16 seismic stations. The Moho geometry is imaged as it deepens from ~40-km depth beneath the foothills and the Lesser Himalaya to ~58-km depth beneath the Higher Himalayan range. A midcrustal low-velocity zone is detected at ~15-km depth along ~55-km horizontal distance and is interpreted as the signature of fluids expelled from rocks descending in the footwall of the Main Himalayan Thrust. Our new image allows structural comparison of the Moho and of the Main Himalayan Thrust geometry along-strike of the Himalayas and documents long-wavelength lateral variations. The general crustal architecture observed on our images resembles that of Central Nepal; therefore, Western Nepal is also expected to be able to host large (M W > 8) megathrust earthquakes, as the 1505 CE event. Plain Language Summary We investigate the Earth's crust in Western Nepal. We use 16 stations to detect waves from faraway earthquakes, and using these we make an image of the structure of the crust. We find that the crust is getting thicker beneath the Himalaya and that fluids are locally present in the upper part of the crust. As this image is similar to the one we know in Central Nepal, where major earthquakes happened, it is possible that major earthquakes will also happen in Western Nepal in the future (without knowing when)

Persistent CO2_2 emissions and hydrothermal unrest following the 2015 earthquake in Nepal

International audienceFluid–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 (CO$_2$) emissions associated with seismic activity, so far only observed in magmatic context. Here we report spectacular non-volcanic CO$_2$ emissions and hydrothermal disturbances at the front of the Nepal Himalayas following the deadly 25 April 2015 Gorkha earthquake (moment magnitude M$_w$ = 7.8). The data show unambiguously the appearance, after the earthquake, sometimes with a delay of several months, of CO$_2$ 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 CO$_2$ from active orogens. Possible pre-seismic effects need further confirmatio

Tectonic significance of the 2021 Lamjung, Nepal, mid-crustal seismic cluster

International audienceSince the Mw 7.9 Gorkha earthquake of April 25, 2015, the seismicity of central and western Nepalese Himalaya has been monitored by an increasing number of permanent seismic stations. These instruments contribute to the location of thousands of aftershocks that occur at the western margin of the segment of the Main Himalayan Thrust (MHT) that ruptured in 2015. They also help to constrain the location of seismic clusters that originated at the periphery of the fault ruptured by the Gorkha earthquake, which may indicate a migration of seismicity along the fault system. We report here a seismic crisis that followed the Lamjung earthquake, a moderate Mw 4.7 event (ML 5.8, MLv 5.3) that occurred on May 18, 2021, about 30 km west of the Gorkha earthquake epicenter at the down-dip end of the locked fault zone. The study of the hypocentral location of the mainshock and its first 117 aftershocks confirms mid-crustal depths and supports the activation of a 30-40° dipping fault plane, possibly associated with the rupture of the updip end of the MHT mid-crustal ramp. The cluster of aftershocks occurs near the upper decollement of the thrust system, probably in its hanging wall, and falls on the immediate northern margin of a region of the fault that has not been ruptured since the 1344 or 1505 CE earthquake. The spatio-temporal distribution of the first 117 aftershocks shows a typical decrease in the associated seismicity rate and possible migration of seismic activity. Since then, the local seismicity has returned to the pre-earthquake rate and careful monitoring has not revealed any large-scale migration of seismicity towards the locked fault segments

Postseismic deformation following the April 25, 2015 Gorkha earthquake (Nepal): Afterslip versus viscous relaxation

International audienceThe postseismic deformation consecutive to the April 25, 2015 Gorkha earthquake (Mw 7.9) is estimated in this paper based on a cGNSS network installed prior to the earthquake and supplemented by 6 cGNSS stations installed after the main shock. Postseismic displacement are obtained from daily time series corrected for interseismic deformation and seasonal variations. The maximum postseismic displacement is found north of the rupture area, where locally it reached 100 mm between the date of the earthquake and late 2016. The postseismic deformation affects the northern part of the rupture area but not the southern part, along the southern part of the Main Himalayan Thrust (MHT). Three hypotheses for the mechanisms controlling postseismic deformation are tested through numerical simulations of the postseismic time series: (i) viscous relaxation, (ii) afterslip, or (iii) a combination of these two mechanisms. We can exclude postseismic deformation controlled by viscous relaxation of a thick deformation zone along the northern and lower flat of the MHT. However, it is impossible to discriminate between postseismic deformation controlled by either afterslip along the MHT (northern part of the rupture zone, crustal ramp, and lower flat of the MHT) or a combination of afterslip along the MHT (northern part of the rupture zone, crustal ramp) and viscous relaxation controlled by a thin (∼3–4 km thick) low-viscosity body centered on the lower flat of the MHT. The occurrence of afterslip along the northern part of the upper flat of the MHT and its longitudinal variations have been established thanks to the densification of GNSS network by our team presented in this paper