Himalayan Thrust Fault Research Articles

Present-day quantification of seismic coupling along the décollement level beneath the Potwar Plateau region in Pakistan western Himalaya

Using GNSS horizontal surface velocities and Sentinel-1 interferometry line-of-sight velocities, we quantify the current velocity field of the Potwar Plateau Salt Range fold-and-thrust belt. From this velocity field indicating a creep of the Potwar Plateau along the Main Himalayan Thrust, we inferred a weak subhorizontal décollement level formed by a massive Precambrian salt layer. To the south of the Plateau, the Salt Range is uplifted along the Salt Range Thrust, conditioned by the presence of an inherited normal fault. The Kalabagh Fault, which forms the western boundary of the Salt Range and Potwar Plateau, exhibits a creep rate of 3.3 mm/yr. Numerical modelling enabled us to characterise the slip distribution and coupling along the faults, showing the presence of a large asperity along the décollement level beneath the Potwar Plateau and several asperities along the eastern part of this basal thrust. The model also indicated an alternation of coupled and decoupled zones along the Kalabagh Fault, suggesting that this strike-slip fault can be characterised by creep and the occurrence of earthquakes and/or slow slip events. Considering the lack of instrumental and historical large-magnitude earthquakes in this area, the Main Himalayan Thrust and Kalabagh Fault are likely to be affected by earthquakes of magnitude Mw larger than 7.5. A slip rate of 20 mm/yr is modelled along the southern and superficial parts of the Salt Range Thrust, which is larger than the 14 mm/yr slip rate along the Main Himalayan Thrust at depth. This observation suggests the existence of a southward flow of massive salt along the Salt Range Thrust.

Stress Evolution of the Main Himalayan Thrust Fault Induced by the Mw 7.8 Gorkha Earthquake

ABSTRACT The Mw 7.8 Gorkha earthquake that occurred in 2015 is the largest thrust event in the middle of the main Himalayan thrust fault zone (MHT) in the past 81 yr. Its impact on the regional tectonic stress state and future seismic risks is a significant scientific issue worthy of in-depth analyses. In this study, we inverted the planar fault-slip model (the planar model), the flat-ramp fault-slip model (the flat-ramp model) and the double flat-ramp fault-slip model (the double flat-ramp model) to analyze the effect of the fault geometry, based on the steepest descent method (SDM) and the layered earth model. Compared with the flat-ramp model, the planar model exhibits a wider slip distribution in the down-dip direction of the main rupture zone, whereas the double flat-ramp model shows a larger coseismic slip on the middle ramp at the depth of 7.5–11.5 km. Those slip differences produce larger stress shadow areas in the above models, but for a blind thrust event induced by a low-dip thrust fault, this does not significantly change the distribution mode of coseismic Coulomb stress change (ΔCFS) in the three models. Namely, there is an obvious stress release in the main rupture but an obvious stress loading in the up-dip and down-dip directions of the main rupture zone. Based on the Burgers rheological model, we calculated the postseismic viscoelastic Coulomb stress change (V−ΔCFS) and the cumulative Coulomb stress change (C−ΔCFS) induced by the Gorkha earthquake in the flat-ramp model and comparatively analyzed the evolution pattern of coseismic and postseismic stresses. Our results indicate that the variation trend of postseismic stress in the lithosphere is opposite to that of the coseismic ΔCFS. The postseismic viscoelastic relaxation promotes the slip of the flat-ramp structure at the depth of 10–25 km, and the stress unloads in the shallow and deep part simultaneously. As a blind thrust event, the coseismic ΔCFS still plays a dominant role in the shallow part after 50 yr, whereas the loading C−ΔCFS in the deep part of the MHT is greatly weakened by the postseismic V−ΔCFS. Seismic risks still exist in the unruptured area on the west side of the mainshock and the shallow Main Frontal thrust.

Seismically active structures of the Main Himalayan Thrust revealed before, during and after the 2015 Mw 7.9 Gorkha earthquake in Nepal

SUMMARY The Mw 7.9 2015 April 25 Gorkha earthquake is the latest of a millenary-long series of large devastating Himalayan earthquakes. It is also the first time a large Himalayan earthquake and its aftershocks were recorded by a local network of seismic stations. In the 5 yr following the main shock, more than 31 000 aftershocks were located by this permanent network within the ruptured area, including 14 362 events with ML greater than 2.5, 7 events with ML > 6, including one large aftershock with Mw 7.2 on 2015 May 12. In 2020, 5 yr after the main shock, the seismicity rate along the ruptured fault segments was still about 5 times higher than the background seismicity before the Gorkha earthquake. Several bursts of earthquakes, sometimes organized in clusters, have been observed from a few days to several years after the main shock. Some of these clusters were located at the same place as the clusters that happened during the decades of interseismic stress build-up that preceded the large earthquake. They also happened in the vicinity of the high frequency seismic bursts that occurred during the main shock. These heterogeneities contribute to a persistent segmentation of the seismicity along strike, possibly controlled by geological structural complexities of the Main Himalayan Thrust fault. We suggest that these pre-2015 clusters revealed the seismo-geological segmentation that influences both the coseismic rupture and the post-seismic relaxation.

Spatially focused erosion in the High Himalaya and the geometry of the Main Himalayan Thrust in Central Nepal (85°E) from thermo-kinematic modeling of thermochronological data in the Gyirong region (southern China)

The subsurface structure of the Himalaya is dominated by the seismically active Main Himalayan Thrust fault, which accommodates about half of the plate convergence between India and Asia. In Central Nepal, geological, geophysical, and geodetic studies indicate that the Main Himalayan Thrust has a flat-ramp-flat geometry, which causes cooling, erosion, and exhumation of rocks to be faster above the mid-crustal ramp than above the adjacent upper and lower flats. However, in the northern High Himalaya the ramp geometry, in particular its width and depth are not well resolved. To place further constraints on the northward extent and depth of the ramp, we present for the first time low-temperature thermochronological data and U-Pb zircon ages from the Gyirong region, China (northern High Himalaya). Our zircon U-Pb data show that the protolith of the High Himalayan orthogneisses is 478 ± 4 Ma old, and was intruded by a large leucogranite at 19.5 ± 0.7 Ma. The Pliocene-Quaternary cooling of the study area is constrained by apatite fission track ages and apatite and zircon (U-Th)/He ages between ~5 Ma and ~ 0.5 Ma, which show a marked trend of southward-younging ages. Together with published cooling ages from Nepal farther south, the ages define a U-shaped pattern across the Main Himalayan Thrust, with ages increasing to the north and south from a minimum in the High Himalaya. A thermo-kinematic model, in which the geometry of the mid-crustal ramp was varied, explains the age data and suggests that the ramp has a dip of ~22°N. Compared to previous models, our new ages require that the ramp has a greater width (~55 km) and reaches a greater depth (~34 km) below the northern High Himalaya. Still farther north, the fault is presumably developed as a gently-dipping ductile shear zone, which forms the prominent reflector visible in seismic reflection data.

Abnormal disappearance of Duoqing Co lake between November 2015 and April 2016, due to far-field aseismic creeping of the southern Yadong-Gulu rift of Tibet, triggered by the 2015 Ms 8.1 Nepal earthquake

ABSTRACT Duoqing Co is a 60 km2 outflow lake in the N-trending Pagri graben, located at the southern end of the Yadong-Gulu rift in Tibet. The water in this lake suddenly disappeared between November 2015 and April 2016, closely following the Ms 8.1 (Mw 7.8) Nepal earthquake in April 2015. Both, geomorphological and remote sensing data indicate the existence of blind faults striking NNE along the east boundary of Duoqing Co lake. There were also several nearly NE-trending extensional cracks preserved in the dried lakebed, apparently formed in response to creeping deformation of the underlying rock. Based on field studies and analysis of meteorological and remote sensing data, it is suggested that this phenomenon cannot be explained by evaporation linked to climate change nor can it be related to human activity. Instead, it is considered that the lake water drained through the extensional cracks formed in the lakebed as it responded to the far-field effects of the 2015 Nepal earthquake. It is proposed that a shift in regional tectonics occurred as a result of the Nepal earthquake, causing a sharp increase in stress accumulation along the seismically locked Bhutan–Sikkim zone on the Main Himalayan Thrust (MHT) fault, which was accommodated by the extension of the Pagri graben in the southern Yadong-Gulu rift. It is believed that the crust may have reached a critical stress-state that resulted in strain hardening and brittle failure throughout the region along the Bhutan–Sikkim segment of the MHT. If so, considering the high potential for tectonic activity along the segment of the MHT, it may be worth paying attention to deformational changes and potential geomorphological precursors that might appear in the seismically locked Bhutan–Sikkim gap to predict future earthquakes.

Afterslip evolution on the crustal ramp of the Main Himalayan Thrust fault following the 2015 Mw 7.8 Gorkha (Nepal) earthquake

Current afterslip models of the Mw 7.8 Gorkha earthquake contradict with the depth ranges of relocated aftershock distribution and inter-seismic locked-creeping transition zone. We use both GPS and interferometric synthetic aperture radar (InSAR) data to image the afterslip evolution in the first 189 post-seismic days, i.e., from 25 April to 1 November 2015. In the study region, 51 GPS stations are divided into three categories based on their observation time: (1) 25 stations with both pre- and post-seismic observations, (2) 16 stations with only post-seismic observations, and (3) 10 stations with only pre-seismic observations. The secular tectonic motion and seasonal variations are corrected carefully and directly using the fitted functions from the pre-seismic position time series. During the inversions, we adopt the widely accepted flat–ramp–flat geometrical model of the Main Frontal Thrust fault (MFT)–Main Himalayan Thrust fault (MHT). The preferred weight ratios between GPS and InSAR observations are elaborately determined through comprehensive analysis of model resolution and fitting residuals. In contrast to previous afterslip models, our preferred afterslip distributes on the crustal ramp of the MHT from depths of 11 to 29 km, which is verified by further inversions using only GPS data and checkerboard resolution tests. The seismic moment released by the afterslip in the first six months is 3.42 × 1019 N m, only approximately 4% of the mainshock moment. The rupture gap between the mainshock and the 12 May aftershock is filled up with post-seismic aseismic slip.

An assessment of present-day crustal deformation in the Kumaun Himalaya from GPS observations

The India-Eurasia collision has created high mountains in the Himalayan plate boundary system, which has led to numerous earthquakes in this region and released part of the elastic strain accumulated over years. In this study, the present-day deformation pattern is derived from geodetic mode Global Positioning System (GPS) measurements obtained during 2005–2016 from 16 campaign sites in the Kumaun province of the central Himalaya. GPS derived horizontal velocity vectors are in the range of 1–9 mm/yr and 27–38 mm/yr with respect to stable India and Eurasia reference frames respectively. To understand the interseismic strain partitioning in the study region, we consider the uniform slip dislocation model over the fault in an elastic, isotropic and homogeneous half-space medium. The model indicates the Main Himalayan Thrust (MHT) fault is locked to a depth of ∼20 km and slips at a rate of 17.2 ± 1 mm/yr with a dip angle of ∼7°. This MHT slip rate build-up corresponds to a moment deficit of 8.4 ± 1 × 1018 Nm/yr due to locking of the fault beneath the Kumaun Himalaya. The total seismic strain accumulation of 61 × 10−6 estimated from the geodetic data suggests the possibility of large earthquakes (typically magnitude 8 and above) with the recurrence interval of around 600 years in the Kumaun Himalayan region.

Bimodal seismicity in the Himalaya controlled by fault friction and geometry

There is increasing evidence that the Himalayan seismicity can be bimodal: blind earthquakes (up to Mw ~ 7.8) tend to cluster in the downdip part of the seismogenic zone, whereas infrequent great earthquakes (Mw 8+) propagate up to the Himalayan frontal thrust. To explore the causes of this bimodal seismicity, we developed a two-dimensional, seismic cycle model of the Nepal Himalaya. Our visco-elasto-plastic simulations reproduce important features of the earthquake cycle, including interseismic strain and a bimodal seismicity pattern. Bimodal seismicity emerges as a result of relatively higher friction and a non-planar geometry of the Main Himalayan Thrust fault. This introduces a region of large strength excess that can only be activated once enough stress is transferred upwards by blind earthquakes. This supports the view that most segments of the Himalaya might produce complete ruptures significantly larger than the 2015 Mw 7.8 Gorkha earthquake, which should be accounted for in future seismic hazard assessments.

Lateral variations of the midcrustal seismicity in western Nepal: Seismotectonic implications

The midcrustal seismicity along the Himalayas depicts some lateral variations. This seismicity is often related to stress build up, at depth, along the downdip extent of the fault segments producing large and great earthquakes. To better characterize these variations, we deployed the first temporary seismological network in West Nepal (HiKNet) as a complement to the Nepalese national network. We used automatic picks, manually validated, to perform absolute and relative location of the earthquakes. The resulting catalog contains 2190 local events, shallow to midcrustal (8–16 km). The comparison between the seismicity and the geologic balanced cross-sections shows that most earthquakes occur within the hanging wall of the Main Himalayan Thrust fault nearby ramps or suspected contacts between Lesser Himalayan slivers. Some of the events associated with these geological structures are clustered within weeks-long earthquake swarms with seismic envelopes migrating at 35–50 m/day. These transients are possibly related to fluid migrations or geodetically unresolved slow slip events. The lateral variations of some of the structures associated to this seismicity are likely partially controlling the extent of the coseismic ruptures during intermediate earthquakes that break partly the locked fault zone, in a similar way to what was reported after the Mw7.8 2015 Gorkha-Nepal earthquake. A better characterization of the faults segmentation, including a better location and extent of the seismogenic segments at depth, is an important input for seismic hazard studies.

Postseismic deformation associated with the 2015 Mw 7.8 Gorkha earthquake, Nepal: Investigating ongoing afterslip and constraining crustal rheology

The 2015 Mw 7.8 Gorkha earthquake has not only imposed effective constraints on the geometrical structures, friction behaviours and seismogenic patterns of the Nepal Himalaya thrustsystems but has also provided valuable insights into the uplift mechanism and lithosphere rheology of the Tibetan Plateau. Here, ∼1.6-year GPS observations are used to reveal the postseismic deformation characteristics following the Gorkha earthquake, investigate the ongoing aseismic afterslip on the Main Himalayan Thrust (MHT) fault and constrain the crustal rheology of the Southern Tibetan Plateau. First, afterslip is considered to be solely responsible for the postseismic deformation (afterslip-only model). The results show that afterslip is anticorrelated with peak coseismic slip areas. One high-afterslip-concentration area, with a peak of ∼24 cm, is distributed downdip of the coseismic rupture, as well as in two other regions: one partially overlapping the mainshock rupture, and the other next to the Mw 7.3 aftershock area. Second, the GPS postseismic observations are inverted to jointly investigate afterslip and viscoelastic deformation (multiple-mechanism model). The afterslip inversion results of the above two models are highly consistent, indicating the dominant contribution of afterslip to surface deformation during the ∼1.6-year postseismic period. Considering the interseismic fault coupling and historical seismicity, no appreciable fault slip associated with the Gorkha earthquake is found to occur both updip and west of the mainshock rupture areas. This reveals that the Gorkha earthquake only unzipped the lower edge of the locked portion of the MHT, leaving the shallow portion and western segment of the seismogenic zone still locked and the Nepal region under high seismic risk. The viscoelastic mechanism contributes minorly to surface deformation during the ∼1.6-year postseismic period. The middle-lower crust is assumed to comprise Maxwell material beneath an elastic ∼25-km-thick upper crust and the optimal viscosity is conservatively estimated to be 1.6 × 1019 Pa s beneath the Southern Tibetan Plateau, which should be robustly constrained with more long-term observations, more effective spatial constraints, and more detailed crustal models.

Joint inversion of GPS, InSAR and teleseismic data sets for the rupture process of the 2015 Gorkha, Nepal, earthquake using a generalized ABIC method

Various observations are made to invert for the source model of the 2015 Gorkha earthquake. Previously published slip models involve different slip distributions and slip amplitudes. To obtain a robust model, we jointly invert for the rupture process of the Gorkha earthquake using three types of observations: Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and teleseismic P wave data. Three constraints (i.e., spatial and temporal smoothness and minimum moment constraints) are applied to regulate the rupture process. The weight ratios between data sets and prior constraints are determined from a new developed generalized Akaike’s Bayesian Information Criterion (gABIC) weight determination method. This gABIC method can simultaneously and objectively weigh various kinds of observations and prior constraints, rather than being limited to the two types in the traditional ABIC method. The inverted slip model reveals the seismic moment of the earthquake is 9.40×1020Nm released primarily in the first 60s, equivalent to Mw 7.92. The rupture front propagates unilaterally along the strike direction and spreads in both the strike and dip directions. The slip distribution shows one large slip patch with two peaks along the strike direction with a maximum slip amplitude of approximately 7.6m and identifies large slips (> 2m) that occur in the updip section of the Main Himalayan Thrust (MHT) fault plane. Our results indicate that the released slip amplitude is larger than the accumulated slip amplitude since 1833, so the 2015 earthquake likely does not overlap with the 1833 earthquake.

Stress change from the 2015 Mw 7.8 Gorkha earthquake and increased hazard in the southern Tibetan Plateau

We examine the co-seismic Coulomb stress change induced by the 2015 Mw 7.8 Gorkha, Nepal earthquake and investigate its influence on the seismic hazard in the southern Tibetan Plateau. We find that the location of the aftershocks coincides with areas where stress was enhanced. This stress enhanced area also covers the west and the shallow fault section updip of the 2015 Gorkha earthquake rupture in the Main Himalayan Thrust fault (MHT), signaling the raising hazard of the seismic gaps in the west and south to Kathmandu. The 2015 Gorkha earthquake strongly interacted particularly with the normal and thrust faults in the distance of around 100–250km from the rupture zone by bringing stress closer to the Coulomb failure. This fault interaction process increased stress by 12kPa and 100kPa at the southern end of the Tingkye and the Nyalam normal faults, respectively, triggering the Ms 5.9 Tingri earhquake on 25 May 2015 and the Ms 5.3 Nyalam earthquake on 26 May 2015. Stress in the Main Frontal Himalaya Thrust fault (MFT), the Main Boundary Thrust fault (MBT) in the Lalitpur area of Nepal was promoted by 40–100kPa. Such stress transfer should facilitate the future rupture on these active faults and raise seismic hazard in the southern Tibetan Plateau.

Using geodetic data to calculate stress changes on faults in the Tibetan Plateau caused by the 2015 Mw7.8 Nepal earthquake

The 2015 Mw7.8 Nepal earthquake occurred on the segment of the main Himalayan thrust fault between the Indian and Eurasian plates, and caused serious casualties. This earthquake may produce a profound impact on the evolution of the Tibetan Plateau and have brought a stress loading to faults within the plateau. In this paper, a high-resolution slip distribution of the 2015 Nepal earthquake is inverted from the InSAR and GPS data in the near field, and is used to compute the evolution of the cumulative Coulomb stress changes on faults in the earthquake-prone zone in the Tibetan Plateau. In the given reasonable parameters, the calculated co- and post-seismic stress changes on faults do not exceed 1.0kPa at the north of latitude 32° in the Tibetan Plateau. The largest positive stress changes occur on the South Tibet Detachment fault, and the magnitudes are much less than 100kPa. The estimated seismicity rate change on the segment of the South Tibet Detachment fault can be up to a level of two hundred thousandths. This indicates that there is a high hazard of earthquake triggering in the South Tibet Detachment fault and its adjacent regions. In the northern and eastern Tibetan Plateau, the estimated seismicity rate changes are lower than a level of one thousandth. However, some faults with a relative high background seismicity rate, such as the Xianshuihe and Longmenshan faults, may have a high hazard of earthquake triggering in the future.

Pre- and post-seismic deformation related to the 2015, [formula omitted] Gorkha earthquake, Nepal

We analyze time series from continuously recording GPS stations in Nepal spanning the pre- and post-seismic period associated to the Mw7.8 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 ~7months after the mainshock released a moment of [12.8±0.5]×1019Nm which represents 17.8±0.8% of the co-seismic moment. The moment released by aftershocks over that period of time is estimated to 2.98×1019Nm. 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±1.1mm/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 Mw7.5 has occurred since 1505.

The 2015 Gorkha earthquake: A large event illuminating the Main Himalayan Thrust fault

AbstractThe 2015 Gorkha earthquake sequence provides an outstanding opportunity to better characterize the geometry of the Main Himalayan Thrust (MHT). To overcome limitations due to unaccounted lateral heterogeneities, we perform Centroid Moment Tensor inversions in a 3‐D Earth model for the main shock and largest aftershocks. In parallel, we recompute S‐to‐P and P‐to‐S receiver functions from the Hi‐CLIMB data set. Inverted centroid locations fall within a low‐velocity zone at 10–15 km depth and corresponding to the subhorizontal portion of the MHT that ruptured during the Gorkha earthquake. North of the main shock hypocenter, receiver functions indicate a north dipping feature that likely corresponds to the midcrustal ramp connecting the flat portion to the deep part of the MHT. Our analysis of the main shock indicates that long‐period energy emanated updip of high‐frequency radiation sources previously inferred. This frequency‐dependent rupture process might be explained by different factors such as fault geometry and the presence of fluids.

Strong ground motion in the Kathmandu Valley during the 2015 Gorkha, Nepal, earthquake

On 25 April 2015, a large earthquake of Mw 7.8 occurred along the Main Himalayan Thrust fault in central Nepal. It was caused by a collision of the Indian Plate beneath the Eurasian Plate. The epicenter was near the Gorkha region, 80 km northwest of Kathmandu, and the rupture propagated toward east from the epicentral region passing through the sediment-filled Kathmandu Valley. This event resulted in over 8000 fatalities, mostly in Kathmandu and the adjacent districts. We succeeded in observing strong ground motions at our four observation sites (one rock site and three sedimentary sites) in the Kathmandu Valley during this devastating earthquake. While the observed peak ground acceleration values were smaller than the predicted ones that were derived from the use of a ground motion prediction equation, the observed peak ground velocity values were slightly larger than the predicted ones. The ground velocities observed at the rock site (KTP) showed a simple velocity pulse, resulting in monotonic-step displacements associated with the permanent tectonic offset. The vertical ground velocities observed at the sedimentary sites had the same pulse motions that were observed at the rock site. In contrast, the horizontal ground velocities as well as accelerations observed at three sedimentary sites showed long duration with conspicuous long-period oscillations, due to the valley response. The horizontal valley response was characterized by large amplification (about 10) and prolonged oscillations. However, the predominant period and envelope shape of their oscillations differed from site to site, indicating a complicated basin structure. Finally, on the basis of the velocity response spectra, we show that the horizontal long-period oscillations on the sedimentary sites had enough destructive power to damage high-rise buildings with natural periods of 3 to 5 s.

Locking degree and slip rate deficit distribution on MHT fault before 2015 Nepal Mw 7.9 earthquake

The spatial pattern and rate of strain accumulation on a fault during the pre- and inter-seismic phases are very important for interpreting the mechanism of earthquakes and evaluating seismic potentials. Here we use global positioning system (GPS) data and the block-dislocation model to invert for the locking degree and slip rate deficit of the Main Himalayan Thrust (MHT) fault in the southern margin of Tibet before the 2015 Mw 7.9 Nepal earthquake. Results show that the locking depth and slip rate deficit increase from the west to the east. Along the western segment of the MHT fault (80°E–84°E), the locking depth is estimated to be 12–17km with a slip rate deficit of 0–5mm/a; along the central Nepal segment (84°E–87°E), the locking depth is 16–21km with a slip rate deficit of 6–10mm/a, whilst along the eastern segment (87°E–90°E), the locking depth increases to 23–26km with a slip rate deficit of 8–13mm/a. The 2015 Nepal earthquake initiated at the boundary between the western and central segments, an area with as where the slip rate deficit varies dramatically from 0 to 9mm/a within 50km resulting in high energy gradients. High strain concentration along the central and eastern segments leads to unilateral propagation of the rupture to the east. Given the paucity of large seismic events over the previous decades and the current high slip rate deficit, seismic hazard on the eastern Nepal segment remains high.

Himalayan megathrust geometry and relation to topography revealed by the Gorkha earthquake

The Himalayan mountain range has been the locus of some of the largest continental earthquakes, including the 2015 magnitude 7.8 Gorkha earthquake. Competing hypotheses suggest that Himalayan topography is sustained and plate convergence is accommodated either predominantly on the main plate boundary fault, or more broadly across multiple smaller thrust faults. Here we use geodetic measurements of surface displacement to show that the Gorkha earthquake ruptured the Main Himalayan Thrust fault. The earthquake generated about 1 m of uplift in the Kathmandu Basin, yet caused the high Himalaya farther north to subside by about 0.6 m. We use the geodetic data, combined with geologic, geomorphological and geophysical analyses, to constrain the geometry of the Main Himalayan Thrust in the Kathmandu area. Structural analyses together with interseismic and coseismic displacements are best explained by a steep, shallow thrust fault flattening at depth between 5 and 15 km and connecting to a mid-crustal, steeper thrust. We suggest that present-day convergence across the Himalaya is mostly accommodated by this fault—no significant motion on smaller thrust faults is required. Furthermore, given that the Gorkha earthquake caused the high Himalayan mountains to subside and that our fault geometry explains measured interseismic displacements, we propose that growth of Himalayan topography may largely occur during the ongoing post-seismic phase.

Surface ruptures of large Himalayan earthquakes in Western Nepal: Evidence along a reactivated strand of the Main Boundary Thrust

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.

Slip in the 2015Mw 7.9 Gorkha andMw 7.3 Kodari, Nepal, Earthquakes Revealed by Seismic and Geodetic Data: Delayed Slip in the Gorkha and Slip Deficit between the Two Earthquakes

We investigate the rupture process of the 25 April 2015 M w 7.9 Gorkha, Nepal, earthquake and its biggest aftershock on 12 May 2015, based on joint inversion of teleseismic body waves, Interferometric Synthetic Aperture Radar, and Global Positioning System measurements. The Gorkha earthquake propagated unilaterally to the southeast along the Himalayan thrust fault (HTF), with coseismic slip separating into patches up-dip and down-dip of the hypocenter. Slip in the up-dip patch initially surrounded a region on the fault that did not slip. About 15 s after being surrounded, this region of the HTF then slipped, filling in the initial slip deficit. The delayed slip accounts for ∼20% of the moment release in the Gorkha earthquake. The inferred coseismic slip in the Kodari earthquake is localized to one patch on the HTF, extending to the south and southeast from the hypocenter and 20–30 km to the northeast of the main slip patch in the Gorkha earthquake. The maximum coseismic slip in both the Gorkha and Kodari earthquakes is ∼4.5 m.