Blind Fault Research Articles

Estimation of activity of a blind fault using the spatial variation of the relative heights of fluvial terraces : An example of the Iwai river above the focal area of the 2008 Iwate-Miyagi Nairiku earthquake

Estimation of activity of a blind fault using the spatial variation of the relative heights of fluvial terraces : An example of the Iwai river above the focal area of the 2008 Iwate-Miyagi Nairiku earthquake

Forced folding and fracturing induced by differential compaction during post-depositional inflation of sandbodies: Insights from numerical modelling

Abstract Three series of numerical models based on the discrete element method were constructed to simulate forced folding and fracturing triggered by postdepositional inflation of fluidised sandbody. The models consist of numerous particles that have relatively low to high interparticle bonds to represent overburden sediments with a relatively low to high cohesion, and cohesionless, frictionless particles to represent fluidised sands. The modelling results show that normal faults were produced due to the upward inflation of sand domes and the resulting flexed overburden, when the cohesion of the host sediments is low. Opening voids were created as a result of strata collapse, when the intrusion-related normal faults terminated within the host sediments as blind faults. Conical fractures that are aligned along sandbody margins were produced, which consist of closed, lower segments with a reverse displacement, and opening, middle-upper segments with a minor to zero shear component. Forced folds were generated in most models with a moderate to high cohesion, resulting in differential compaction in the overlying sediments that can account for the formation of fold-related fractures, which are either shear, hybrid or pure tensile, depending on their structural positions. The amplitude of forced folds is closely associated with both cohesion and thickness of sediments in the overburden, whilst fold wavelength is mainly controlled by sediment cohesion. Based on the modelling results, three types of preferential sites for the storage of injected sands were suggested, which are believed to be instructive for subsurface sandbody detection and prediction. This study demonstrates that differential compaction induced by sand inflation can play an important role in overburden folding and fracturing.

Multi-Sensor Geodetic Observations and Modeling of the 2017 Mw 6.3 Jinghe Earthquake

The Mw 6.3 Jinghe earthquake struck Xingjiang Province, China, on 8 August 2017 (05:15:04 UTC); the epicenter was near the Kusongmuxieke Piedmont Fault (KPF) of the northern Tian Shan Mountains. We used multi-source and multi-track satellite Synthetic Aperture Radar (SAR) imagery and Interferometric SAR (InSAR) techniques to reconstruct the coseismic displacement field from different line-of-sight geometries. To reduce the phase artifacts, we employed multi-temporal scenes acquired by Sentinel-1, and reconstructed the coseismic deformation through a temporal averaging strategy. Together with a single interferometric pair obtained using the Phased Array type L-band Synthetic Aperture Radar 2 (PALSAR2) sensor aboard the Advanced Land Observing Satellite 2 (ALOS2), we obtained five displacement maps with slightly different viewing geometries; all of which were used to constrain a geodetic inversion to retrieve the fault geometry parameters and slip distribution. Based on the focal mechanism and regional geology, we constructed multiple fault models that differ in dip direction (south and north dipping), and various striking angles. Both models fit the InSAR displacement maps, but have slip distributions of different depths. The slip depth of the south dipping model, with a dip of ~42°, is the most consistent with the relocated earthquake sequence and regional geological structure. Through the geodetic inversion, the maximum slip (0.25 m) occurred at 14.05 km and the associated rake was 89.56°. The result implies that the seismogenic fault is a blind thrust fault north of KPF (towards the foreland). Considering the relative locations of the suggested blind fault, the KPF, and the continuing north to south (N–S) shortening of the Tian Shan Mountains, this fault could be formed by the northward propagation of the regional fold-thrust belt.

Geophysical researches for identification of hidden faults in the western region of Banesh, Fars, Iran

Possessing seismic information on active faults is required for earthquake risk estimation. In earthquake prone areas such as Zagros Mountains in which active basement faults are covered by Phanerozoic sedimentary cover, we can use geophysical data such as special resistivity and employ Schlumberger array as an inexpensive and quick method to identify blind faults with high accuracy. For this purpose, 30 vertical electric soundings and 10 profiles were implemented on Banesh Plain in Fars Province of Iran. Using data from geoelectric sections combined with qualitative data, we determined blind faults and sub fractures. The results show two faults zones along east-southwest and northwest-southeast lines and some sub faults and fractures along the Zagros flow.

Insight into the 2017 Mainling Mw 6.5 earthquake: a complicated thrust event beneath the Namche Barwa syntaxis

On November 18, 2017, the Mainling Mw 6.5 earthquake occurred on the northern Namche Barwa syntaxis and was the largest earthquake in the syntaxis and surrounding areas since the Zayu Mw 8.4 earthquake in 1950. Due to inconvenient access and the severe environment in the Namche Barwa syntaxis area, the motions and tectonic structures of most faults remain unclear. Sparsely distributed seismic observation stations make the seismogenic fault and focal mechanism of the Mainling earthquake controversial. We adopt interferometric synthetic aperture radar (InSAR) to invert the slip distribution of this event by defining the fault geometry with relocated aftershocks. Our preferred model suggests that the 2017 Mainling earthquake ruptured two blind faults beneath the Namche Barwa syntaxis. The ruptures were dominated by thrusts with slight right-lateral strike-slip components. The slips on the two faults are equivalent to moment magnitudes of Mw 6.12 and Mw 6.34, with maximum dislocation magnitudes of 0.36 m and 0.43 m, respectively. The model fits well with the InSAR observations and the distribution of aftershocks. The results from the Coulomb stress simulation indicate that the stress loading caused by strong historical events promoted the occurrence of the 2017 Mainling earthquake. Compared with the seismogenic faults of the Mainling earthquake, the larger thrust faults in the southern Namche Barwa syntaxis can generate larger earthquakes. Therefore, we assume that Mw > 6.5 earthquakes may occur beneath the Namche Barwa syntaxis and that the seismic risk has been further promoted by historical events.

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.

Origin and quantification of diffuse CO2 and H2S emissions at Crater Hills, Yellowstone National Park

We characterized volatile emissions based upon diffuse soil degassing measurements and fumarolic gas chemistry at Crater Hills, a thermally-altered area adjoining the Sour Creek resurgent dome that is located within the Yellowstone Caldera. The objective of this study was to investigate the source and flux of CO2 and H2S gases to improve our understanding of both the total emissions and origin of the spatial distribution. The total emission of CO2 estimated using the sequential Gaussian simulation method (sGs) was 66 to 109 t day−1 with 95% confidence, which is an underestimation due to the: (1) inability to measure a high flux area on a steep slope, and (2) absence of measurements from fumarole and hot pool emissions. Based on gas chemistry data obtained for a fumarole at Crater Hills in 2007, the proportion of CO2 calculated to be derived from magma would be at least 38%, but could be as high as 50%. The spatial distribution of prominent geothermal features with the highest gas flux are broadly consistent with the regional fault pattern and, therefore, likely reflect the pattern of blind faults and/or fractures covered by overlying alluvium. The estimated emission of H2S was 0.39 t day−1, based on the linear correlation between H2S and CO2. The heat output was also estimated to be ~35 MW with an average heat flux of ~100 W m−2 based upon CO2-H2O-heat relations.

Late Cenozoic structural deformation and evolution of the central-southern Longmen Shan fold-and-thrust belt, China: Insights from numerical simulations

Since the Late Cenozoic, the central-southern Longmen Shan (LMS) region is characterized by a two-décollement thrust-fold system with a localized upper evaporite weaker layer just beneath the Sichuan Basin and the lower stronger décollement beneath the LMS. To date, the role of upper décollement and erosion in shaping the structural deformation of the LMS remains unknown. The investigation of this question may shed lights on the assessment of the structural evolution and the further discrimination of mountain building mechanism. In this work, a 2D finite difference method is applied with regards to the variations of viscosity/friction angle, and lengths of upper décollement under different erosion rates. Two types of numerical models are setup, with purely lower-upper frictional (FF) décollements and lower-frictional upper-viscous (FV) décollements. Tectonic evolution with FF models exhibits large similarity with that of FV models. Meanwhile, our results underline the presence of in-sequence fault propagation procedures during most of the stages. The development of the boundary ramp, the Range Frontal Blind fault, is strongly induced by a prerequisite with upper décollement of necessary length. The activation of Beichuan/Pengguan faults or blind ramp piercing to the surface is extremely mechanically sensitive to the viscosity of the salt décollement, erosion rate, and the length of the upper décollement. Out-of-sequence thrusting, shallow and possible basal accretion processes beneath the Sichuan Basin are found when deformation transfers from the thick-skinned LMS to the thin-skinned Sichuan Basin at the very late stage, supporting upper crustal shortening dominates the mountain building mechanism.

Secrets of a Los Angeles blind thrust fault reveal increased seismic risk

Secrets of a Los Angeles blind thrust fault reveal increased seismic risk

Uncertainty Analysis of Faulting and Folding on Near Surface Aquifers

With advances in near surface geophysical techniques, notably Airborne Electromagnetics (AEM), great strides have been made in mapping near-surface (0-200m) hydrostratigraphy, including aquitards and aquifers. However, an important uncertainty in the mapping of groundwater systems and resource estimation is the potential impact of faults to; Generate fault parallel fracture transmissivity;Reduce across-fault permeability; andProvide connectivity from one aquifer to another across aquitards.It is vital to differentiate between faults and fault propagation folds associated with “blind faults”. As AEM is an evolving technology a set of criteria based on sound structural geomechanical and structural geologic has been developed to discriminate between faults and folds.Even with consistent and robust modeling there are fundamental uncertainties that require appropriate modeling using stochastic fault seal analysis. An extensive database of calibrations has been developed for the prediction of liquid hydrocarbon free water levels (FWL). Based on hundreds of hindcasting models FWL can be predicted with better than 10m accuracy.Despite this high accuracy prediction the juxtaposition area of across fault reservoir vs reservoir juxtaposition can vary by two or three orders of magnitude. The juxtaposition area is a key term in the calculation of across- fault Darcy Flow. Variation in area is strongly nonlinear and dependent on accounting for the thickness of aquitards and variation along strike of fault displacement. It is vital that geologically valid faults are analyzed with the appropriate parameter uncertainty. Examples from the Broken Hill Managed Aquifer Project and the Sydney Basin will be used to illustrate the process.

Geometry and kinematics of buried structures in the piedmont of the central Longmen Shan: implication for the growth of the Eastern Tibetan Plateau

Structural geometry and kinematics are important to understand the growth of the Tibetan Plateau margins. In this study, several high-resolution seismic reflection profiles were combined with well data and the fault-bend fold theory to determine the geometric and kinematic characteristics of the Pengxian buried structures located in the piedmont of central Longmen Shan (LMS). Seismic interpretations indicate that the Pengxian buried structures were involved in multi-period, superimposed deformation during the Cenozoic. The structural deformation of the frontal LMS was controlled by a Triassic detachment fault (f 2 ), which extended forward at a low angle, and formed a fault-bend fold during the Late Miocene. The high-angle Pengxian blind fault developed on the f 2 with little offset during the Pleistocene. Three-dimensional imaging of the Pengxian buried structures shows that the f 2 horizontally propagated into the western Sichuan Basin with a fault slip of c . 3.3–3.5 km. The Longquan Shan (LQS) anticline has a total slip of c . 3.0 km since its initiation. Our study suggests that the average slip rate of the f 2 is <0.4 mm a −1 in the central LMS front. The Pengxian buried structures and the LQS anticline have been slowly shortening since the Late Cenozoic, which could provide a constraint on the growth pattern of the eastern Tibetan Plateau margin.

Geodetic Changes Associated with Crustal Deformation on the Andean Backarc of San Juan, Argentina

In this work we have analyzed the results of temporal geodetic variations associated with tectonic deformation on the Andean backarc in southern San Juan province, Argentina. This area is affected by the subduction of the Juan Fernandez Ridge and has the greatest seismic hazard in Argentina. Since 2000, we have monitored the variations of gravity annually with high precision (g4D) in the active deformation belt, around the city of San Juan (31°S to 32°S, 68°W to 69°W). Temporal gradients of g4D have been associated with active structures and interpreted as subsurface mass redistributions as a result of accumulation and release of stress. These data, in addition to the topographic deformations measured by the Global Positioning System (GPS) and the identification of structures in the crystalline basement from the geophysical models, allow us interpret mechanisms of deformation in the contact between the Precordillera and Pampean Ranges in the foreland of San Juan. In this area, after more than a decade of low activity, in 2012 the Tulum Fault System (TFS) was reactivated, generating crustal events of moderate magnitude including an earthquake of Mw 5.0 (January 18, 2012). According to our interpretation, the fault system accumulated elastic deformation for more than 10 years expressed as height increases at gravity stations (decrease of g4D) until the year 2011. The January 18, 2012 earthquake released part of the energy at the same time as a coseismic increase of g4D. This should be reflected in regional tectonic subsidence from the point of view of conventional gravimetric models. However, GPS reoccupation of the benchmarks located in this fault system indicates a generalized increase in altitude of the fault system in the last 15 years. TFS behavior could be assumed as topographic flexural displacements induced by a system of blind faults. We analyzed the time series of a permanent Global Navigation Satellite System (GNSS) station located to the north of the TFS. The GRLS station (31.61oS; 68.32oW) presents slow variation in the vertical and north components. The variation began on December 20, 2011 reaching its maximum amplitude on January 18, 2012. The geodetic variations coincide with the period of seismic reactivation.

The Seismic Potential in the Seismic Gap Between the Wenchuan and Lushan Earthquakes Revealed by the Joint Inversion of Receiver Functions and Ambient Noise Data

AbstractBased on a dense seismic array deployed to monitor the seismic gap between the Wenchuan and Lushan earthquakes, a joint inversion of dispersion curves and receiver functions yielded a fine velocity structure in this region with the highest resolution of approximately 20 km. The results show that the upper crust of the Songpan‐Ganzi terrane overthrusted onto the basement of the Sichuan Basin, indicating that the crustal shortening model may be the major mechanism that accounts for the growth of the Longmenshan. The results also show a low‐velocity zone (Vs < 3.5 km/s) and thickened crust (>67 km) beneath the seismic gap, which could be associated with partial melting. The heat transferred from the partial melting at the base of the seismic gap may increase the temperature of the faults to as high as 300–400 °C and cause the segment of the Longmenshan Fault in the seismic gap to become “aseismic.” In contrast, the Shuangshi‐Dachuan Fault, Dayi Fault, and other blind faults to the east‐southeast of the seismic gap are located above a high‐velocity body. The seismogenic environment of these faults is similar to the segments ruptured during the Wenchuan and Lushan earthquakes, both of which occurred above the westward extension of the strong Sichuan Basin. Higher seismic activity was also observed along these faults than that above the low‐velocity zone. Therefore, we deduce that a major earthquake on these faults is possible and that the magnitude could be larger than that of the Dayi earthquake (M 6.2).

Effects of the January 2018 seismic sequence on shear-wave splitting in the upper crust of Marathon (NE Attica, Greece)

On the 15th of January 2018, a moderate Mw = 4.2 earthquake occurred in the area of Marathon (NE Attica). This event is of great interest, given that it is located in a region characterized by low seismic activity, even though there are previously identified faults (e.g. Penteli fault). The sequence started 7 days before the mainshock and lasted until the end of the January 2018. In the present study, waveforms from stations of the Hellenic Unified Seismological Network were utilized to explore the seismotectonic regime, using data recorded since 2012. While the focal mechanism of the mainshock is in agreement with known local faults, relocated hypocenters indicate the existence of blind faults. In addition, the proximity of two stations, DION and PTL, to the January 2018 sequence provided a unique opportunity to investigate seismic anisotropy of the upper crust in the study area, for the first time. Analysis of local events between 2012 and 2018 led to the revelation of an anisotropic layer beneath Marathon. However, a complex anisotropy regime is identified. The polarization direction of the Sfast in DION presents stable characteristics and is oriented according to the regional maximum horizontal stress component. However, PTL exhibits rotation to a direction perpendicular to the one that conforms to stress orientation, possibly associated with local faults of similar direction. Time-delays through the whole period are uncommonly low, for such studies, in both stations. Nevertheless, this changes for the January 2018 sequence in DION. Higher values of time-delays permit the establishment of trends that could be attributed to stress accumulation and relaxation, related to the Mw = 4.2 event, in agreement with the Anisotropic Poro-Elasticity model.

Source Model and Stress Disturbance of the 2017 Jiuzhaigou Mw 6.5 Earthquake Constrained by InSAR and GPS Measurements

Seismogenic fault geometry, especially for a blind fault, is usually difficult to derive, based only on the distribution of aftershocks and interference fringes of Interferometric Synthetic Aperture Radar (InSAR). To better constrain the fault geometry of the 2017 Jiuzhaigou Mw 6.5 earthquake, we first carried out a nonlinear inversion for a single fault source using multi-peak particle swarm optimization (MPSO), Monte Carlo (MC), and Markov Chain Monte Carlo (MCMC) algorithms, respectively, with constraints of InSAR data in multiple SAR viewing geometries. The fault geometry models retrieved with different methods were highly consistent and mutually verifiable, showing that a blind faulting with a strike of ~154° and a dip angle of ~77° was responsible for the Jiuzhaigou earthquake. Based on the optimal fault geometry model, the fault slip distribution jointly inverted from the InSAR and Global Positioning System (GPS) data by the steepest descent method (SDM) and the MC method showed that the slip was mainly concentrated at the depth of 1–15 km, and only one slip center appeared at the depth of 5–9 km with a maximum slip of about 1.06 m, some different from previous studies. Taking the shear modulus of μ = 32 GPa, the seismic moment derived from the distributed slip model was about 7.85 × 1018 Nm, equivalent to Mw 6.54, which was slightly larger than that from the focal mechanism solutions. The fault spatial geometry and slip distribution could be further validated with the spatial patterns of the immediate aftershocks. Most of the off-fault aftershocks with the magnitude > M2 within one year after the mainshock occurred in the stress positive stress change area, which coincided with the stress triggering theory. The static Coulomb stress, triggered by the mainshock, significantly increased at the Tazang fault (northwest to the epicenter), and at the hidden North Huya fault, and partial segments of the Minjiang fault (west of the epicenter).

Rupture process of the 2015 Pishan earthquake from joint inversion of InSAR, teleseismic data and GPS

An earthquake of M w6.4 occurred in Pishan County in Xinjiang Province, northwestern Tibetan Plateau, on July 3, 2015. The epicenter was located on an active blind thrust system located at the northern margin of the Western Kunlun Mountain Orogenic Belt southwest of the Tarim Basin. We constructed a shovel-shaped fault model based on the layered-crust model with reference to the seismic reflection profile, and obtained the rupture process of the earthquake from the joint inversion of Interferometric Synthetic Aperture Radar (InSAR) measurements, far-field waveform data, and Global Positioning System (GPS) data. The results show that the seismic fault dips southward with a strike of 109°, and the rupture direction was essentially northward. The fault plane rupture distribution is concentrated, with a maximum recorded slip of 73 cm. The main features of the fault are as follows: low inclination angle (25°–10°), thrust slip at a depth of 9–13 km, rupture propagation time of about 12 s, no significant slip in soft or hard sedimentary layers at 0–4 km depth and propagation from the initial rupture point to the surrounding area with no obvious directionality. The InSAR time-series analysis method is used to determine the deformation rate in the source region within 2 years after the earthquake, and the maximum value is ~17 mm yr−1 in the radar line-of-sight direction. Obvious post-earthquake deformation is evident in the hanging wall, with a similar trend to the coseismic displacement field. These results suggest that the Pishan earthquake has not completely released the accumulated energy of the region, given that the multilayer fold structure above the blind fault is still in a process of slow uplift since the earthquake. Post-earthquake adjustment models and aftershock risk analysis require further study using more independent data.

Development of Detachment Folds in the Mexican Ridges Foldbelt, Western Gulf of Mexico Basin

AbstractExamples of natural folds growing in a homogenous mechanical stratigraphy of alternating competent and incompetent thin layers of fine‐ and coarse‐grained sediments are examined, and the fold growth process is quantified. Our analysis reveals that the overall response to loading of siliciclastic sequences corresponds to that of flexural flow and parallel‐to‐bedding heterogeneous pure shear. Folds start out as low‐amplitude sinusoidal disturbances that rapidly become finite‐amplitude folds of heterogeneous strain. We also derive the following scaling relations: (i) degree of amplification scales with both the height above the detachment and strain, (ii) wavelength selectivity broadens with increasing strain, and (iii) deposition of syn‐sedimentary geometries is function of strain. These relations are a natural consequence of idealized area‐preserving laws of fold growth. From these results we devise a method to estimate fold strain by means of an amplitude versus depth diagram. We are also able to define a progression of fold shape change as a function of the fundamental parameter strain. Initially, structures grow by limb rotation and the selective amplification of a single dominant wavelength giving rise to sinusoidal folds. When strain reaches ~8%, softening/plastic yielding around hinges results in the development of sharp fold profiles. Limbs lock their dips at 35°–45°, suggesting that growth in this stage is permitted by hinge mobility along ramps and blind faults. Moreover, hinge migration causes fold development to accelerate spontaneously. These findings suggest that conclusions relating periods of accelerated erosion/uplift in contractional structures to tectonic processes should be treated with caution.

Making EEMD more effective in extracting bearing fault features for intelligent bearing fault diagnosis by using blind fault component separation

Making EEMD more effective in extracting bearing fault features for intelligent bearing fault diagnosis by using blind fault component separation

Architecture, deformation style and petrophysical properties of growth fault systems: the Late Triassic deltaic succession of southern Edgeøya (East Svalbard)

AbstractThe Late Triassic outcrops on southern Edgeøya, East Svalbard, allow a multiscale study of syn‐sedimentary listric growth faults located in the prodelta region of a regional prograding system. At least three hierarchical orders of growth faults have been recognized, each showing different deformation mechanisms, styles and stratigraphic locations of the associated detachment interval. The faults, characterized by mutually influencing deformation envelopes over space‐time, generally show SW‐ to SE‐dipping directions, indicating a counter‐regional trend with respect to the inferred W‐NW directed progradation of the associated delta system. The down‐dip movement is accommodated by polyphase deformation, with the different fault architectural elements recording a time‐dependent transition from fluidal‐hydroplastic to ductile‐brittle deformation, which is also conceptually scale‐dependent, from the smaller‐ (3rd order) to the larger‐scale (1st order) end‐member faults respectively. A shift from distributed strain to strain localization towards the fault cores is observed at the meso to microscale (<1 mm), and in the variation in petrophysical parameters of the litho‐structural facies across and along the fault envelope, with bulk porosity, density, pore size and microcrack intensity varying accordingly to deformation and reworking intensity of inherited structural fabrics. The second‐ and third‐order listric fault nucleation points appear to be located above blind fault tip‐related monoclines involving cemented organic shales. Close to planar, through‐going, first‐order faults cut across this boundary, eventually connecting with other favourable lower‐hierarchy fault to create seismic‐scale fault zones similar to those imaged in the nearby offshore areas. The inferred large‐scale driving mechanisms for the first‐order faults are related to the combined effect of tectonic reactivation of deeper Palaeozoic structures in a far field stress regime due to the Uralide orogeny, and differential compaction associated with increased sand sedimentary input in a fine‐grained, water‐saturated, low‐accommodation, prodeltaic depositional environment. In synergy to this large‐scale picture, small‐scale causative factors favouring second‐ and third‐order faulting seem to be related to mechanical‐rheological instabilities related to localized shallow diagenesis and liquidization fronts.

Seismic imaging of the Main Frontal Thrust in Nepal reveals a shallow décollement and blind thrusting

Because great earthquakes in the Himalaya have an average recurrence interval exceeding 500 yr, most of what we know about past earthquakes comes from paleoseismology and tectonic geomorphology studies of the youngest fault system there, the Main Frontal Thrust (MFT). However, these data are sparse relative to fault segmentation and length, and interpretations are often hard to validate in the absence of information about fault geometry. Here, we image the upper two km of strata in the vicinity of the fault tip of the MFT in central Nepal (around the town of Bardibas) applying a pre-stack migration approach to two new seismic reflection profiles that we interpret using quantitative fault-bend folding theory. Our results provide direct evidence that a shallow décollement produces both emergent (Patu thrust) and blind (Bardibas thrust) fault strands. We show that the décollement lies about 2 km below the land surface near the fault tip, and steps down to a regional 5 km deep décollement level to the north. This implies that there is significant variation in the depth of the décollement. We demonstrate that some active faults do not reach the surface, and therefore paleoseismic trenching alone cannot characterize the earthquake history at these locations. Although blind, these faults have associated growth strata that allow us to infer their most recent displacement history. We present the first direct evidence of fault dip on two fault strands of the MFT at depth that can allow terrace uplift measurements to be more accurately converted to fault slip. We identify a beveled erosional surface buried beneath Quaternary sediments, indicating that strath surface formation is modulated by both climate-related base level changes and tectonics. Together, these results indicate that subsurface imaging, in conjunction with traditional paleoseismological tools, can best characterize the history of fault slip in the Himalaya and other similar thrust fault systems.