Blind Fault Research Articles

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.

Sentinel-1 observation of the 2017 Sangsefid earthquake, northeastern Iran: Rupture of a blind reserve-slip fault near the Eastern Kopeh Dagh

New satellites are now revealing InSAR-based surface deformation within a week after natural hazard events. Quick hazard responses will be more publically accessible and provide information to responding agencies. Here we used Sentinel-1 interferometric synthetic aperture radar (InSAR) data to investigate coseismic deformation associated with the 2017 Sangsefid earthquake, which occurred in the southeast margin of the Kopeh Dagh fault system. The ascending and descending interferograms indicate thrust-dominated slip, with the maximum line-of-sight displacement of 10.5 and 13.7 cm, respectively. The detailed slip-distribution of the 2017 Sangsefid Mw6.1 earthquake inferred from geodetic data is presented here for the first time. Although the InSAR interferograms themselves do not uniquely constrain what the primary slip surface is, we infer that the source fault dips to southwest by analyzing the 2.5 D displacement field decomposed from the InSAR observations. The determined uniform slip fault model shows that the dip angle of the seimogenic fault is approximately 40°, with a strike of 120° except for a narrower fault width than that predicted by the empirical scaling law. We suggest that geometric complexities near the Kopeh Dagh fault system obstruct the rupture propagation, resulting in high slip occurred within a small area and much higher stress drop than global estimates. The InSAR-determined moment is 1.71 × 1018 Nm with a shear modulus of 3.32 × 1010 N/m2, equivalent to Mw 6.12, which is consistent with seismological results. The finite fault model (the west-dipping fault plane) reveals that the peak slip of 0.90 m occurred at a depth of 6.3 km, with substantial slip at a depth of 4–10 km and a near-uniform slip of 0.1 m at a depth of 0–2.5 km. We suggest that the Sangsefid earthquake occurred on an unknown blind reverse fault dipping southwest, which can also be recognised through observing the long-term surface effects due to the existence of the blind fault.

Coseismic landslides triggered by the 8th August 2017 Ms 7.0 Jiuzhaigou earthquake (Sichuan, China): factors controlling their spatial distribution and implications for the seismogenic blind fault identification

On 8th August 2017, a magnitude Ms 7.0 earthquake struck the County of Jiuzhaigou, in Sichuan Province, China. It was the third Ms ≥ 7.0 earthquake in the Longmenshan area in the last decade, after the 2008 Ms 8.0 Wenchuan earthquake and the 2013 Ms 7.0 Lushan earthquake. The event did not produce any evident surface rupture but triggered significant mass wasting. Based on a large set of pre- and post-earthquake high-resolution satellite images (SPOT-5, Gaofen-1 and Gaofen-2) as well as on 0.2-m-resolution UAV photographs, a polygon-based interpretation of the coseismic landslides was carried out. In total, 1883 landslides were identified, covering an area of 8.11 km2, with an estimated total volume in the order of 25–30 × 106 m3. The total landslide area was lower than that produced by other earthquakes of similar magnitude with strike-slip motion, possibly because of the limited surface rupture. The spatial distribution of the landslides was correlated statistically to a number of seismic, terrain and geological factors, to evaluate the landslide susceptibility at regional scale and to identify the most typical characteristics of the coseismic failures. The landslides, mainly small-scale rockfalls and rock/debris slides, occurred mostly along two NE-SW-oriented valleys near the epicentre. Comparatively, high landslide density was found at locations where the landform evolves from upper, broad valleys to lower, deep-cut gorges. The spatial distribution of the coseismic landslides did not seem correlated to the location of any known active faults. On the contrary, it revealed that a previously-unknown blind fault segment—which is possibly the north-western extension of the Huya fault—is the plausible seismogenic fault. This finding is consistent with what hypothesised on the basis of field observations and ground displacements.

Estimation of Stress/Strain State of Tectonic Structures Using V P /V S Ratios: A Case Study of Seismically Active Zones of the Greater Caucasus, Kura Depression, Transcaucasia, and the Western Caspian Region

Earthquakes, which result from tectonic movements within the bowels of the Earth, reflect spatiotemporal changes in the stress field of the geologic medium. Weak regional earthquakes indicate the activation of some elements of seismotectonic zones and preparation of strong earthquakes. The travel times of Pand S-waves and V P /V S ratios are also a source of information on the regional tectonic setting; sometimes this information helps to reveal blind faults. The paper analyzes local earthquakes in a seismically active region, which comprises the Greater Caucasus, the Kura Depression, Transcaucasia, and the western Caspian Region. Based on a large database of hypocenter parameters, P- and S-wave travel times, and areal V P /V S ratio distributions, we identified seismotectonic zones with characteristic anomalous features and outlined gradient zones where V P /V S anomalies reverse sign. It is found that the V P /V S ratio field complies with the regional stress field and deep seismotectonic setting. Zones where anomalies reverse sign (gradient zones) are most likely confined to blind faults. Areas of the high gradient of the field can be put down to blind tectonic structures (faults) or the incipient sources of future strong earthquakes. V P /V S ratio monitoring and mapping will make it possible to identify the source zones of forthcoming earthquakes.

Geodetic Evidence for a Blind Fault Segment at the Southern End of the San Jacinto Fault Zone

AbstractThe San Jacinto Fault (SJF) splits into several active branches southeast of Anza, including the Clark fault and the Coyote Creek fault. The Clark fault, originally believed to terminate at the southern tip of the Santa Rosa Mountains, was suggested to extend further to the southeast to a junction with the Superstition Hills fault based on space geodetic observations and geologic mapping. We present new interferometric synthetic aperture radar and GPS data that confirm high deformation rates along the southeastern extent of the Clark fault. We derive maps of horizontal and vertical average velocities by combining data from the ascending and descending satellite orbits with an additional constraint provided by the azimuth of the horizontal component of secular velocities from GPS data. The resulting high‐resolution surface velocities are differentiated to obtain a map of maximum shear strain rate. Joint inversions of InSAR and GPS data suggest that the hypothesized blind segment of the Clark fault and the Coyote Creek fault have slip rates of 13 ± 3 mm/yr and 5 ± 4 mm/yr, respectively. The blind southern segment of the Clark fault thus appears to be the main active strand of the SJF, posing a currently unrecognized seismic hazard.

Geological and geophysical evidences of the polyphase structural evolution of the Southern Precordillera (31°42′S-69°24′W), central-western Argentina

The evolution of the western margin of South America comprised different tectonic regimes responsible of the present geological composition and structure of the Central Andes. Remote sensing, geological and geophysical data (aeromagnetic and seismological data) have been processed to understand the polyphase structural evolution of the Argentine Southern Precordillera. The outcrops exposed in the Sierra de Barreal (31°42′S-69°24′W) show evidences of a complex structural evolution related to the superposition of three orogenies from the Early Paleozoic to nowadays. The Chanic orogeny (Late Devonian) is characterized by two systems of non-coaxial ductile folds that affect Early Paleozoic rocks of the Ciénaga del Medio Group. The recognized rotation in the shortening direction (from NW-SE to ENE-WSW) is interpreted here as a result of a sinistral transpressive regime. The San Rafael orogeny (Early Permian) is represented by back-thrusts and blind faults associated to asymmetric folding that involves the deformation of Upper Carboniferous rocks (Majaditas Formation). Finally, the compressive Andean orogeny associated to the Pampean flat-slab subduction segment has been locally controlled by inherited pre-Andean anisotropies. This is evidenced by: 1-the structural style of western-southern Argentine Precordillera, which in the study area is represented by faulted blocks and top-to-west vergent back-thrusts; 2- NW-SE deflections and associated magnetic anomalies in the Argentine Precordillera fold-and-thrust belt; 3-neotectonic structures developed in a sinistral transpressive system; 4-the crustal seismicity associated to NW-SE trending structures with left-lateral component motion.

Seismotectonics of the 2013 Lushan Mw 6.7 earthquake: Inversion tectonics in the eastern margin of the Tibetan Plateau

AbstractOn 20 April 2013, an unexpected Mw 6.7 earthquake occurred in Lushan County at the southern Longmen Shan, the eastern margin of the Tibetan Plateau. A high‐resolution seismic reflection profile was combined with near‐surface geological data, earthquake relocation and geodetic measurements, and a recent deep artificial seismic reflection profile to identify the active fault and seismotectonics of this earthquake. Three‐dimensional imaging of the aftershocks was used to identify two planar faults that together form a y shape (f1 and f2). Seismic interpretations suggest that fault f1 did not break through the overlying Mesozoic and Cenozoic rocks and is a typical blind fault. Geodetic measurements suggest that the coseismic deformation is consistent with the geometry and kinematics of shear fault‐bend folding. Deep seismic data indicate the syndepositional nature of fault f1 a preexisting normal fault older than the Triassic, which underwent positive inversion tectonics during the Late Cenozoic. A thrust fault f3 converges with f1 at a depth of approximately 12 km with an accumulated slip 3.6 km. This 2013 Lushan earthquake triggered by blind faults is a hidden earthquake. Blind and reactivated faults increase the potential risk and uncertainty related to earthquakes in the eastern margin of the Tibetan Plateau.

How do horizontal, frictional discontinuities affect reverse fault-propagation folding?

The development of new reverse faults and related folds is strongly controlled by the mechanical characteristics of the host rocks. In this study we analyze the impact of a specific kind of anisotropy, i.e. thin mechanical and frictional discontinuities, in affecting the development of reverse faults and of the associated folds using physical scaled models. We perform analog modeling introducing one or two initially horizontal, thin discontinuities above an initially blind fault dipping at 30° in one case, and 45° in another, and then compare the results with those obtained from a fully isotropic model. The experimental results show that the occurrence of thin discontinuities affects both the development and the propagation of new faults and the shape of the associated folds. New faults 1) accelerate or decelerate their propagation depending on the location of the tips with respect to the discontinuities, 2) cross the discontinuities at a characteristic angle (∼90°), and 3) produce folds with different shapes, resulting not only from the dip of the new faults but also from their non-linear propagation history. Our results may have direct impact on future kinematic models, especially those aimed to reconstruct the tectonic history of faults that developed in layered rocks or in regions affected by pre-existing faults.

Analyzing the Fault Injection Sensitivity of Secure Embedded Software

Fault attacks on cryptographic software use faulty ciphertext to reverse engineer the secret encryption key. Although modern fault analysis algorithms are quite efficient, their practical implementation is complicated because of the uncertainty that comes with the fault injection process. First, the intended fault effect may not match the actual fault obtained after fault injection. Second, the logic target of the fault attack, the cryptographic software, is above the abstraction level of physical faults. The resulting uncertainty with respect to the fault effects in the software may degrade the efficiency of the fault attack, resulting in many more trial fault injections than the amount predicted by the theoretical fault attack. In this contribution, we highlight the important role played by the processor microarchitecture in the development of a fault attack. We introduce the microprocessor fault sensitivity model to systematically capture the fault response of a microprocessor pipeline. We also propose Microarchitecture-Aware Fault Injection Attack (MAFIA). MAFIA uses the fault sensitivity model to guide the fault injection and to predict the fault response. We describe two applications for MAFIA. First, we demonstrate a biased fault attack on an unprotected Advanced Encryption Standard (AES) software program executing on a seven-stage pipelined Reduced Instruction Set Computer (RISC) processor. The use of the microprocessor fault sensitivity model to guide the attack leads to an order of magnitude fewer fault injections compared to a traditional, blind fault injection method. Second, MAFIA can be used to break known software countermeasures against fault injection. We demonstrate this by systematically breaking a collection of state-of-the-art software fault countermeasures. These two examples lead to the key conclusion of this work, namely that software fault attacks become much more harmful and effective when an appropriate microprocessor fault sensitivity model is used. This, in turn, highlights the need for better fault countermeasures for software.

Primary surface rupture of the 1950 Tibet-Assam great earthquake along the eastern Himalayan front, India

The pattern of strain accumulation and its release during earthquakes along the eastern Himalayan syntaxis is unclear due to its structural complexity and lack of primary surface signatures associated with large-to-great earthquakes. This led to a consensus that these earthquakes occurred on blind faults. Toward understanding this issue, palaeoseismic trenching was conducted across a ~3.1 m high fault scarp preserved along the mountain front at Pasighat (95.33°E, 28.07°N). Multi-proxy radiometric dating employed to the stratigraphic units and detrital charcoals obtained from the trench exposures provide chronological constraint on the discovered palaeoearthquake surface rupture clearly suggesting that the 15th August, 1950 Tibet-Assam earthquake (Mw ~ 8.6) did break the eastern Himalayan front producing a co-seismic slip of 5.5 ± 0.7 meters. This study corroborates the first instance in using post-bomb radiogenic isotopes to help identify an earthquake rupture.

INSAR AND GPS EARTHQUAKE SOURCE PARAMETER INVERSION FOR THE 2016 Mw6.4 MEINONG, TAIWAN EARTHQUAKE

AbstractOn February 6, 2016, an Mw6.4 earthquake struck the Meinong District of Kaohsiung city in Taiwan, China. Various researches have been conducted on the earthquake. Most of these researches are based on seismic data and no consensus has been reached on the fault structure and focal parameters yet. Surface displacement obtained by interferometric synthetic aperture radar (InSAR) is widely used in earthquake studies because of its high resolution and accuracy with large and continuous coverage. Therefore, this study selects InSAR and GPS data to investigate the focal mechanism and slip distribution of the 2016 Meinong earthquake. Using the dual‐track differential InSAR (D‐InSAR) technology, we extract the coseismic deformation field of this earthquake with SAR data (both the ascending and descending) from satellite ALOS2 and the ascending data from satellite Sentinal‐1A. The results show that the maximum deformation occurs in the west of the epicenter, with an uplift around 11.2 cm.The uniform dislocation model and multiple peak particle swarm optimization (MPSO) algorithm are employed to determine the fault geometry parameters of this earthquake based on the InSAR and GPS data. The results show that the rupture is a reverse fault with sinistral strike‐slip with the average slip angle of 51.5°. The seismic source is at 22.920°N, 120.420°E, and a depth of 12 km. The rupture plane is about 15 km long with a strike angle of 307° and a dip angle of 16.5°. The optimal dip angle (15.7°), weighting ratio (18:1) between GPS and InSAR and the smoothing factor (0.06) obtained by the grid iteration method together with the non‐uniform model and the non‐negative least squares method are used to obtain the detailed slip distribution. The results show that the maximum value of dip slip and strike slip are 51.7 cm and 55.3 cm, respectively. The moment magnitude of the non‐uniform model is Mw6.38, slightly smaller than the result of GCMT (Mw6.4). The comparison between our research and previous research and the analysis of the regional faults indicate that a single fault geometry is more reasonable and it can fit both the GPS and InSAR data well. We also find that the ruptured fault is a blind fault located in the area among Zouchen fault and Chishan fault with an ES‐WN strike and dipping toward ES. So we believe this fault should have some relation with the 2010 Mw6.3 Jiashian earthquake.

Influence of a Sub-Seismic Blind Faults on the Safety Assessment of CO2 Geological Storage in Deep Saline Aquifer

Within the European context, CO2 storage operations shall address the potential impacts of largescale CO2 storage through risk assessment. The key risks identified for this onshore CO2 storage site were the migration through sub-seismic blind faults.To quantify the CO2 migration along such sub-seismic blind fault, the impact of the fault is investigated both for its mechanical and flow contribution.In this study, a blind sub-seismic fault was assumed near a CO2 injection well. The study area is in the eastern part of the Paris Basin and targeting the Lower Triassic Sandstone formation which is deemed adequate for CO2 injection. The arbitrary geometry of the fault (with limited throw c.a. 1 m), was assumed across the expected migration pathway of the injected CO2. The fault is assumed to extend vertically between the storage aquifer and a control aquifer.The flow-geomechanics model coupled with an uncertainties management computes the failure probability, i.e. the probability of mechanical failure in the fault vicinity. Such probability of failure is characterized by low to very low probability of occurrence which requires a large number of simulations to enable its evaluation. Each failure scenario models the CO2 migration from a storage horizon to a control aquifer when altering the mechanical parameters of the fault damage zones and fault core. To limit CPU requirements associated with the flow-geomechanics model, it is replaced by a fast surrogate model in order to evaluate the numerous simulations needed to evaluate the probability of failure. They show that limited CO2 migration is occurring within the fault but no breakthrough in the control aquifer. The injection induces limited pressure disturbance resulting in small ground level heave in the order of a millimeter at the end of the 30-year injection period at an average rate of about 0.8 Mtpa.For each geological layer, several uncertain mechanical parameters were considered to illustrate the approach: the permeability, the young modulus and Poisson ratio of the core and damaged zones of the fault, the shale content within the fault core, the critical gas saturation and capillary entry pressure of the fault damaged zones. The key response of interest was the safety factor based upon the Mohr-Coulomb failure criteria.An initial local sensitivity analysis identified that only four of the 18 uncertain input variables have a significant influence on the response of the model. A Sobol design of experiments was then launched with the four remaining parameters in order to construct a surrogate model. Gaussian process surrogates were chosen because of their capability of giving an approximation of the prediction error, which is useful to assess the quality of the surrogate along the domain of interest. Furthermore, the error of prediction can also be taken into account when computing probabilities. From this surrogate, it was concluded that there is a strong influence of the mechanical integrity of the base of the sub-seismic blind fault (Sobol Sensitivity Indices). Based on the surrogate model of the mechanical safety factor (elastic behavior i.e. no loss of integrity), a Monte-Carlo experiment was used to evaluate the probability of mechanical failure of the fault at the base of the storage cap rock.The most influential parameter on the mechanical safety factor is the anisotropy ratio between horizontal stresses. Within the considered domain of variations of the parameter, the unsafe domain (mechanical failure) may be reached at the upper bound of the uncertain domain and the failure probability was estimated to be lower than 10-3 for this case. A confirmation run with the detail numerical model at the upper bound of the uncertain domain confirmed that the mean behavior is inside the safe domain. Consequently, the uncertainty of the surrogate model was further reduced and showed no failure within the uncertain domain (failure probability below 10-6 for this case): the mechanical failure of the base of the sub-seismic blind fault is very unlikely.

Multiple geophysical methods examining neotectonic blind structures in the Maradona valley, Central Precordillera (Argentina)

A high-resolution superficial geophysical study was carried out in an area of the retroarc region of the Andes mountains, located in the southwest of San Juan Province (31°45′ S, 68°50′ W), Central Precordillera of Argentina. The main objectives of this study were to confirm the presence of blind neotectonic structures and characterize them by observing variations in magnetic susceptibility, density and p-wave velocities. Geological evidence demonstrates the existence of a neotectonic fault scarps affecting Quaternary alluvial deposits in eastern piedmont of de Las Osamentas range, in addition to direct observation of the cinematic of this feature in several natural exposures. The Maradona valley is characterized by the imbricated eastern-vergence Maradona Fault System that uplifts Neogene sedimentary rocks (Albarracín Formation) over Quaternary (Late Pleistocene-Holocene) alluvial deposits. The combined application of different geophysical methods has allowed the characterization of a blind fault geometry also identified on a natural exposure. The magnetic data added to the gravimetric model, and its integration with a seismic profile clearly shows the existence of an anomalous zone, interpreted as uplifted blocks of Miocene sedimentary rocks of Formation Albarracín displaced over Quaternary deposits. The application and development of different geophysical methods, together with geological studies allow to significantly improving the knowledge of an area affected by Quaternary tectonic activity. Finally, this multidisciplinary study, applied in active blind structures is very relevant for future seismic hazard analysis on areas located very close to populated centers.

A 3D structural analysis of the Goliat field, Barents Sea, Norway

The Goliat field consists of Middle to Late Triassic reservoirs which exploit an elongate anticline (the Goliat anticline) in the hanging wall of the Troms-Finnmark Fault Complex (TFFC), offshore Norway. The area is affected by a dense network of multiple trending fault populations which historically have inhibited seismic resolution owing to persistent fault shadow. Seismic investigations utilising a multi-azimuth three-dimensional survey (EN0901) allow much crisper delineation of seismic features previously unattainable by vintage single-azimuth surveys. Three dominant fault populations are identified in the area, two of which parallel TFFC segments, the Alke–Goliat (WSW–ENE) and the Goliat–Tornerose (NNE–SSW) segments. The Goliat field is located within a zone of intersection between both segments. A third E–W trending fault population, the Hammerfest Regional population, is likely influenced by the offshore extension of the Trollfjord-Komagelv Fault Complex (TKFZ). A local NW–SE trending fault population, the Goliat Central, affects the Goliat anticline and partitions Alke–Goliat and Goliat–Tornerose subsidiary faults resulting in curvilinear traces. Several cross-cutting relationships between fault populations are observed and may provide fluid compartmentalisation in the reservoirs. Compilation of regional transects and the EN0901 survey provides new insight into the evolution of the Goliat anticline which is underlain by a fault-bound basement terrace that became established in the Late Palaeozoic. The structure is interpreted to have formed due to vertical segmentation of the TFFC and cores the overlying broad anticline. The western limb of the Goliat anticline likely formed by differential compaction, whereas the eastern limb is primarily a result of hanging wall roll-over linked to variable listric to ramp-flat-ramp fault geometry. Rifting took place in the Palaeozoic (Carboniferous to Permian?), and in the Mesozoic, possibly as early as the Late Triassic, with a major event in the Late Jurassic to Early Cretaceous. Minor reactivations continued into the Late Cretaceous, and possibly the Early Cenozoic. Mesozoic syn-kinematic geometries in the hanging wall of the Goliat–Tornerose TFFC segment are consistent with deposition during up section propagation of a blind fault, over which, a monocline was established and later breached. Jogs (abrupt orientation changes) in fault traces, transverse folds (associated with displacement maxima/minima) and vertical fault jogs suggest the TFFC existed as a greater number of segments prior to amalgamation during the Late Triassic to Jurassic. A phase of Barremian inversion created local compression structures above blind extensional faults, and deeper seated buttressing against large faults. Polygonal faults affect the Late Cretaceous to Early Cenozoic successions.

Quaternary deformation in the Gorubathan recess: Insights on the structural and landscape evolution in the frontal Darjiling Himalaya

The Gorubathan recess in the Darjiling frontal Himalaya is an unusual and unique region in the Himalaya as the Ramgarh thrust (RT) rather than the MFT defines the mountain front and the width of the Himalayan arc or the N-S aerial distance between the foreland (27.0947°N, 88.8744°E) and the Tibet Plateau (27.6781°N, 89.1175°E) is merely ∼70 km here. South of the mountain front, blind faults have deformed several Quaternary alluvial fans and formed scarps over a ∼35 km N-S zone. We first used field observations and fold-thrust belt concepts to re-interpret thrust nomenclature and geometry in the recess by correlating structures and lithology with the adjacent Dharan salient across the salient-recess separating Gish Transverse fault (GTF) and renamed the locally recognized faults to their regional equivalents. We then used Real Time Kinematic Global Navigation Satellite Systems (RTKGNSS) corrected SRTM 30 m C-Band data to compute geomorphic indices along with their uncertainties. Our results indicated that neotectonic deformation has generated an active but mature landscape in the Gorubathan recess. Quantitative topographic profiles based on RTKGNSS measurements reveal neotectonic warping and formation of geomorphic scarps on the fan surfaces. Geomorphic scarps form due to motion along thrusts but are not actual fault scarps. These profiles were used to model topographic growth and perturbation of fan slopes by active thrusting and fault-propagation folding by Boundary Element Method based dislocation modelling. Results indicate activity through much of the Quaternary on the Ramgarh thrust, a hanging wall imbricate and a rejoining splay in its footwall as well as the Main Boundary thrust. Comparison of modelled and measured topographic profiles in the recess points to erosion of the fault-related scarp-tops in the deformed alluvial fans on the windward side of the SW monsoon in both the Gorubathan and the Samsing-Matiali fans. Our integrated methodology improves the understanding of coupled fault-generated deformation and topographic growth and may be applied across the entire Himalayan front.

Numerical simulation study on the impact of the Wenchuan earthquake to the seismic environment of the Lushan earthquake

The finite element numerical simulation method was used to investigate the impact of the Wenchuan earthquake on the seismic environment of the southern Longmenshan Mountain. The three-dimensional viscoelastic geological model, including the Longmenshan front-range fault, central fault, back-range fault, the Bayan Kala block, and the South China block, was constrained by the results of the geological survey, coseismic dislocation, and the GPS observation of Wenchuan and Lushan earthquakes. We studied how the calculation results were influenced by gravity potential energy caused by high altitude topography, the low velocity layer in the middle crust and the rupture of the Beichuan-Yingxiu and Jiangyou-Guanxian faults, by modifying the parameters of a model. The Wenchuan earthquake was simulated by reducing the Young’s modulus of the possible generating fault. We calculated the Coulomb stress of the Dayi blind fault, Dachuang-Shuangshi fault, and the “Y”-shaped blind thrust fault between the two faults above. The simulation results show that the Coulomb stress has a similar distribution on the fault planes of the southern Longmenshan faults zone caused by the Wenchuan earthquake, considering the low-velocity layer or the double-rupture surface. The Coulomb stress on the top of the Dachuan-Shuangshi fault surface was about 0 MPa, and it gradually reduced towards the southwest direction. At the source of the Lushan earthquake, the Coulomb stress was about −0.02 MPa. For the west branch of “Y”-shaped fault, the Coulomb stress on the fault surface was significantly reduced and was about −0.02–−0.03 MPa at the source of the Lushan earthquake; after the Wenchuan earthquake, the Coulomb stress on the east branch of the “Y”-shaped fault plane was about −0.01–0.03 MPa; for the Dayi fault, the Coulomb stress was increasingly obvious on the fault surface from the southwest to the northwest after the Wenchuan earthquake. Therefore, the Wenchuan earthquake could reduce the risk of an earthquake on the Dachuan-Shuangshi fault and the west branch of “Y”-shaped fault earthquake, but it increases the risk on the east branch of “Y”-shaped fault and the Dayi fault. The maximum Coulomb stress near the source was located on the Dayi fault. Considering the terrain and the simultaneous failure of the Beichuan-Yingxiu and Jiangyou-Guanxian, the presence of the low velocity layer increases the Coulomb stress on the four faults, and obviously increases towards the east branch of the “Y”-shaped fault and the Dayi fault. If the terrain and the low velocity layer are considered, compared with only the ruptured Beichuan-Yingxiu fault, the Beichuan-Yingxiu and Jiangyou-Guanxian faults’ simultaneous rupture increased the risk of earthquakes on the Dachuan-Shuangshi fault, the east branch of “Y”-shaped fault, and the Dayi hidden fault, but reduced the risk of the west branch of “Y” type fault. In any of the above models, the Coulomb stress changes on the four fault planes of southern Longmenshan Mountains were influenced by terrain. In the case of certain crust, mantle medium properties, and spatial structure, the existence of the low velocity layer on the west of the Longmenshan fault zone, the topographic features of the Qinghai-Tibet Plateau to the Sichuan Basin, and the conditions for the simultaneous rupture of the central fault and the front-range fault during the Wenchuan earthquake, are the important factors that affect the Wenchuan earthquake to the Lushan earthquake. Among them, the terrain and low velocity layer have the most significant influence on the Coulomb failure stress for the area around the Lushan earthquake area caused by the Wenchuan earthquake. The effect of terrain features is more obvious in the north section of the selected section, yet the effect of the Wenchuan earthquake simultaneous rupture planes is smaller.

Modelling of the Kopili Fault based on slip rate, moment rate and seismic activity in Mikir Hills Plateau of Northeastern India

ABSTRACTThe recurrence period of maximum magnitude earthquake in a seismogenically active formation along the Kopili Fault has been estimated having adequate dependence on the slip rate, moment rate and its seismicity pattern. Here, the frequency–magnitude cum fault area–maximum magnitude relations play a key role with input parameters pertinent to the Kopili Fault zone. Subsequently, where the fault slip rate estimates are not available, the seismic activity is studied from the seismic moment release. The results of this study show that the return period has a strong relation with the fault length, slip rate, strain drop and rigidity. This study ascertains the activity rate in terms of the return period as ∼50 ± 5 years with the moment release of 2.12E+23 dyne-cm from the most active 80-km fault length considering Mw 5.5 as reference magnitude under the Kopili Fault zone that may produce a maximum magnitude of Mw ∼ 7.6. Finally, we conclude that these models can be used to study the rate of seismicity of the active faults in Northeast India which will provide us prime inputs for seismic hazard analysis of the region and is especially significant for estimating expected return period for poorly known faults or blind faults that lack surface expression.

Behavior of Abu-Jir Fault Zone in Al-Thirthar Valley and near Habbaniya Lake Areas – Comparative Study Using Seismic Reflection Sections

A comparative study of behavior of Abu-Jir fault zone in Al-Thirthar Valley and near Al-Habbaniya Lake areas has been carried out using seismic reflection sections. Interpretation of the seismic sections showed some similarities and differences in behavior of Abu-Jir Fault Zones in both areas. The seismic sections exhibit simple flower structure in the fault zone of both Al-Thirthar Valley and near Habbaniya Lake but the flower structure in later is simpler than the former. The fault zone in Al-Thirthar Valley behaves as blind fault, whereas it reaches to the Earth's surface near Habbaniya Lake. Abu-Jir Fault Zone near Habbaniya Lake is wider than that in Thirthar Valley area. Syn-rifting due to faulting and slightly thickening has begun within both areas; occur between Alan and Hartha reflectors in Thirthar area and between Alan and Dammam reflectors near Habbaniya Lake. Abu-Jir Fault Zone near Habbaniya Lake suffered to strike slip movement less than that in Thirthar Valley because it is far from the collision suture zone between Arabian plate and Turkish and Iranian plates.

A Comparative Study of Source Complexity of Two Moderate-Sized Earthquakes in Southern Taiwan: 2016 Mw6.4 and 2010 Mw6.2 Earthquakes

We study the complex rupture history of the 2016 Mw6.4 Meinong earthquake and the 2010 Mw6.2 Jiaxian earthquake in southern Taiwan by simultaneously inverting near-field strong-motion records, local and teleseismic broadband waveforms along with long-period surface waves. The focal mechanism results reveal that both earthquakes may rupture two different blind faults. The rupture of the 2016 event is dominated by the strike-slip motion with minor thrust faulting, while the 2010 event is a relatively high-angle thrust earthquake. Our preferred finite-fault source models suggest that the coseismic rupture history of the 2016 event should be relatively more complex than the 2010 event. During the 2016 event, two main patches of slip characterize the coseismic slip history. The cumulative seismic moment within 12 s of rupture is about 5.36 × 1018 N m, which is 1.9 times of the moment of the 2010 event. The slip of the 2016 event is associated with relatively larger perk slip of 0.7 m at shallower depth (~18–19 km), higher average slip of 0.3 m and faster slip rate with the maximum value of 3.4 m/s. Our kinematic models can shed some light on the cause of the difference in seismic hazard between these two earthquakes, and will be further applied to analyze the deep blind fault structure in southern Taiwan.

Multi-Stack Persistent Scatterer Interferometry Analysis in Wider Athens, Greece

The wider Athens metropolitan area serves as an interesting setting for conducting geodetic studies. On the one hand, it has a complex regional geotectonic characteristic with several active and blind faults, one of which gave the deadly M w 5.9 Athens earthquake on September 1999. On the other hand, the Greek capital is heavily urbanized, and construction activities have been taking place in the last few decades to address the city’s needs for advanced infrastructures. This work focuses on estimating ground velocities for the wider Athens area in a period spanning two decades, with an extended spatial coverage, increased spatial sampling of the measurements and at high precision. The aim is to deliver to the community a reference geodetic database containing consistent and robust velocity estimates to support further studies for modeling and multi-hazard assessment. The analysis employs advanced persistent scatterer interferometry methods, covering Athens with both ascending and descending ERS-1, ERS-2 and Envisat Synthetic Aperture Radar data, forming six independent interferometric stacks. A methodology is developed and applied to exploit track diversity for decomposing the actual surface velocity field to its vertical and horizontal components and coping with the post-processing of the multi-track big data. Results of the time series analysis reveal that a large area containing the Kifisia municipality experienced non-linear motion; while it had been subsiding in the period 1992–1995 (−12 mm/year), the same area has been uplifting since 2005 (+4 mm/year). This behavior is speculated to have its origin on the regional water extraction activities, which when halted, led to a physical restoration phase of the municipality. In addition, a zoom in the area inflicted by the 1999 earthquake shows that there were zones of counter-force horizontal movement prior to the event. Further analysis is suggested to investigate the source and tectonic implications of this observation.