Critical Stress State Research Articles

Impact of particle elongation on the behavior of round and angular granular media: Consequences of particle rotation and force chain development

We developed a 2D discrete element model to simulate shearing response of elongated round and angular particles. Model was calibrated and validated by comparing numerical results with corresponding biaxial shearing tests on circular (round) and hexagonal (angular) rods at microscopic and macroscopic scales. Then, the systematic effect of particles aspect ratio was investigated on quasi-static shearing response of round and angular granular materials. Macroscopically, we observed a nonlinear tendency wherein as the aspect ratio decreased from 1, the critical state stress ratio initially increased, reaching a maximum, followed by a decreasing trend. This effect was more prominent in round samples. Microscopically, decreasing the aspect ratio from 1 reduced particle rotations and increased the mean coordination number. Elongated particles exhibited significant contact anisotropy, forming irregular force chains, facilitating interparticle sliding, and reducing overall strength. Additionally, we explored the impact of the interparticle friction coefficient. A high value of interparticle friction coefficient led to a monotonically increasing strength with elongation, underscoring the importance of accurately calibrating microscopic friction coefficients.

Frequency dependence of acoustic emission with particle interaction and failure process in dry sands during triaxial compression

To investigate the behaviour of sands at the particle-scale during shear failure, a series of triaxial compression tests was conducted on dry sands with varying confining stresses and relative densities. These tests were complemented by acoustic emission (AE) monitoring to analyse the frequency-dependent AE signals and characterise particle interactions in terms of crushing (>100 kHz) and rearrangement (<100 kHz). The results reveal a linear relationship between the relative breakage index (Br) and the logarithm of cumulative high-frequency AE hits, regardless of the relative density. With this, particle crushing, as well as particle rearrangement, is demonstrated to occur throughout the entire shearing. Particle crushing increases more rapidly to prevail from the very beginning to the general yield, thereafter at a slower increasing rate to become less predominant. Its evolution in the post-peak regime is related to the failure pattern, which accumulates at a constant and most rapid rate when the specimen is ‘barrelling’; however, it decays rapidly in the presence of shear banding and eventually reaches an equilibrium state with particle rearrangement once the critical stress state is attained. Accordingly, two types of approximate evolution trends are suggested to estimate particle crushing upon shearing depending on the failure patterns.

Inhibitory action of B-type natriuretic peptide on adrenocorticotropic hormone in patients with acute coronary syndrome.

In addition to the classical actions of hemodynamic regulation, natriuretic peptides (NPs) interact with various neurohumoral factors that are deeply involved in the pathophysiology of cardiovascular diseases. However, their effects on the hypothalamic-pituitary-adrenal (HPA) axis, which is activated under acute high-stress conditions in acute coronary syndrome (ACS), remain largely unknown. We investigated the impact of plasma B-type NP (BNP) on plasma adrenocorticotropic hormone (ACTH)-cortisol levels during the acute phase of ACS ischemic attacks. The study population included 436 consecutive patients with ACS for whom data were collected during emergency cardiac catheterization. Among them, biochemical data after acute-phase treatment were available in 320 cases, defined as the ACS-remission phase (ACS-rem). Multiple regression analyses revealed that plasma BNP levels were significantly negatively associated with plasma ACTH levels only during ACS attacks (P < 0.001), but not in ACS-rem, whereas plasma BNP levels were not significantly associated with plasma cortisol levels at any point. Accordingly, covariance structure analyses were performed to clarify the direct contribution of BNP to ACTH by excluding other confounding factors, confirming that BNP level was negatively correlated with ACTH level only during ACS attacks (β = -0.152, P = 0.002), whereas BNP did not significantly affect ACTH in ACS-rem. In conclusion, despite the lack of a significant direct association with cortisol levels, BNP negatively regulated ACTH levels during the acute phase of an ACS attack in which the HPA axis ought to be activated. NP may alleviate the acute stress response induced by severe ischemic attacks in patients with ACS.NEW & NOTEWORTHY BNP negatively regulates ACTH during a severe ischemic attack of ACS in which hypothalamic-pituitary-adrenal axis ought to be activated, indicating an important role of natriuretic peptides as a mechanism of adaptation to acute critical stress conditions in humans.

Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

AbstractThe in situ stress state within fault zones is technically challenging to characterize. At the Bedretto Underground Laboratory in the Swiss Alps, the breakouts observed in an array of eight inclined boreholes penetrating a fault zone offer a unique opportunity to characterize the fault‐associated spatial stress variations. Synthesizing multiple geophysical logs, natural geologic structures intersecting these boreholes are identified, revealing a hierarchy of a major fault zone along with secondary structures. Within the boreholes, breakout rotations occur over multiple scales, spanning individual fractures and the entire major fault zone. We first estimate and rule out the effect of the fracture‐induced anisotropy on the breakout rotations, which are attributed mainly to the stress variations. Based on the stress field around a circular borehole and Mohr‐Coulomb failure criterion, the observed breakout azimuths are used to invert the stress information. Results show that the stress field outside the fault zone features a stress ratio (quantifying the relative stress magnitude) of about 0.9, an inclined overburden stress (inclination: 12°∼18°), and a maximum horizontal principal stress (SHmax) oriented N100∼120°E. Within the fault zone, a substantial reduction of the stress ratio and complicated stress rotations are constrained, likely induced by the stress drop on local fractures. As a result, less critical stress state inside the major fault zone is expected. Our work provides a semi‐quantitative estimation of the in‐situ stress variations around fault zones in the absence of direct stress measurements, which is beneficial to a number of scientific and engineering applications.

Impact of the Food-Related Stress Conditions on the Expression of Enterotoxin Genes among Staphylococcus aureus.

Staphylococcus aureus is one of the most important foodborne pathogens. S. aureus has the capability to produce a variety of toxins, including staphylococcal enterotoxins (SEs). The aim of this study was to evaluate the survival rate of S. aureus cells and analyze enterotoxins gene expression after exposure to osmotic stress and acidic/alkaline stress. To determine survival rates, the traditional plate counting method and flow cytometry were used. The expression levels of the enterotoxin genes were performed by quantitative reverse transcription PCR (RT-qPCR). Expression changes differed depending on the stressors chosen. The obtained results in this study showed the effect of critical food-related stress conditions on SE gene expression in S. aureus. The study showed different expression levels of the tested enterotoxins genes depending on the stress. The most tested enterotoxin genes (seg, sei, and selo) after exposure to pH = 4.5 stress have similar expression as in the optimal condition. After alkaline treatment (pH = 9.6), a similar expression gene value as for the optimal condition was observed. The analysis of gene expression in response to stress caused by NaCl, showed that the expression of selp decreased, whereas selu, selm, and selo genes increased. A significantly decreased expression of the sea gene was observed.

Systematic effect of particle roundness/angularity on macro- and microscopic behavior of granular materials

Roundness/angularity is a vital shape descriptor that significantly impacts the mechanical response of granular materials and is closely associated with many geotechnical problems, such as liquefaction, slope stability, and bearing capacity. In this study, a series of biaxial shearing tests are conducted on dual-size aluminum circular and hexagonal rod material. A novel image analysis technique is used to estimate particle kinematics. A discrete element model (DEM) of the biaxial shearing test is then developed and validated by comparing it with the complete experimental data set. To systematically investigate the effect of roundness/angularity on granular behavior, the DEM model is then used to simulate eight non-elongated convex polygonal-shaped particles. Macroscopically, it is observed that angular assemblies exhibit higher shear strengths and volumetric deformations, i.e., dilations. Moreover, a unique relationship is observed between the critical state stress ratio and particle roundness. Microscopically, the roundness shows a considerable effect on rotational behavior such that the absolute mean cumulative rotation at the same strain level increases with roundness. A decrease in roundness results in relatively stronger interlocking, restricting an individual particle’s free rotation. Furthermore, the particles inside the shear band exhibit significantly higher rotations and are always associated with low coordination numbers. Generally, the geometrical shape of a particle is found to have a dominant effect on rotational behavior than coordination number.Graphical

Changes in the Bacterial Community Composition of Cultivated Soil after Digging up Operations for Laying a Pipeline

Our study aimed to evaluate the impact of the pipeline installation on the bacterial composition in cultivated soil by metagenomic analyses performed before the excavation and in the following three years. Differential abundance analysis was obtained using DESeq2 from the GAIA pipeline to verify the bacteriological diversity in soils collected after the reference year (2013). Soil samples presented a different distribution of taxa, especially in 2014, in which a further allocation at the phylum and family levels was observed compared to the previous year (2013). The phyla Bacteroidetes and Firmicutes increased significantly, while the phylum Actinobacteria, most abundant in 2013, showed reduced abundance; moreover, Chloroflexi and Planctomycetes decreased considerably, and Verrucomicrobia was absent. The significant differences in the taxonomic composition and structure of the soil microbial community were due to critical stress conditions following the soil excavations. The bacterial communities were capable of profound physiological and genetic changes, implementing different mechanisms for survival and adaptation to an environment with changed conditions. The implication of changes in microbial diversity before and after the mechanical insult of soil has been determined.

Invisible damage identification and danger warning for steel box girders using the metal magnetic memory method

Locating invisible damage and detecting the critical stress state leading to structural damage or failure are essential tasks in structural diagnosis. In this study, a four-point bending static loading experiment was conducted on two steel box girder members—one nondestructive, whereas the other had a butt weld—based on the metal magnetic memory (MMM) method. The correlation between the buckling failure position of the steel box girders and distribution characteristics of the magnetic signals was analyzed. The critical stress states of the elastic and elastic–plastic or elastic–plastic and plastic stages were statistically described using characteristic magnetic parameters. The results showed that the extremum position of the magnetic signal variationΔHpywell correlated with the location of the stress concentration. TheΔHpyextremum could locate the invisible damage position and predict the failure position, while the extremum of the average valueΔHpyaand integral value AofΔHpyvalue could accurately diagnose the critical stress state of steel box girders in the elastic and elastic–plastic or elastic–plastic and plastic stages. The proposed magnetic charge model could explain the distribution characteristics of the magnetic signals on the butt weld. These results are expected to provide a basis for effectively evaluating invisible damage using the MMM method.

Analysis of the Shear Strength of a Soil-Geosynthetic Interface

Abstract The paper presents the results of the shear strength measurements of a soil-geosynthetic interface. The tests were executed using a large-size direct shear test apparatus. A total of 5 different samples of materials were tested, i.e., ash, sand, well-graded gravel, fine poorly-graded gravel, and medium poorly-graded gravel. These materials were reinforced using different types of geosynthetics, i.e., Thrace WG80 black woven geotextile, Tencate Miragrid GX55/30 woven geogrid, and Thrace TG3030S rigid polypropylene geogrid. An interface coefficient α, which represents the ratio of the soil-geosynthetic interface shear strength to the shear strength of unreinforced material sample, was determined for given combination of the material and geosynthetics. The coefficient α reached a greater value in the critical stress state than in the peak stress state for sands and gravels reinforced using GX55/30 and TG3030S geogrids. The value of the coefficient α was in a range of 0.87 - 1.04 for gravels and 1.03 - 1.19 for sand. The black woven geotextile was used as the reinforcement only in samples of sand and ash. The results pointed to the different behaviour of these materials in the testing of the interface shear strength.

A cohesive strengthening/damage-plasticity model and its application for DEM numerical investigation in hardening/softening of rock and soil

In this study, a cohesive strengthening/damage-plasticity model (CS/DPM) based on the discrete element method (DEM) was proposed to better simulate the hardening/softening effect and shear dilatancy/shrinkage characteristic of rock and soil. Firstly, on the basis of the original cohesive damage-plasticity model (CDPM), a cumulative strengthening variable was defined to characterize the evolution of bonded contact stiffness and strength with plastic strain at the grain scale. According to the hyperbolic yield criterion, the critical stress state of the bond during the transition from elastic to plastic was determined. The strengthening and damage thresholds were introduced separately to control the extent of hardening and softening. On this basis, the shear shrinkage effect between particles was considered and incorporated into the development of the constitutive model algorithm as a stress return mechanism. Subsequently, a series of simulation experiments were carried out and the results were compared with the existing experimental or numerical solutions to verify the correctness and feasibility of the model. In addition, the impact of different contact parameters on the mechanical response was analyzed in a comparative manner to provide a reference for studying the hardening/softening behavior and failure mechanism of rock and soil from the mesoscopic scale.

Predictive machine learning approaches for the microstructural behavior of multiphase zirconium alloys

Zirconium alloys are widely used in harsh environments characterized by high temperatures, corrosivity, and radiation exposure. These alloys, which have a hexagonal closed packed (h.c.p.) structure thermo-mechanically degrade, when exposed to severe operating environments due to hydride formation. These hydrides have a different crystalline structure, than the matrix, which results in a multiphase alloy. To accurately model these materials at the relevant physical scale, it is necessary to fully characterize them based on a microstructural fingerprint, which is defined here as a combination of features that include hydride geometry, parent and hydride texture and crystalline structure of these multiphase alloys. Hence, this investigation will develop a reduced order modeling approach, where this microstructural fingerprint is used to predict critical fracture stress levels that are physically consistent with microstructural deformation and fracture modes. Machine Learning (ML) methodologies based on Gaussian Process Regression, random forests, and multilayer perceptrons (MLP) were used to predict material fracture critical stress states. MLPs, or neural networks, had the highest accuracy on held-out test sets across three predetermined strain levels of interest. Hydride orientation, grain orientation or texture, and hydride volume fraction had the greatest effect on critical fracture stress levels and had partial dependencies that were highly significant, and in comparison hydride length and hydride spacing have less effects on fracture stresses. Furthermore, these models were also used accurately predicted material response to nominal applied strains as a function of the microstructural fingerprint.

A State-Dependent Hypoplastic Bounding Surface Model for Gassy Sand

As a kind of partially saturated soil often containing dissolved gas with a high degree of saturation (S &gt; 85%), gassy sand sediment widely exists in the marine environment all over the world. Due to the effect of gas dissolution and exsolution on the pore fluid compressibility, its stress–strain and pore pressure responses are quite different from those of common saturated and unsaturated soils when subjected to undrained loading. Since almost all the gas bubbles are occluded in the pore water of offshore gassy sand, the matric suction may be neglected, and an undrained constitutive model for gassy sand is developed based on the existing hypoplastic bounding surface model for saturated sand. Both Boyle’s and Henry’s laws are employed in the model to characterize the equilibrium behavior of the gas compressibility and solubility, and then the equivalent compressibility coefficient of the pore fluid is obtained. To avoid unrealistic volumetric expansion, the concepts of the critical state and state-dependent dilatancy stress ratio are incorporated to describe its ultimate shear strength and dilatancy characteristics, respectively. Finally, the triaxial undrained test results on gas-charged sand from Hangzhou Bay are analyzed at three initial saturation degrees of 85%, 90%, and 100%, and two effective confining pressures of 50 kPa and 200 kPa. Moreover, carbon dioxide (CO2) was selected in the test, and the samples were loose with a relative density of 30%. It is noted that a good agreement is achieved between the simulation results and the experimental data, including the influence of gas content and confining pressure on the shear dilatancy and the mean effective stress increase at the beginning of the effective stress path, among others.

Laboratory hydraulic fracturing in layered tight sandstones using acoustic emission monitoring

Understanding the propagating mechanism of complex fracture network is essential for optimizing the hydraulic fracturing design scheme. Laboratory hydraulic fracturing experiments were performed on three tight sandstone specimens with bedding plane perpendicular, oblique and parallel to the axial direction, respectively. Before hydraulic fracturing, three specimens were first loaded close to the critical stress state under 22.5 MPa confining pressure. The entire hydraulic fracturing process was monitored by acoustic emission (AE) and ultrasonic measurement. The experimental results show that the hydraulic fracturing behaviors were significantly influenced by bedding structure and water infiltration. The bedding plane plays an important role on breakdown pressure, distribution characteristics of AE activities and hydraulic fracture networks. The bedding-oblique specimen exhibits lower breakdown pressure than those of the bedding-parallel and bedding-perpendicular specimens. AE activities in the bedding-oblique specimen is characterized by lower b-value, while higher b-value is found in other two specimens. X-ray computed tomography images of the broken specimens show an excellent agreement between AE hypocenters and macroscopic fractures, indicating geometrical morphology of macroscopic fractures is closely related to the bedding plane and well imaged by AE hypocenters. A bedding-parallel shear fault formed in the bedding-oblique specimen. Complex shear fault linking several bedding-parallel echelon arrays or branches formed in the bedding-parallel and bedding-perpendicular specimens. Water infiltration affects the hydraulic fracturing behaviors by inducing AE events initiate at transition region of wet-dry boundary around the central borehole. In view of the formation of fracture network and the risk of induced seismicity, the influence of bedding structure and water infiltration should be taken into consideration under given in-situ stress regime during the hydraulic fracturing design scheme.

The proneness assessment of a mineral deposit to tectonic rockburst: A case-study of Yuzhnoe deposit

The procedure of the proneness assessment to tectonic rockburst (with fault-slip mechanism) is insufficiently described in guidelines and regulatory documents, thus, additional investigations are required in this regard. The tectonic rockburst proneness criteria at Yuzhnoe deposit, Primorye are considered to be the location of large faults in the modern stress field and the critical stress state of rock mass. It is found that the maximum (compression) principal stress, reconstructed using the methods of tectonophysics and the data on rock mass faulting, fits the orientation of the maximum compression in the stress field, determined by instrumental measurements. In this stress field, in the plane nearby the plane of the effective maximum shear stresses, a large fault exists and is traced underground and on ground surface for more than 10 km. The analysis of geodynamics at the deposit, deformation of boundaries of underground excavations, core discing in wells far from the underground excavations, as well as rock bursts both at the study deposit and at the similar Nikolaevskoe deposit nearby shows that the level of the natural stresses at Yuzhnoe deposit is rather high and the rock mass is everywhere in the critical stress state. Yuzhnoe deposit is assumed to be to tectonic rockburst prone as both criteria are fulfilled: the rock massif is in the critical stress state and there is large fault which lies comfortably for mutual movement of rock blocks.The studies were carried out using facilities of the Center for Shared Use of Scientific Equipment at the&nbsp;Center for Processing and Storage of Scientific Data of the Far East Branch of the Russian Academy of&nbsp;Sciences, supported by the Ministry of Science and Higher Education of the Russian Federation, Project&nbsp;No. 075-15-2021-663.

Critical State and the Loosest Jammed State of Granular Materials

Solid-state (i.e., jammed) granular soils can be prepared into different densities characterised by the mean pressure p and the solid fraction ϕ (i.e., different p-ϕ combinations). The limits for jammed states (i.e., the range of possible p-ϕ) are studied theoretically in the literature or through isotropic compression simulations with the discrete element method (DEM). Shearing also causes unjamming and the critical state is an important reference state for shear deformation. How the jamming limits from isotropic compression tests are related to the critical state is examined in this paper by DEM simulations. Two methods are used to generate isotropic samples. One is the isotropic compression method, which is mainly used for studying jamming in the literature. Possible jammed states from this method lie between two compression lines. The varying-friction methods can generate samples with a larger range of p-ϕ. Isochoric shear tests are conducted on isotropic specimens prepared with both methods. Some specimens reach liquefaction (p′≈ 0) and the others reach the critical state. The obtained critical state p-ϕ line is found to be the same as the loosest jammed state line from the isotropic compression method. Additionally, the critical state stress state is also well described by a Coulomb-type equation in the octahedral profile.

Structural Analysis of Different Types of Chassis Designs for a Multi-Axle Heavy Commercial Vehicle

In recent years, the increasing capacity observed in air transportation requires enhanced security measures. The Aircraft Rescue and Fire Fighting (ARFF), which is one of the measures that are excessively taken in aviation, is constantly developing along with the sector, and it follows the technology closely, consequently increases its capacity day by day. In order to provide both civil and military air transportation safety, ARFF vehicles, which are specifically designed to operate in unusual boundary conditions with an ease. ARFF vehicles have high engine power in order to reach the crash site as soon as possible, aside from the high-power engine, they are equipped with all-wheel drive system (AWD) to increase mobility in rough terrain. Also, they are equipped with off road compatible tires, and it has high carrying capacity to carry extinguishing agents to be used to effectively respond to fire. In order for the vehicle to operate under these unusual boundary conditions, the frame component which works as carrier of the vehicle must be manufactured with materials which have high strength, and low weight and the frame should be as modular as possible. In this study, two different chassis designs were examined. The designs were created with the PTC Creo 3D modelling program for a multi-axle driven ARFF vehicle. Numerical calculations of the loads on the chassis were made with reference to the contact points of the chassis, rectangular-circular hollow section and different types of critical stress conditions. Finite element (FE) analysis for two different chassis conducted according to the calculation results. The main purpose of this study is to evaluate the stress conditions of different types of ladder chassis under static axle load according to two different critical loads and to determine the stresses acting on the chassis. As a result of the determination, engineering evaluations were made by evaluating the suitability of the chassis for production.

Improve the Accuracy in Numerical Modeling of Suspended Sediment Concentrations in the Hangzhou Bay by Assimilating Remote Sensing Data Utilizing Combined Techniques of Adjoint Data Assimilation and the Penalty Function Method

Suspended sediment dynamics play an important role in controlling nearshore and estuarine geomorphology and the associated ecological environments. Modeling the transport of suspended sediment is a complicated and challenging research topic. The goal of this study is to improve the accuracy of modeling the suspended sediment concentrations (SSCs) with newly developed techniques. Based on a three-dimensional suspended cohesive sediment transport model, the transport of suspended sediment and SSCs are simulated by assimilating SSCs retrieved from the Geostationary Ocean Color Imager (GOCI) with the adjoint data assimilation in the Hangzhou Bay, a typical strong tidal estuary along the coast of the East China Sea. To improve the effect of the data assimilation, the penalty function method, in which the reasonable constraints of the estimated model parameters are added to the cost function as penalty terms, will be introduced for the first time into the adjoint data assimilation in the SSCs modeling. In twin experiments, the prescribed spatially varying settling velocity is estimated by assimilating the synthetic SSC observations, and the results show that the penalty function method can further improve the effect of data assimilation and parameter estimation, regardless of synthetic SSC observations being contaminated by random artificial errors. In practical experiments, the spatially varying settling velocity is firstly estimated by assimilating the actual GOCI-retrieved SSCs. The results demonstrate that the simulated results can be improved by the adjoint data assimilation, and the penalty function method can additionally reduce the mean absolute error (MAE) between the independent check observations and the corresponding simulated SSCs from 1.44 × 10−1 kg/m3 to 1.30 × 10−1 kg/m3. To pursue greater simulation accuracy, the spatially varying settling velocity, resuspension rate, critical shear stress and initial conditions are simultaneously estimated by assimilating the actual GOCI-retrieved SSCs to simulate the SSCs in the Hangzhou Bay. When the adjoint data assimilation and the penalty function method are simultaneously used, the MAE between the independent check observations and the corresponding simulated SSCs is just 9.90 × 10−2 kg/m3, which is substantially less than that when only the settling velocity is estimated. The MAE is also considerably less than that when the four model parameters are estimated to be without using the penalty function method. This study indicates that the adjoint data assimilation can effectively improve the SSC simulation accuracy, and the penalty function method can limit the variation range of the estimated model parameters to further improve the effect of data assimilation and parameter estimation.

A new model for predicting hydraulic fracture penetration or termination at an orthogonal interface between dissimilar formations

Vertical height growth of hydraulic fractures (HFs) can unexpectedly penetrate a stratigraphic interface and propagate into neighboring layers, thereby resulting in low gas-production efficiency and high risk of groundwater contamination or fault reactivation. Understanding of hydraulic fracture behavior at the interface is of pivotal importance for the successful development of layered reservoirs. In this paper, a two-dimensional analytical model was developed to examine HF penetration and termination behavior at an orthogonal interface between two dissimilar materials. This model involves changes in the stress singularity ahead of the HF tip, which may alter at the formation interface due to material heterogeneity. Three critical stress conditions were considered to assess possible fracture behavior (i.e., crossing, slippage, and opening) at the interface. Then, this model was verified by comparing its theoretical predictions to numerical simulations and three independent experiments. Good agreement with the simulation results and experimental data was observed, which shows the validity and reliability of this model. Finally, a parametric study was conducted to investigate the effects of key formation parameters (elastic modulus, Poisson's ratio, and fracture toughness) between adjacent layers. These results indicate that the variation in the introduced parameters can limit or promote vertical HF growth by redistributing the induced normal and shear stresses at the interface. Among the three studied parameters, Poisson's ratio has the least influence on the formation interface. When the fracture toughness and elastic modulus of the bounding layer are larger than those of the pay zone layer, the influence of fracture toughness will dominate the HF behavior at the interface; otherwise, the HF behavior will more likely be influenced by elastic modulus.

Seismic analysis of the Xiluodu reservoir area and insights into the geometry of seismogenic faults

The Xiluodu (XLD) reservoir is the second largest reservoir in China and the largest in the Jinsha River basin. The occurrence of two M > 5 earthquakes after reservoir impoundment has aroused great interest among seismologists and plant operators. We comprehensively analyzed the seismicity of the XLD reservoir area using precise earthquake relocation results and focal mechanism solutions and found that the seismicity of this area was weak before impoundment. Following impoundment, earthquake activity increased significantly. The occurrence of M ≥ 3.5 earthquakes within five years of impoundment also appear to be closely related to rapid rises and falls in water level, though this correlation weakened after five years because earthquake activity was far from the reservoir area. Earthquakes in the XLD reservoir area are clustered; near the dam (Area A), small faults are intermittently distributed along the river, while Area B is composed of multiple NW-trending left-lateral strike-slip faults and a thrust fault and Area C is composed of a NW-trending left-lateral strike-slip main fault and a nearly EW-trending right-lateral strike-slip minor fault. The geometries of the deep and the shallow parts of the NW-trending fault differ. Under the action of the NW-trending background stress field, a series of NW-trending left-lateral strike-slip faults and NE-trending thrust faults in critical stress states were dislocated due to the stress caused by reservoir impoundment. The two largest earthquakes in the XLD reservoir area were tectonic earthquakes that were directly triggered by impoundment.

Precursory Tidal Triggering of the 2011 Mw 9.0 Tohoku Aftershocks

Precursory earth tidal triggering is believed to influence earthquake timing preferentially when a region is critically stressed. However, whether and how the recurrence of aftershocks after a giant earthquake is affected by tidal triggering remains perplexing. To provide insight into this study, we utilized the Schuster test to explore the tidally induced stress variation correlated with the 2011 Mw 9.0 Tohoku earthquake aftershock sequence by determining the tidal phase angle at the occurrence time of events and the periodic characteristics of the aftershocks. Our results show that the aftershocks were triggered by short-period tides, including semidiurnal and diurnal tides. The rupture associated with the mainshock likely resulted in a critical stress state in the focal region, which is conducive to tidal triggering. We subdivided the aftershock catalog into several subsets, using a depth of 30 km and a magnitude of 5 as discriminators. The analysis of these subsets reveals that weaker and deeper earthquakes are best correlated with Earth tides, which will be helpful to investigate the mechanisms of tidal correlation.