Principal Stress Axes Research Articles

Crustal stress near the Yakutat microplate collision from probabilistic earthquake focal mechanisms

Earthquake source characteristics provide a valuable constraint on crustal stress and regional plate tectonics. Earthquake source studies in Yukon and northern British Columbia have been limited by sparse seismic network coverage. In this work, we leverage recent seismic network improvements to estimate focal mechanisms for small and moderate-magnitude (M>2.0) earthquakes from P-wave first-motion polarity data. We invert these data within a probabilistic framework to rigorously quantify mechanism uncertainties. Subsequently, probabilistic earthquake focal mechanisms are used as input for inversions for the orientation of principal stress axes, and stress shape ratio, for spatial windows throughout the region. We implement a novel, data-driven approach to propagate focal mechanism uncertainty in stress inversion. Our results improve the spatial coverage of existing earthquake focal mechanisms and enable an orogenic-scale study of crustal stress near the Yakutat-North America collision. Overall, the region is characterized by a transpressive stress regime. Maximum horizontal compressive stress orientations exhibit a pattern orthogonal to the Yakutat collision syntaxis. Stress inversion results also reveal an abrupt change in orientation east-to-west, which we interpret as a change in stress regime across the Fairweather-Connector-Totschunda fault system that is likely related to coupling between the subducting Yakutat slab and North America. This work improves our understanding of potential earthquake hazards in southwestern Yukon and the surrounding region.

Influence mechanism of the principal stress axis rotation effect on the propagation of blast-induced cracks under in-situ stress

In-situ stress plays a pivotal role in controlling both the propagation speed and trajectory of blast-induced cracks. Previous studies have primarily placed emphasis on the qualitative analysis of the propagation morphology of such cracks, but the principal stress axis rotation effect under static-dynamic coupling loading was ignored. Consequently, the mechanical mechanism of the propagation of blast-induced cracks has not been figured out yet. In this study, a combination of laboratory tests, theoretical analysis, and numerical simulations was employed to explore the influence mechanism of the principal stress axis rotation effect on the propagation of blast-induced cracks under in-situ stress. The results suggest that the hydrostatic pressure does not change the propagation trajectory of blast-induced cracks, but a higher hydrostatic pressure will inhibit both their propagation speed and length. In contrast, the deviatoric stress can change the propagation trajectory of blast-induced cracks, and a higher deviatoric stress makes it easier for blast-induced cracks to deflect to the maximum loading stress direction. Besides, the propagation of blast-induced cracks exhibits different features in different stages under in-situ stress. In the zone near the blast source, the dynamic stress is dominant; the maximum principal stress of the mass points is distributed in the radial direction; and blast-induced cracks expand mainly along the radial direction. On the contrary, in the zone far from the blast source, the static load is dominant; the maximum stress direction of the mass point alters to the maximum loading stress direction; and blast-induced cracks deflect from the radial direction to the maximum loading stress direction. Therefore, the propagation of blast-induced cracks under in-situ stress is a dynamic process in which dynamic and static stresses compete for crack initiation, and the changes in both stress value and principal stress direction are identified as the main reasons for the deflection of blast-induced cracks.

Analysis of the mechanical behavior of asphalt pavement under multiple coupling effects

To reveal the damage mechanism of asphalt pavement caused by the spatial rotation of the stress principal axis, and the influence range of excess pore water pressure on the pavement under the coupling action of multiple fields, the finite element software was used to build the three-dimensional highway asphalt pavement models. Under the consideration of gravity's impact on dynamic computation results, the study investigated the variation patterns of the direction cosines of the principal stress axes of asphalt structural pavement elements under the action of moving loads, as well as the distribution laws of excess pore water pressure. The results indicate that the influence of gravity on dynamic computation results increases gradually with the depth of the pavement, reaching an error of first principal stress of 40.1 % at the bottom of the lower layer. During the movement of loads, the pavement elements directly under the load undergo rotation of the principal stress axes in the xy, xz, and yz planes under different physical fields. With increasing pavement depth, the angle between the first principal stress and the z-axis primarily fluctuates between −90° to 0° and 90° to 180°. The cosine of the angle between the first principal stress and the x-axis and z-axis undergoes instantaneous positive and negative conversion, which can easily lead to transverse and longitudinal cracks on the road surface. The influence widths of excess pore water pressure along the longitudinal and transverse directions of the pavement increase with the pavement depth, with maximum widths of 5.426 m and 2.075 m, respectively. These findings offer new insights into the mechanisms behind the formation of transverse and longitudinal cracks on asphalt pavement and hold significant implications for the improvement of asphalt design methods.

Cemented sand under hollow cylinder multiaxial loading

Improvement of granular soils mechanical properties can be achieved by the addition of bonding agents. In this research, low amount of Portland cement was added to a sand and its beneficial shear strengthening effects were evaluated under a range of multiaxial stress paths. The influence of the orientation of the principal axes of stress and strain on the stress-strain response and failure of cemented sand has only been scarcely investigated. Therefore, this experimental investigation reports the results of a series of consolidated drained hollow cylinder torsional tests with constant principal stress path direction, ασ, varying from 0° to 90° Results were compared with the shear behaviour of the uncemented sand tested under similar loading conditions. Results show that the addition of cement to the sand matrix increases the soil strength for all multiaxial stress path directions. The suitability of two multiaxial strength criteria for reproducing the shape of the failure envelope as a function of the orientation of principal stress axis ασ has also been analysed.

Analytical model and stress behavior of consolidated load bearing geotextile tubes

Accurately predicting stress-strain characteristics is crucial to ensuring the regulated capacity and controlled deformation of the tubes during and after construction. However, research on the shear strength of geotextile tubes under surcharge loading, especially after dewatering, is insufficient. This study proposes an analytical model with a Stress-State Boundary (SSB) and Yield Function to comprehensively describe the stress-strain behavior of Load-Bearing Geotextile Tubes (LGTs). The SSB is designed to predict the initial state of stress in the infill soil prior to load application, while the Yield Function is formulated to express the shear stress path experienced by the LGT before fabric failure. The model considers various factors that affect LGT behavior, including diverse soil mechanical parameters, nonlinear fabric stiffness, initial tension due to self-weight and principal stress axes rotation. Results show that a decrease in Poisson's ratio corresponds to an increase in failure stress. Moreover, it was demonstrated that the axial failure strain can be influenced by the geotextile linear or nonlinear behavior. Notably, the study highlights that tube height and inclination angle significantly affect the geotextile's confining effect. Beyond theoretical contributions, the analytical model serves as a valuable tool for optimizing geotextile tube design and execution, contributing to project success and longevity through enhanced structural stability.

Physics‐Based Forecasts of Eruptive Vent Locations at Calderas

AbstractConstraining stresses in the Earth's crust in volcanic regions is critical for understanding many mechanical processes related to eruptive activity. Dike pathways, in particular, are shaped by the orientation of principal stress axes. Therefore, accurate models of dike trajectories and future vent locations rely on accurate estimates of stresses in the subsurface. This work presents a framework for probabilistic constraint of the stress state of calderas by combining three‐dimensional physics‐based dike pathway models with observed past vent locations using a Monte Carlo approach. The retrieved stress state is then used to produce probability maps of future vent opening across a caldera. We test our stress inversion and vent forecast approach on synthetic scenarios, and find it successful depending on the distribution of the available vents and the complexity of the volcano's structural history. We explore the potential and limitations of the approach, show how its performance is sensitive to the assumptions in the models and available prior information, and discuss how it may be applied to real calderas.

Tectonic Stress Analysis of Shivnath-Salena Area, Using Stress Response Structure

The geological mapping of the Shivnath-Salena area and structure analysis of the same area were conducted in the LesserHimalayan sequence, Far West Nepal, unveiling the characteristics of kinematics and the associated stress field in the region. Thearea comprises of the Salena Formation and Chachura Formation. The Salena Formation is sandwiched between the PachkoraThrust and Dudulakhan Thrust, and the Chachura Formation occurs between the North Dadeldhura Thrust and Pachkora Thrust.The Salena Formation is composed of light grey and white quartzite and black slates that are intensely deformed, with folds. The Malena Anticline and the Lamalek Syncline are the intraformational folds observed in the study area. Superimposed folding was also observed. The Chachura Formation comprises Paleocene to Late Eocene green sandstone, grey and purple shale, and approximately 1 m thick grey fossiliferous limestone. This study interprets stress-response structures to show the tectonic stress field in the Salena Formation. Twenty-seven fault slip data (slickenside) were collected in the field and used to determine the stress regime by applying the stress inversion technique. The direction of the maximum principal stress axes is interpreted as NE-SW. The average value of the stress index R' is about 0.30, indicating an extensional tectonic stress regime in the study area. As σ1 is vertical, R = R' in the study area suggests normal faulting in an extensional regime.

Deformation Characteristics and Noncoaxial Behavior of Fiber-Reinforced Soil under Pure Principal Stress Axis Rotation

Deformation Characteristics and Noncoaxial Behavior of Fiber-Reinforced Soil under Pure Principal Stress Axis Rotation

Dynamic response of a permafrost railway subgrade with 3D train-track-subgrade-ground model simulations

Studying train-induced response characteristics is essential for safely operating permafrost railway subgrades. A three-dimensional thermal-mechanical coupling nonlinear dynamic model of train-track-subgrade-ground relationships was established to analyse the train-induced dynamic stress, acceleration and stress path characteristics of a permafrost railway subgrade, and field monitoring data were used to verify this model. The differences between the 2D and 3D models are also discussed, along with the seasonal changes, train speed, axle load, and train type affecting permafrost subgrades. The main results are as follows. First, the vibration load significantly impacts the subgrade 6 m below the sleeper, producing distinct vertical dynamic stress waves due to the wheels and bogies. Dynamic compression stress dominates the subgrade and is influenced by the train structure, speed, and sleeper spacing. While the 2D model tends to underestimate the dynamic stress in shallower layers, it concurs with the 3D model in deeper subgrade dynamics within a 10% margin of error. Then, the principal stress axis of the subgrade soil rotates synchronously with train movements, exhibiting regular stress paths in the YZ plane (longitudinal section) with depth-dependent variations in the stress cycles and deviatoric stress. Finally, predominantly originating from sleeper-induced vibrations, the subgrade vibration acceleration varies with the train speed, sleeper spacing, and season and is most pronounced in the vertical direction. This study provides theoretical guidance for the vibration response of permafrost subgrades on the Qinghai-Tibet Railway (QTR).

Experimental study on the influence of principal stress axes rotation on sand mechanical behavior and non-coaxiality effects

The soil around the foundations of offshore engineering experienced the principal stress axes rotation caused by the wave loadings. To investigate the influence of the principal stress axes rotation on the soil stress-strain relationship, a series of pure principal stress axes rotation tests under drained/undrained conditions were carried out with a hollow cylinder apparatus. The undrained test results show that the deviatoric stress, relative density, and starting point of rotation affect the mechanical properties of sand. Through the analysis of pore water pressure accumulation in the 1st cycle, it was observed that more pore pressure accumulates in Quadrants II and III, while relatively less accumulates in Quadrants I and IV. Under drained and undrained conditions, the non-coaxial behavior of the soil differs. The non-coaxiality can be significantly observed in the drained test, and the direction of strain increment tends to the stress increment direction when the cycle number increases.

An anisotropic hypoplastic model for clays considering the non-coaxial behavior

Previous experimental tests on clays have confirmed the non-coincidence of principal strain increments with principal stress axes even though the stress rate is colinear with the current stress. To simulate the non-coaxial behavior of saturated clay subjected to monotonic proportional or non-proportional loading with constant principal stress rate directions, an anisotropic hypoplastic model is presented in this paper. The model is proposed by incorporating an improved anisotropic asymptotic state boundary surface (ASBS) and a non-coaxial asymptotic strain rate direction into the original explicit-ASBS hypoplastic model. The non-coaxial flow results from a non-coaxial stress rate defined by a Gram-Schmidt orthogonalization process based on a reference stress tensor. The capability of the proposed model is demonstrated by simulating the test data performed in hollow cylindrical apparatus (HCA) on Wenzhou clay and Shanghai clay with different drainage conditions and initial consolidation states. In addition, a series of numerical stress probing tests have been conducted to gain further insights into the properties of the proposed model.

Fault Networks in Triaxial Tectonic Settings: Analog Modeling of Distributed Continental Extension With Lateral Shortening

AbstractTriaxial deformation is a general feature of continental tectonics, but its controls and the systematics of associated fault networks remain poorly understood. We present triaxial analog experiments mimicking crustal thinning resulting from distributed longitudinal extension and lateral shortening. Contemporary longitudinal extension and lateral shortening are related by the principal horizontal strain ratio (PHSR). We investigate the effect of crustal geometry, rheology and strain rate on deformation localization, faulting regime and pattern, and PHSR in brittle and brittle‐viscous crustal‐scale models. We find that in brittle models the fault networks reflect the basal boundary condition and fault‐density scales inversely with brittle layer thickness. In brittle‐viscous models, as strain rate (ė) decreases, (a) Three fault patterns emerge: conjugate sets of strike‐slip faults (ė > 3 × 10−4 s−1, PHSR > 0.31), sets of parallel oblique normal faults (ė = 0.3–3 × 10−4 s−1, PHSR = 0.15–0.25), horst‐and‐graben system (ė < 0.3 × 10−4 s−1, PHSR < 0.1). (b) The strain localization increases systematically and gradually. We interpret the strain rate dependent of faulting regimes to be controlled by vertical coupling between the model upper mantle and model upper crust resulting in spontaneous permutation of principal stress axes. Rate‐dependency of strain localization can be related to mechanical coupling between the upper and lower crust. We identify the following parameters controlling triaxial tectonic deformation: upper crustal thickness and friction coefficient, lower crustal thickness and viscosity, as well as strain rate. We test our models and predictions against natural prototypes (Tibet, Anatolia, Apennines, and Basin and Range Province) thus providing new perspectives on triaxial deformation.

Analysis of strength characteristics of loess before and after freezing using a hollow cylinder torsional shear apparatus

This paper aims to comprehensively analyze the influence of the principal stress angle rotation and intermediate principal stress on loess's strength and deformation characteristics. A hollow cylinder torsional shear apparatus was utilized to conduct tests on remolded samples under both normal and frozen conditions to investigate the mechanical properties and deformation behavior of loess under complex stress conditions. The results indicate significant differences in the internal changes of soil particles, unfrozen water, and relative positions in soil samples under normal and frozen conditions, leading to noticeable variations in strength and strain development. In frozen state, loess experiences primarily compressive failure with a slow growth of cracks, while at normal temperature, it predominantly exhibits shear failure. With the increase in the principal stress angle, the deformation patterns of the soil samples under different conditions become essentially consistent, gradually transitioning from compression to extension, accompanied by a reduction in axial strength. The gradual increase in the principal stress axis angle (α) reduces the strength of the generalized shear stress and shear strain curves. Under an increasing α, frozen soil exhibits strain-hardening characteristics, with the maximum shear strength occurring at α = 45°. The intermediate principal stress coefficient (b) also significantly impacts the strength of frozen soil, with an increasing b resulting in a gradual decrease in generalized shear stress strength. This study provides a reference for comprehensively exploring the mechanical properties of soil under traffic load and a reliable theoretical basis for the design and maintenance of roadbeds.

Earth's Free Surface Complicates Inference of Absolute Stress From Earthquake‐Induced Stress Rotations

AbstractThe stress redistribution from an earthquake can produce localized measurable rotations of the principal stress axes if the absolute level of differential stress in the crust is on the order of the earthquake stress drop. Two simple analytic solutions have been developed to estimate the differential stress from an observed stress rotation. However, each has assumptions that may not be accurate near Earth's free surface. I model synthetic earthquakes in an elastic half‐space, and show that the assumptions of the methods are accurate for strike‐slip earthquakes, and for deep dip‐slip earthquakes. However, they are incorrect for shallow dip‐slip earthquakes. I introduce a free surface correction for one of the methods for dip‐slip earthquakes. I revise an analysis of stress rotations due to great subduction zone earthquakes, including this correction. The results support the original conclusion of near complete stress drop for many shallow subduction zone earthquakes.

ПОЛЯ НАПРУЖЕНЬ ТА ГЕОЛОГІЧНА СТРУКТУРА ЗАХІДНОГО ЗАМИКАННЯ ГОРЛІВСЬКОЇ АНТИКЛІНАЛІ ДОНБАСУ ЧАСТИНА 2. ПОЛЯ НАПРУЖЕНЬ І ДЕФОРМАЦІЙ

Background. In tectonophysical studies, particular importance is placed on the mechanism of formation of large complex deformation structural elements, and the Horlivka anticline of the Donets Basin serves as such an example. Its importance lies not only in its complex structural composition and development mechanism but also in its potential impact on safe and efficient coal mining. This study aims to analyse the geological structure, stress and strain fields of the western closure of the Horlivka anticline that could be useful in forecasting geological factors for coal mining operations. Methods. The paleostress analysis was performed on the collected fault orientation and kinematic data using Gushchenko's kinematic method based on the analysis of tectonic displacement vectors along slickensides. Mesoregional stress field characteristics were reconstructed by statistical processing of local stereographic solutions. The relative age chronology and staging of tectonic stresses were studied using the stress monitoring method. The characteristics of the principal axes of the total strain field were processed using the GEOS software. Results. Mesoregional stress field is characterised by a subhorizontal NW–SE-oriented maximum principal stress axis and a subhorizontal NE–SW-oriented minimum principal stress axis. This stress regime is classified as a strike-slip regime and is the youngest one (Alpine orogeny) for the Donets Basin. The evolution of tectonic loading conditions of the studied structure is characterised by a deformation series of six stress regimes from the oldest (normal) to the youngest (strike-slip). Stretching axis of the strain ellipsoid is oriented NW and N–S, and shortening axis is oriented NE, nearly orthogonal to the anticline axis. Strike- and oblique-slip faulting regimes of the total strain field were determined for most of the study area, and the deformation has occurred under shear conditions, as indicated by the Lode–Nadai coefficient. Conclusions. A structural pattern of deformation elements, including a conjugate strike-slip fault system of the shear zone, a dome-shaped fold, and longitudinal thrusts in its limbs, may be interpreted as a single pattern of structural paragenesis developed by right-lateral displacements along the longitudinal strike-slip fault system within the Horlivka anticline paraxial part. Fragments of mutual symmetry between the stress and strain fields can be taken as evidence of their genetic relationship.

Sand liquefaction in simple shear tests

Liquefaction is a phenomenon marked by a rapid loss of soil strength and stiffness, which generally occurs in loose saturated sandy deposit during earthquake because of the generation of excess pore water pressure. Several experimental researches concluded that liquefied soil behaves as a fluid during ground movement, but after the earthquake motion ceases, due to the dissipation of excess pore water pressure, the liquefied soil recovers its initial stiffness and returns to behave as a solid. Liquefaction resistance of sandy soil can be studied by means of Cyclic Simple Shear (CSS) test or Cyclic Triaxial (CTX) tests. While CTX tests are widely used in liquefaction studies due to their simplicity, CSS tests are more representative of stress conditions produced during an earthquake by simulating the continuous rotation of the principal stress axes. In this research the preliminary results of CSS tests carried out with confining rings on the sandy samples retrieved in the South of Sicily are reported. The apparatus is described in detail. All samples used were obtained from the same type of Italian sand by using same preparation method to minimize the number of factors influencing the results. Moreover, all tests were conducted by a single operator. All experimental results are reported in the plane cyclic resistance ratio (CRR) and number of cycles where liquefaction occurs (Nliq) in order to assess the liquefaction phenomenon.

Fault stability analysis and its application in stress inversion quality assessment

Fault stability analysis plays an important role in assessing the potential hazard of faults and in studying the mechanism of earthquake occurrence. Fault stability depends on the magnitude of the normal and shear stresses imposed on the fault by the tectonic stress and rock friction, while the magnitude of the normal and shear stresses is related to the spatial orientation of the fault normal with respect to the three principal stress axes, so it is easier to understand the variation of fault stability with its orientation by expressing the stability of different faults in the principal axis coordinate system. In this paper, we first developed a method to plot the stability of faults with different orientations in the principal stress axis coordinate system, then investigated the influence of the magnitude of principal stresses and friction on fault instability, and reached the conclusion that the instability is mainly affected by the relative magnitude of principal stresses (shape ratio). Finally, we proposed to use fault stability as an indicator to evaluate the quality of inverted stress obtained from fault slip data or earthquake focal mechanisms, that is, to evaluate the reliability of the inverted stress according to the compatibility of stress and fault stability. It is described in detail in terms of measured fault slip data from two regions.

GEODYNAMIC REGIMES IN THE LAPTEV SEA REGION ACCORDING TO THE LATEST SEISMOLOGICAL DATA

The results of the analysis of focal mechanisms and the general distribution of earthquake epicenters in the Laptev Sea region were presented. For four groups of clusters of events with known focal mechanisms, the directions of the principal stress axes were calculated by the formal stress inversion method. The distributions of earthquake epicenters and crustal thickness were compared. It has been revealed that, according to seismological data, the prolongation of the extension axis of the Gakkel Ridge on the Laptev Sea shelf is currently located in the vicinity of the group of extension detachments, which is extended along the eastern boundary of the Anisin, Zarya, and Belkovsko-Svyatonossky rift chains. The older extension axis, located along the group detachments marking the eastern boundary of the Ust-Lena and Omoloy rift systems, and continuing the axis of the Gakkel Ridge, is currently much less active, realizing residual stresses near its intersection with the Khatanga-Lomonosov fault zone in the northwestern parts of the shelf and with the Lena-Taimyr zone of boundary uplifts – in the southwestern. Near the Lena delta extension axes are oriented along the Olenekskaya and Bykovskaya channels and the border of the Siberian Platform, forming the extension conditions in the eastern part and the strike-slip regime in the western part of the vicinity of the Lena delta.

The Cause of the Earthquake (4.7 mb) and Recent Stress Regime Occurring in Al-Refaei Area, Mesopotamian plain, August 18th, 2017

Al-Refaei area is considered one of the highest seismically activity in the Mesopotamian Plain according to its seismic history. Depending on the International seismological Center, four seismic swarms were diagnosed in this area; these are: August 2004, January 2013, May-Jun 2017, and August 2017. Based on three focal mechanism solutions of magnitude greater than 4 mb, the stresses in the area were analysed. The results of focal mechanisms suggest that the faults movements in the study area show reverse movements, which is generated by compression force. To determine the directions of principal stress axes that affecting the faults in the area, the Rotational Optimization method were used. The directions of the three stress axes are: 02°/32°, 10°/302°, and 80°/136° for σ1, σ2, and σ3, respectively. The collision between the Arabian and Iranian plates is the main cause of seismic activity in Al-Refaei area. Therefore, Al-Refaei active fault is responsible for most the earthquakes in the area, and responsible for the earthquake that occurred in August 18th, 2017.

Discrete Element Simple Shear Test Considering Particle Shape

The particle shape has significant effects on the slip and rotation of particles in the shear of geomaterials, which is an important factor in the deformation and strength of geomaterials. This paper employed particle flow code (PFC3D) to simulate the simple shear test, and ellipsoidal particles with different aspect ratios were prepared to study the effects of particle shape on the mechanical behavior and fabric evolution of granular materials under complex stress paths. The numerical results show that the particle shape has a significant effect on the peak strength, dilatancy, non-coaxiality, and other mechanical properties of granular materials. The contact fabric evolves from orthotropy to transverse isotropy under the principal stress axes rotation. This paper will provide a reference for natural granular materials with different shapes in the study of mechanical behavior and the micro-constitutive model.