Recent research has paid little attention to the shear damage of discontinuities with different joint wall material (DDJM). In this paper, we present an investigation on the evolution of the shear ...behaviour and the damage of three typical types of natural DDJM in a sliding-prone stratum of China. Experimental direct shear tests were performed on 14 pairs of natural DDJM specimens to examine the changes in the shear strengths and surface damages of the DDJM with increasing normal stresses and an increasing number of shear cycles by evaluating surface damages via damage zone distribution, damage area percentage, and variation of joint roughness coefficient (JRC). The results indicate that the differences in the shear damage between the two halves are closely related to the difference in strength of the two joint walls of the DDJM specimens with similar initial JRC values of the two joint surfaces. Simultaneously, parallel numerical direct shear tests were conducted in
PFC3D
. The performance of the numerical modeling was examined by comparing the parameters of shear strength, damage area and damage depth of DDJM specimens with those obtained in the experimental direct shear tests. Then the validated models were used to explore the evolution of the damage depth of DDJM specimens during the shearing process. The results demonstrate that the proposed numerical approach has the ability to reproduce the shear behavior and damage of DDJM reasonably and could be used to examine the internal damage of DDJM which are not easy to investigate via laboratory experiments.
We investigate the upper mantle seismic discontinuities at 410 and 660 km depth beneath the Indian Ocean Geoid Low (IOGL). To map the discontinuities' topography, we use differential travel times of ...PP and SS waves and their precursors. Our final data set consists of 37 events with Mw ≥ 5.8, which densely cover our investigation area, also with crossing ray paths. We use array methods to detect the low‐amplitude precursor signals. The best quality data show a deepened 410 km discontinuity in the center of the IOGL as well as a mostly elevated 660 km discontinuity beneath the northern Indian Ocean, which we interpret as a hot anomaly currently residing in the mantle transition zone. We conclude that the largest negative geoid anomaly might be caused by a combined effect of hot material in the midmantle below the innermost IOGL and cold material below 660 km farther south.
Key Points
PP and SS precursors are used to investigate the topography of mantle transition zone discontinuities beneath the Indian Ocean Geoid Low
A hot midmantle anomaly is found beneath the center of the geoid low and a cold anomaly below the transition zone farther south
Superposition of the effects of a hot midmantle anomaly and a cold lower mantle anomaly cause the large Indian Ocean Geoid Low
Recent research has paid little attention to weakening of discontinuities with different joint wall material (DDJM) induced by wetting and drying cycles. Experimental investigations were conducted to ...examine the effects of wetting–drying cycles on weakening of three typical types of natural planar DDJM collected from a sliding-prone formation in the Three Gorges reservoir region of China. Deterioration of the corresponding four types of discontinuities with identical joint wall material (DIJM) were also been studied for comparison. The weakening laws of DDJM were revealed from macro- and microperspectives by testing on mineral compositions, microstructures and water absorption rate of joint walls as well as shear strength of discontinuities. The results indicate that the repeated wetting–drying cycles lead to obvious changes in microstructure and increases of macro water absorption rate for the studied joint walls with the changing degree closely related to their clay mineral contents; the peak shear strengths of DDJM gradually decreases with the wetting and drying cycles for a certain normal stress with the weakening rate between that of the DIJM with two joint wall materials of the DDJM, while closer to that of DIJM with the weak half materials of the DDJM; significant correlations have been found between the variation coefficient of rock microstructure of joint walls and the macrodeterioration coefficient of DDJM; the changing rates of multi-scale parameters before and after six wetting–drying cycles vary obviously, indicating the weakening is slowed down after six wetting–drying cycles for the studied discontinuities.
Highlights
Effects of wetting-drying cycles on weakening discontinuities with different joint wall material were studied.
Deterioration of the discontinuities with identical joint wall material were also been investigated for comparison.
The weakening laws were revealed from macro and micro perspectives.
Significant correlations have been found between the variation coefficient of microstructure of joint walls and the macro deterioration coefficient.
The results could provide a basis to better understand the stability evolution of rock slopes with soft and hard rock interbedded.
The problem of frictional discontinuities in rock masses is addressed computationally in the context of the NMM (numerical manifold method). The use of the NMM allows the use of nonmatching meshes to ...discretize the problem domain. To impose frictional contact conditions, the Uzawa-type ALM (augmented Lagrange method), which inherits the merits of the LMM (Lagrange multiplier method) and the PM (penalty method), is incorporated into the NMM, and a numerical model termed as Uzawa-type augmented Lagrangian numerical manifold method (UALNMM) is proposed. In the UALNMM, two layers of iterations are adopted, which are Newton-Raphson iterations for the inner layer and augmentation iterations for the outer layer. The implementation of the UALNMM is discussed in details. Four benchmark examples of frictional/frictionless contact problems and an example of rock slope with multiple frictional fractures are used to validate the accuracy and robustness of the UALNMM. The proposed UALNMM deserves further investigation for three dimensional rock mechanics problems with frictional contact.
We present new constraints on the upper mantle transition zone structure beneath eastern and southeastern Tibet based on P wave receiver functions for a large broadband data set from two very dense ...seismic arrays. A clear depression of both the 410 km and 660 km discontinuities is detected west of the Red River fault relative to the east. The correlated topographic variations across the Red River fault are indicative of temperature changes in the upper mantle above the transition zone, which suggests that the fault is a deep‐rooted structure that penetrates into the upper mantle and separates Indochina from South China. West of the Red River fault, the transition zone thickness under the Tengchong volcano is found to be normal compared to the global average. This strongly suggests that the intraplate volcano may originate from slab tearing of the eastward subducting Indian plate at shallow depths in the upper mantle rather than from dehydration of a flattened plate within the transition zone. Our results further show that the 660 km discontinuity is significantly depressed under the western Yangtze Craton and that the transition zone therefore thickens by up to 20 km. This thickening is suggestive of lowered temperatures associated with a remnant of detached lithosphere in response to overlying asthenospheric escape flow in and around the western Yangtze Craton. In addition, we find that the transition zone thickness beneath much of the Sichuan Basin is similar to the global average.
Key Points
Topographic variation on the two discontinuities across the Red River fault zone
Tengchong is likely caused by a slab tear of the eastward subducted Indian plate
A detached lithosphere at 660 km depth below western Yangtze Craton
Global WGM-12 gravity and EMAG2v3 magnetic models were used to present new information of the Estonian crust. The Estonian Precambrian crystalline basement, composed of Paleo-Meso Proterozoic ...metamorphic and igneous rocks, is covered by a Paleozoic sedimentary rock deposit 100–800 m thick. The present work aims to map structural patterns and recognise physical layers of the crust over Estonia using global potential models. The gravimetric data was used to identify the depth of the Moho and Conrad discontinuities. The magnetic data has been processed to calculate the Curie point depth (CPD), which was then used to estimate heat flow (HF) values in the study area. The depth of the Moho indicates an average value of 60 km, while the average depth of the Conrad discontinuity is around 18 Km. The depth of the CPD reveals an average value of 15 km. Different derivative techniques were applied over the residual part of both potential models, to delineate and map geophysical lineaments. The subsurface of Estonia has been divided into six petrological-structural zones: Tallinn, Alutaguse, Jõhvi, West-Estonian, Tapa and South-Estonian. Geophysical lineaments in each zone subsurface division delineates a NW-SE trend, perpendicular to the NE-SW oriented Fenno-Sarmatian collision. To visualise the anomalous crustal variations over Estonia, different profiles showing varying values of potential fields, CPD and heat flow are presented, especially around the Precambrian Rapakivi granitic plutons and in the Paldiski-Pskov deformation zone, due to the high contrast observed by mapping the different results over these areas.
•A spectral analysis and modelling of the global potential fields in Estonia has been carried out.•A reconnaissance of the physical layers of the Estonian crust (i.e., Moho and Conrad discontinuities, Curie depth point) was performed.•Heat Flow tends to increase around the zones where the Elastic thickness and the Moho discontinuity are deeper, and possible fractured zones arise (i.e., deformation zones).•Potential geophysical lineaments in Estonia seem to have a tectonic significance due to the processes of the Svecofennian orogeny.
In this study, a total of 13,080 P receiver functions and 2800 S receiver functions recorded by 116 stations in Southeast Tibet, are used to determine the depths of the Moho, the ...lithosphere–asthenosphere boundary (LAB), and the 410km and 660km discontinuities. The results show that the LAB depth increases from 80–100km beneath southern Yunnan to 140–180km beneath Sichuan. Most of the 410km discontinuity is flat and at 410km depth, with a slight depression of 10–20km beneath the Longmenshan (LMS) Fault, the central Sichuan–Yunnan (SY) diamond-shaped block, and southeastern Yunnan. For the 660km discontinuity, the greatest depth occurs beneath the LMS and southern Yunnan, reaching 670–680km. The deepening of 660km discontinuity is attributed to compositional heterogeneity within the MTZ or dynamic pressure on the discontinuity. Additionally, the velocity of movement in the crust and upper mantle beneath SY is different. In Yunnan, crustal movement is southeastward, whereas across the Red-River fault, part of the crustal motion is redirected southwest by the westward rollback of the mantle beneath Myanmar. The interaction between the asthenosphere moving northeastward from Myanmar and southeastward from Tibet near latitude 26°N causes the crust and lithospheric mantle beneath southern Yunnan to have different movement directions.
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•Receiver function shows LAB depth from 70 to 180km beneath Southeast Tibet.•The 410 and 660km interfaces are mostly flat, with depression by 10–20km.•Geodynamical mode in Yunnan is different from that beneath LMS.
Ocean swell activities excite body‐wave microseisms that contain information on the Earth's internal structure. Although seismic interferometry is feasible for exploring structures, it faces the ...problem of spurious phases stemming from an inhomogeneous source distribution. This paper proposes a new method for inferring seismic discontinuity structures beneath receivers using body‐wave microseisms. This method considers the excitation sources of body‐wave microseisms to be spatially localized and persistent over time. To detect the P‐s conversion beneath the receivers, we generalize the receiver function analysis for earthquakes to body‐wave microseisms. The resultant receiver functions are migrated to the depth section. The detected 410‐ and 660‐km mantle discontinuities are consistent with the results obtained using earthquakes, thereby demonstrating the feasibility of our method for exploring deep‐earth interiors. This study is a significant step toward body‐wave exploration considering the sources of P‐wave microseisms to be isolated events.
Plain Language Summary
The ocean waves excite persistent and random ground motions called microseisms. Since this excitation is independent of seismic activities, this wavefield has information about seismic structures that earthquakes never have. For the deep structure, such as the mantle and core, body‐wave microseisms are more suitable than surface‐wave microseisms because body‐wave microseisms have better sensitivity. Previous studies using body‐wave microseisms mainly adopted the cross‐correlation analysis known as seismic interferometry. This method assumes that the microseisms are excited everywhere. However, the inhomogeneous source distribution of body‐wave microseisms causes artifacts for exploration by seismic interferometry. We developed a new method which circumvents this problem. Assuming that the body‐wave microseisms are spatially isolated, this method extracted the P‐s converted waves beneath receivers from body‐wave microseisms. The 3‐Dimensional imaging result of extracted P‐s converted waves shows both 410‐ and 660‐km mantle discontinuities, consistent with results using earthquakes. This study shows the potential of body‐wave microseisms for exploring the deep earth structure.
Key Points
The P‐S waves at mantle discontinuities were extracted from the ambient noise excited by the ocean swells
We developed the source deconvolution method to generalize a receiver function method to P‐wave microseisms
The migration result of P‐S waves was consistent with previous studies, showing the potential of P‐wave microseisms to seismic structures
A new thin beam-column element with capability to model multiple embedded discontinuities is developed, in which the mechanical failure process is simulated, leading to nonlinear analysis of frame ...structural systems. The physical and mathematical principles are based on a Lagrangian variational formulation of the mechanical behavior of thin beam-columns with multiple strong axial and rotation embedded discontinuities adapted to represent strain localization. These strong embedded discontinuities represent the material failure process as hinges for beams and cracking or crushing for bars. The developed variational formulation is derived from a decomposed continuous incremental plasticity model with hardening, softening and elastoplastic behavior, from which the strong embedded discontinuities, that represent the strain concentration at non-predetermined localization zone, are naturally obtained. The boundary value problem, which is valid throughout the domain of the structural member, is obtained. Closed form solutions for bars and thin beams with multiple strong embedded discontinuities for any boundary condition are obtained by solving the proposed variational formulation. Closed form solutions are particularized to a basic system without rigid-body displacement modes, which naturally leads to a symmetric flexibility matrix and thus a symmetric stiffness matrix of the thin beam-column element with capability to model multiple embedded discontinuities with arbitrary locations. This matrix is more efficient than those developed with the finite element method, since it is naturally condensed, its coefficients are defined values, not in terms of integrals, and any type of loads are modelled. Representative examples of beams and frames validate the capability of the formulated thin beam-column element for modelling damage. In these examples, the load-displacement curves are in agreement with those reported in the literature.
•A new thin beam-column element with capability to model multiple discontinuities.•Physical and mathematical principles based on a Lagrangian formulation.•Boundary value problem and closed form solutions naturally obtained.•A symmetric stiffness matrix for a thin beam-column element with discontinuities.•Nonlinear analysis of frame structural systems with the developed element.
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