Seismic body waves from distant earthquakes, which propagate near vertically beneath recording stations, provide tools for imaging shallow Earth structures with high vertical resolution. The most ...commonly used techniques such as P and S wave receiver functions utilize mode conversions from P to S waves or vice versa to retrieve information on the gradients of elastic properties in the crust and upper mantle. Here we demonstrate the feasibility and advantage of utilizing reflection signals through an improved method of teleseismic P wave coda autocorrelation. We recover clear reflections independently on vertical and radial components, which provide complementary constraints on the subsurface structures. Field data from two stations from different geological settings are analyzed, one of which is an ice station in Antarctica and the other is a bedrock station on the Kaapvaal craton in South Africa. The results from both analyses show the feasibility of the method to unveil P and S wave reflection signals from the ice‐rock interface and the Moho discontinuity. Extensive synthetic experiments are set up to corroborate our results.
Key Points
High‐frequency P and S wave reflections extracted using station autocorrelations
Improved processing allows access to shallow reflections
Successful application to ice and bedrock situations
This work investigates systematically traction- and stress-based approaches for the modeling of strong and regularized discontinuities induced by localized failure in solids. Two complementary ...methodologies, i.e., discontinuities localized in an elastic solid and strain localization of an inelastic softening solid, are addressed. In the former it is assumed a priori that the discontinuity forms with a continuous stress field and along the known orientation. A traction-based failure criterion is introduced to characterize the discontinuity and the orientation is determined from Mohr's maximization postulate. If the displacement jumps are retained as independent variables, the strong/regularized discontinuity approaches follow, requiring constitutive models for both the bulk and discontinuity. Elimination of the displacement jumps at the material point level results in the embedded/smeared discontinuity approaches in which an overall inelastic constitutive model fulfilling the static constraint suffices. The second methodology is then adopted to check whether the assumed strain localization can occur and identify its consequences on the resulting approaches. The kinematic constraint guaranteeing stress boundedness and continuity upon strain localization is established for general inelastic softening solids. Application to a unified stress-based elastoplastic damage model naturally yields all the ingredients of a localized model for the discontinuity (band), justifying the first methodology. Two dual but not necessarily equivalent approaches, i.e., the traction-based elastoplastic damage model and the stress-based projected discontinuity model, are identified. The former is equivalent to the embedded and smeared discontinuity approaches, whereas in the later the discontinuity orientation and associated failure criterion are determined consistently from the kinematic constraint rather than given a priori. The bi-directional connections and equivalence conditions between the traction- and stress-based approaches are classified. Closed-form results under plane stress condition are also given. A generic failure criterion of either elliptic, parabolic or hyperbolic type is analyzed in a unified manner, with the classical von Mises (J2), Drucker–Prager, Mohr–Coulomb and many other frequently employed criteria recovered as its particular cases.
A number of possible hypotheses have been proposed to explain the origin of mid-lithospheric discontinuities (MLDs), typically characterized by ∼2-6% reductions in seismic shear wave velocity (VS) at ...depths of 60 km to ∼150 km in the cratonic sub-continental lithospheric mantle (SCLM). One such hypothesis is the presence of low-shear wave velocity, hydrous and carbonate mineral phases. Although, the presence of hydrous silicates and carbonates can cause a reduction in the shear wave velocity of mantle domains, the contribution of volatile metasomatism to the origins of MLDs has remained incompletely evaluated. To assess the metasomatic origin of MLDs, we compiled experimental phase assemblages, phase proportions, and phase compositions from the literature in peridotite + H2O, peridotite + CO2, and peridotite + H2O + CO2 systems at P-T conditions where hydrous silicate and/or carbonate minerals are stable. By comparing the experimental assemblages with the compiled bulk peridotite compositions for cratons, we bracket plausible proportions and compositions of hydrous silicate and carbonate mineral phases that can be expected in cratonic SCLMs. Based on the CaO and K2O contents of cratonic peridotite xenoliths and the estimated upper limit of CO2 content in SCLM, ≤∼10 vol.% pargasitic amphibole, ≤∼2.1 vol.% phlogopite and ≤∼0.2 vol.% magnesite solid solution can be stable in the SCLM. We also present new elasticity data for the pargasite end member of amphibole based on first principles simulations for more accurate estimates of aggregate VS for metasomatized domains in cratonic mantle. Using the bracketed phase compositions, phase proportions, and updated values of elastic constants for relevant mineral end members, we further calculate aggregate VS at MLD depths for three seismic stations in the northern continental U.S. Depending on the choice of background wave speeds of unmetasomatized peridotite and the cratonic geotherm, the composition and abundance of volatile-bearing mineral phases bracketed here can explain as much as 2.01 to 3.01% reduction in VS. While various craton formation scenarios allow formation of the amphibole and phlogopite abundances bracketed here, presence of volatile-bearing phases in an average cratonic SCLM composition cannot explain the entire range of velocity reductions observed at MLDs. Other possible velocity reduction mechanisms thus must be considered to explain the full estimated range of shear wave speed reduction at MLD depths globally.
•High P-T peridotite ± H2O ± CO2 phase relations vs sub-continental mantle compositions.•≤10% pargasite, ≤2% phlogopite, ≤0.2% magnesite causes mid-lithospheric discontinuity (MLD) velocity reductions.•First-principles calculations yield pargasite Vs at MLD to be 4-8% slower than previous estimates.•A fraction of MLD VS reduction can occur via volatile-bearing minerals like pargasite.•Volatile metasomatism at MLD depths can occur during craton formation.
In recent years, there has been a growing interest in analyzing and quantifying the effects of random inputs in the solution of ordinary/partial differential equations. To this end, the spectral ...stochastic finite element method (SSFEM) is the most popular method due to its fast convergence rate. Recently, the stochastic sparse grid collocation method has emerged as an attractive alternative to SSFEM. It approximates the solution in the stochastic space using Lagrange polynomial interpolation. The collocation method requires only repetitive calls to an existing deterministic solver, similar to the Monte Carlo method. However, both the SSFEM and current sparse grid collocation methods utilize global polynomials in the stochastic space. Thus when there are steep gradients or finite discontinuities in the stochastic space, these methods converge very slowly or even fail to converge. In this paper, we develop an adaptive sparse grid collocation strategy using piecewise multi-linear hierarchical basis functions. Hierarchical surplus is used as an error indicator to automatically detect the discontinuity region in the stochastic space and adaptively refine the collocation points in this region. Numerical examples, especially for problems related to long-term integration and stochastic discontinuity, are presented. Comparisons with Monte Carlo and multi-element based random domain decomposition methods are also given to show the efficiency and accuracy of the proposed method.
Kinetic‐Scale Current Sheets in the Solar Wind at 5 AU Vasko, I. Y.; Alimov, K.; Phan, T. D. ...
Journal of geophysical research. Space physics,
June 2024, 2024-06-00, 20240601, Letnik:
129, Številka:
6
Journal Article
Recenzirano
Odprti dostop
We present statistical analysis of 16,903 current sheets (CSs) observed over 641 days aboard Ulysses spacecraft at 5 AU. We show that the magnetic field rotates across CSs through some shear angle, ...while only weakly varies in magnitude. The CSs are typically asymmetric with statistically different, though only by a few percent, magnetic field magnitudes at the CS boundaries. The data set is classified into about 90.6% non‐bifurcated and 9.4% bifurcated CSs. Most of the CSs are proton kinetic‐scale structures with the half‐thickness of non‐bifurcated and bifurcated CSs within respectively 200–2,000 km and 500–5,000 km or 0.5–5λp and 0.7–15λp in units of local proton inertial length. The amplitude of the current density, mostly parallel to magnetic field, is typically within 0.05–0.5 nA/m2 or 0.04–0.4JA in units of local Alfvén current density. The CSs demonstrate approximate scale‐invariance with the shear angle and current density amplitude scaling with the half‐thickness, Δθ≈16.6°λ/λp0.34 ${\Delta }\theta \approx 16.6{}^{\circ}\,{\left(\lambda /{\lambda }_{p}\right)}^{0.34}$ and J0/JA≈0.14λ/λp−0.66 ${J}_{0}/{J}_{A}\approx 0.14\,{\left(\lambda /{\lambda }_{p}\right)}^{-0.66}$. The matching of the magnetic field rotation and compressibility observed within the CSs against those in ambient solar wind indicate that the CSs are produced by turbulence, inheriting its scale‐invariance and compressibility. The estimated asymmetry in plasma beta between the CS boundaries is shown to be insufficient to suppress magnetic reconnection through the diamagnetic drift of X‐line. The presented results will be of value for future comparative analysis of CSs observed at different distances from the Sun.
Plain Language Summary
Current sheets (CSs) are coherent structures potentially contributing to solar wind heating and particle acceleration. These structures are highly‐likely produced by turbulence, but the alternative hypothesis of coronal origin has not been ruled out. The analysis of CSs at different distances from the Sun may potentially shed light onto the origin and contribution of these structures to solar wind heating and particle acceleration. While there are comprehensive analyses of solar wind CSs at 1 AU and near the Sun, there is still no equivalent analysis of these structures well beyond 1 AU. In this study we present an extensive statistical study of CSs observed aboard Ulysses around 5 AU. We demonstrate that the CSs are predominantly rotations of the magnetic field, typically occurring on proton kinetic scales and exhibiting approximate scale‐invariance. We provide strong evidence that the CSs are produced by turbulence and inherit the scale‐invariance and magnetic field compressibility typical of turbulence. The presented results will be valuable for understanding solar wind turbulence, solar wind heating and particle acceleration at different distances from the Sun.
Key Points
Current sheets (CSs) are predominantly rotations of the magnetic field on proton kinetic scales
Scale‐invariant properties strongly indicate that CSs are produced by turbulence
The asymmetry of CSs is typically insufficient to suppress magnetic reconnection
•Introducing comprehensive literature review about deformation measurement technique in the presence of discontinuities.•The techniques are grouped together as per their relevance.•Discussing DIC ...based techniques for measuring discontinuous deformation.•Future research areas for discontinuous deformation measurement are discussed.
Digital Image Correlation (DIC) is the heart of the remote deformation measurement techniques using images. It is applied to many different applications from medical imaging to engineering problems for measuring deformation remotely with high accuracy. Whereas it is highly successful in measuring continuous deformation, it has severe limitation of failure in measuring discontinuous deformation. Most of the real world problems contains discontinuous deformation and this makes the DIC application restricted to lab environment only. This study presents the first comprehensive literature review on remote deformation measurement in the presence of discontinuities using images. Different methodologies to overcome the limitation of DIC are analysed and categorized systematically. Finally, based on the reviewed literature, a broader outlook and possible future research directions in this area of research are presented.
A finite element method is introduced to simulate surface tension dominated flow of two immiscible fluids featuring an enriched space for capturing both strong and weak pressure discontinuities. The ...proposed enriched finite element space is created utilizing the standard finite element shape functions. Discontinuities are captured by adding merely one additional degree of freedom per each node of the elements cut by the interface. Being local to the cut elements, these additional degrees of freedom are eliminated before assembling the global system of equations following a condensation procedure. The method is stabilized introducing a procedure for improving the conditioning of the enriched pressure contribution to the stiffness matrix in small-cut situations. An improved smoothing strategy based on an artificial diffusion equation is proposed to enhance the performance of the method on rather coarse meshes. A series of three-dimensional two-phase fluid flow benchmarks are solved to assess the performance of the method. Particular attention is paid to surface tension dominated cases. The method is verified by showing its accuracy in capturing strong pressure discontinuity at the interface of a spherical droplet as well as its capability in handling large pressure gradient discontinuity in a hydrostatic liquid–gas container. The method is further validated by simulating oscillations of a slightly disturbed spherical droplet. The mass conservation property of the method and the effect of the smoothing procedure on the result is assessed by simulating the oscillations of a prolate droplet. Ultimately, the method is tested in a more challenging setting by simulating the rising gas bubble inside a liquid domain.
•Stabilized pressure-enriched fixed-grid finite element method.•Accurate capturing of the discontinuity in the pressure and its gradient.•Condensation of the additional degrees of freedom.•Stabilization within the variational multi-scale framework.•Taylored for modeling the droplet dynamics.
Regional‐scale geologic structures characteristic of mantle lithosphere within cratons found in continent interiors are interpreted using geo‐registered diverse data sets from the Slave craton of ...northwest Canada. We developed and applied a new method for mapping seismic discontinuities in three dimensions using multiyear observations at sparse, individual broadband receivers. New, fully 3‐D conductivity models used all available magnetotelluric data. Discontinuity surfaces and conductivity models were geo‐registered with previously published P‐wave and surface‐wave velocity models to confirm first‐order structures such as a midlithosphere discontinuity. Our 3‐D model to 400 km depth was calibrated by “drill hole” observations derived from xenolith suites extracted from kimberlites. A number of new structural discontinuities emerge from direct comparison of coregistered data sets and models. Importantly, we distinguish primary mantle layers from secondary features related to younger metasomatism. Subhorizontal Slave craton layers with tapered, wedge‐shaped margins indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. Mapping of conductivity and metasomatism in 3‐D, the latter inferred via mineral recrystallization and resetting of isotopic ages in xenoliths, indicates overprinting of the primary layered structures. The observed distribution of relatively conductive mantle at 100–200 km depths is consistent with pervasive metasomatism; vertical “chimneys” reaching to crustal depths in locations where kimberlites erupted or where Au mineralization is known.
Key Points
The craton was assembled by stacking of Archean lithospheric layers
Craton margins are characterized by dipping discontinuities and wedge tectonics
Metasomatized mantle is conductive and cross‐cuts constructional layers
The slope stability has already become one of the most important factors directly related to the safety production and development of a mine, which is principally controlled by the properties of ...geological discontinuities of slope, such as bedding planes, faults, cleavage and joints. In this study, a new approach for the evaluation of slope stability in large open-pit mines was presented based on a comprehensive investigation of the geometry and shear strength of the geological discontinuities. This approach follows three steps: (1) graded kinematic analysis using stereographic projection techniques; (2) field estimation of shear strength of discontinuities with the consideration of the impact of uncertainty and scale dependency; and (3) slope stability assessment based on deterministic analysis by FoS and the probabilistic analysis by PoF. A case study, namely, the Yangtaowu Slope in the Dexing Copper Mine, was selected for the application of this approach. The stability assessment result shows good agreement with field observation, illustrating the ability of the proposed method in effectively predicting the stability of open-pit mine slopes. Furthermore, the comparison of the calculated results based on the precise and generalized calculation models demonstrated that the accuracy of the geometric model has a high influence on the stability assessment.
In this study, we identify and characterise how organisations have responded, in ways ranging from restoration to radical change, to discontinuities in their product-based service (PBS) supply chains ...during the COVID-19 pandemic.
Following a theoretical approach that integrates transilience and panarchy theory as a response strategy in PBS supply chains, our qualitative study involved collecting data through 19 semi-structured interviews at six manufacturing firms during the first 6 months of the COVID-19 pandemic (i.e., March to August 2020) and triangulating the findings with the secondary data and that from an industry workshop. Following an inductive approach, we performed thematic data analysis in Nvivo software package.
The findings suggest characterising discontinuities in PBS supply chains as unmanageable external supply-side, demand-side or interactional discontinuities or other manageable deliberate or forced organisational discontinuities. Following that characterisation, we developed a conceptual framework combing both resilience and transformation into new service opportunities.
We gained insights into the first-response abilities and ways of coping among manufacturing firms during the COVID-19 pandemic. Though our findings capture a contemporary, eye-of-the-storm perspective on future directions, a longitudinal study on the pandemic could further validate and extend the modes of response that complement mitigation with the ability to accelerate change or innovation of internal process or external service offerings.
Combining current literature with lessons learned from the firms' immediate responses, this paper's overview and characterisation of discontinuities following the COVID-19 outbreak in PBS supply chains demonstrate how manufacturing firms can foster transilience. As such, it integrates product-based supply chain discontinuities into the domain of service-based supply chains.
PDF
