Anisotropy In Multilayers Research Articles

Perpendicular magnetic anisotropy in multilayers arising from the interplay of thermal strains and diffusion-driven plastic deformation

Magnetic films with perpendicular magnetic anisotropy (PMA) are the basis for efficient memory-storage and future spintronic devices. PMA predominantly stems from surface effects, e.g., symmetry reduction, at the magnetic-layer interface and quickly decays with increasing layer thickness. Strong PMA is thus typically observed in sub-nanometer multilayers with alternating magnetic and noble metals. Using in-situ temperature-dependent measurements of ferromagnetic resonance it is demonstrated that strong PMA can be achieved in Si/Ni multilayers with individual layer as thick as 16 nm. Here, PMA is imposed by thermal strains acting via magnetoelastic coupling and directly depends on the annealing temperature and time, which regulate the diffusion-driven plastic deformation. This opens a way to PMA-film fabrication that avoids complex and resource-intensive production of atomically thin multilayers.

Multilayer anisotropy along the Alaska-Aleutians Subduction zone

SUMMARY Increasing evidence from seismic methods shows that anisotropy within subduction zones should consist of multiple layers. To test this, we calculate and model shear wave splitting across the Alaska-Aleutians Subduction Zone (AASZ), where previous studies have argued for separate layers of anisotropy in the subslab, slab and mantle wedge. We present an updated teleseismic splitting catalogue along the span of the AASZ, which has many broad-band seismometers recently upgraded to three components. Splitting observations are sparse in the Western Aleutians, and fast directions are oriented generally trench parallel. There are significantly more splitting measurements further east along the AASZ. We identify six regions in the Central and Eastern Aleutians, Alaskan Peninsula and Cook Inlet with a high density of splits suitable for multilayered anisotropy analyses. These regions were tested for multilayer anisotropy, and for five of the six regions we favour multiple layers over a single layer of anisotropy. We find that the optimal setup for our models is one with a dipping middle layer oriented parallel to palaeospreading. A prominent feature of our modelling is that fast directions above and below the dipping layer are generally oriented parallel to the strike of the slab. Additionally, we lay out a framework for robust and statistically reliable multilayer shear wave splitting modelling.

Acoustic field simulation-based ultrasonic water immersion non-destructive testing for titanium alloy plate

ABSTRACT Titanium alloy plate is widely used in automotive, aerospace, medical and other fields. Ultrasonic testing is a highly efficient non-destructive testing method for the inspection of titanium alloy components. However, there are strong distortion and attenuation of sound wave propagation in titanium alloys due to the presence of anisotropy and inhomogeneities, making it difficult to detect tiny defects in such components. In addition, some configuration parameters, such as the water path depth also affect the distribution of the acoustic field in the detected components. It is necessary to predict the acoustic field distribution to achieve a better testing accuracy. An acoustic field simulation method based on a multi-Gaussian beam is proposed, which can model the focused acoustic field in a multilayer anisotropy medium. The relationship between the acoustic focused area and the water path depth was explored and optimised comprehensively. C-scan imaging of specimen with flat bottom holes was performed at different water path depths. The results show that the proposed method can optimise the configuration parameters to improve the accuracy of the flaw sizing. This study provides an effective method for testing titanium alloy plate.

Automated shear-wave splitting analysis for single- and multi- layer anisotropic media

Shear-wave velocity anisotropy is present throughout the earth. The strength and orientation of anisotropy can be observed by shear-wave splitting (birefringence) accumulated between earthquake sources and receivers. Seismic deployments are getting ever larger, increasing the number of earthquakes detected and the number of source-receiver pairs. Here, we present a new Python software package, SWSPy, that fully automates shear-wave splitting analysis, useful for large datasets. The software is written in Python, so it can be easily integrated into existing workflows. Furthermore, seismic anisotropy studies typically make a single-layer approximation, but in this work we describe a new method for measuring anisotropy for multi-layered media, which is also implemented. We demonstrate the performance of SWSPy for a range of geological settings, from glaciers to Earth's mantle. We show how the package facilitates interpretation of an extensive dataset at a volcano, and how the new multi-layer method performs on synthetic and real-world data. The automated nature of SWSPy and the discrimination of multi-layer anisotropy will improve the quantification of seismic anisotropy, especially for tomographic applications. The method is also relevant for removing anisotropic effects, important for applications including full-waveform inversion and moment magnitude analysis.

Inducing or suppressing the anisotropy in multilayers based on CoFeB

Controlling the uniaxial magnetic anisotropy is of practical interest for a wide variety of applications. We study ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ single films grown on various crystalline orientations of ${\mathrm{LiNbO}}_{3}$ substrates and on oxidized silicon. We identify the annealing conditions that are appropriate to induce or suppress in-plane uniaxial anisotropy. Anisotropy fields can be increased by annealing up to 11 mT when using substrates with anisotropic surfaces. They can be decreased to below 1 mT when using isotropic surfaces. In the first case, the observed increase of the anisotropy originates from the biaxial strain in the film caused by the anisotropic thermal contraction of the substrate when back at room temperature after strain relaxation during annealing. In the second case, anisotropy is progressively removed by applying successive orthogonal fields that are assumed to progressively suppress any chemical ordering within the magnetic film. The method can be applied to CoFeB/Ru/CoFeB synthetic antiferromagnets, but the tuning of the anisotropy comes with a decrease of the interlayer exchange coupling and a drastic change in the exchange stiffness.

Seismic anisotropy in the mantle of a tectonically inverted extensional basin: A shear-wave splitting and mantle xenolith study on the western Carpathian-Pannonian region

Information on seismic anisotropy in the Earth's mantle can be obtained from (1) shear-wave splitting analyses which allow to distinguish single or multi-layered anisotropy and delay time of the fast and slow polarized wave can indicate its thickness, and (2) studying mantle peridotites where seismic properties can be inferred from lattice preferred orientation of deformed minerals. We provide a detailed shear-wave splitting map of the western part of the Carpathian-Pannonian region (CPR), an extensional basin recently experiencing tectonic inversion, using splitting data. We then compare the results with seismic properties reported from mantle xenoliths to characterize the depth, thickness, and regional differences of the anisotropic layer in the mantle.Mantle anisotropy is different in the northern and the central/southern part of the western CPR. In the northern part, the lack of azimuthal dependence of the fast split S-wave indicates a single anisotropic layer, which agrees with xenolith data from the Nógrád-Gömör volcanic field. Systematic azimuthal variations in several stations in the central areas point to multiple anisotropic layers, which may be explained by two distinct xenolith subgroups described in the Bakony-Balaton Highland. The shallower layer probably has a ‘fossilized’ lithospheric structure, representing former asthenospheric flow, whereas the deeper one reflects structures attributed to present-day convergent tectonics, also observed in the regional NW-SE fast S-wave orientations. In the Styrian Basin at the western rim of the CPR, results are ambiguous as shear-wave splitting data hint at the presence of multiple anisotropic layers. Spatial coherency analysis of the splitting parameters places the center of the anisotropic layer at ~140–150 km depth under the Western Carpathians, which implies a total thickness of ~220–240 km. Thicknesses estimated from seismic properties of xenoliths give lower values, pointing to heterogeneously distributed anisotropy or different orientation of the mineral deformation structures.

Seismic anisotropy accrued by seven unusually deep local earthquakes (between 50 and 60 km) in the Albertine Rift: implications of asthenospheric melt upwelling

We investigated the primary mechanisms triggering the S-wave splitting (SWS) of seven unusually deep local earthquakes (between 50 and 60 km) which originated in the lithosphere beneath the Rwenzori region. We attempted to develop an understanding of the relationship between anisotropic structures in the lithosphere and tectonic deformation processes. A total of 12 out of 44 waveforms showed evidence of SWS on their polarization diagrams. The fast-wave direction (φ) and delay-time (δt) were estimated using the covariance matrix and the cross-correlation coefficient methods, respectively. We observed a clockwise rotation of φ-directions (NW - SE and ~ENE - WSW) at stations located in the rift valley. We related this pattern of φ-directions to anisotropic fabric, probably lattice-preferred orientation of preexisting olivine, whose a-axes are aligned with ESE absolute plate motion (APM) vector. At stations located outside the rift valley, however, we observed WNW - ESE and NNW - SSE patterns of φ-directions. We associated these patterns to the shape-preferred orientation of structures frozen in the lithosphere that are aligned with the present-day APM direction. We observed δt values ranging between 0.04 ± 0.01s and 0.43 ± 0.02 s, which decrease with distance away from the rift axis. This further supported our concept that the anisotropy observed at stations located on the moving plate is related to aligned melt inclusions frozen in the surrounding lithosphere. We further observed that the δt values increase linearly with ray-path length, which could indicate a fairly uniform anisotropy between 50-km and 60-km depth. Our study reported no evidence of multi-layer anisotropy beneath the Rwenzori region.

Shear-wave splitting beneath Fennoscandia — evidence for dipping structures and laterally varying multilayer anisotropy

SUMMARYThe geodynamic evolution of Fennoscandia in northern Europe (Finland, Sweden and Norway) is coined by ca. 3 Ga history of tectonic processes including continental growth in its central and eastern parts and Neogene uplift processes of the Scandinavian mountains (Scandes) located along its western edge. Many details are still under debate and we contribute with new findings from studying deep-seated seismic anisotropy. Using teleseismic waveforms of more than 260 recording stations (long-running permanent networks, previous temporary experiments and newly installed temporary stations) in the framework of the ScanArray experiment, we present the most comprehensive study to date on seismic anisotropy across Fennoscandia. The results are based on single and multi-event shear-wave splitting analysis of core refracted shear waves (SKS, SKKS, PKS and sSKS). The splitting measurements indicate partly complex, laterally varying multilayer anisotropy for individual areas. Consistent measurements at permanent and temporary recording stations over several years and for seismic events of specific source regions allow us to robustly constrain dipping anisotropic structures by adding systematic forward modelling. Although the data coverage is partly limited to only few source regions, our findings support concepts of continental growth due to individual episodes of (paleo-) subduction, each affecting a plunging of the anisotropic fast axis direction due to collisional deformation. Along the northern Scandes the fast axis direction (ϕ) is parallel to the mountain range (NE-SW), whereas an NNW-SSE trend dominates across the southern Scandes. In the south, across the Sorgenfrei–Tornquist Zone, a NW-SE trend of ϕ dominates which is parallel to this suture zone. The Oslo Graben is characterized by an NNE-SSW trend of ϕ. In northern Norway and Sweden (mainly Paleoproterozoic lithosphere), a dipping anisotropy with ϕ towards NE prevails. This stands in contrast to the Archean domain in the NE of our study region where ϕ is consistently oriented NNE-SSW. In the Finnish part of the Svecofennian domain, a complex two-layer anisotropy pattern is found which may be due to lateral variations around the seismic stations and which requires a higher data density than ours for a unique model building. Based on these findings our study demonstrates the importance of long recording periods (in the best case > 10 yr) to obtain a sufficient data coverage at seismic stations, especially to perform meaningful structural modelling based on shear-wave splitting observations.

Receiver function analysis reveals layered anisotropy in the crust and upper mantle beneath southern Peru and northern Bolivia

Subduction systems play a key role in plate tectonics, but the deformation of the crust and uppermost mantle during continental subduction remains poorly understood. Observations of seismic anisotropy can provide constraints on dynamic processes in the crust and uppermost mantle in subduction systems. The subduction zone beneath Peru and Bolivia, where the Nazca plate subducts beneath South America, represents a particularly interesting location to study subduction-related deformation, given the along-strike transition from flat to normally dipping subduction. In this study we constrain seismic anisotropy within and above the subducting slab (including the overriding plate) beneath Peru and Bolivia by examining azimuthal variations in radial and transverse component receiver functions. Because anisotropy-aware receiver function analysis has good lateral resolution and depth constraints, it is complementary to previous studies of anisotropy in this region using shear wave splitting or surface wave tomography. We examine data from long-running permanent stations NNA (near Lima, Peru) and LPAZ (near La Paz, Bolivia), and two dense lines of seismometers from the PULSE and CAUGHT deployments in Peru and Bolivia, respectively. The northern line overlies the Peru flat slab, while the southern line overlies the normally dipping slab beneath Bolivia. We applied harmonic decomposition modeling to constrain the presence, depth, and characteristics of dipping and/or anisotropic interfaces within the crust and upper mantle. We found evidence for varying multi-layer anisotropy, in some cases with dipping symmetry axes, underneath both regions. The presence of multiple layers of anisotropy with distinct geometries that change with depth suggests a highly complex deformation regime associated with subduction beneath the Andes. In particular, our identification of depth-dependent seismic anisotropy within the overlying plate crust implies a change in deformation geometry, dominant mineralogy, and/or rheology with depth, shedding light on the nature of deep crustal deformation during orogenesis.

Upper mantle anisotropy inferred from shear wave splitting beneath the Eastern Indian Shield region

We estimate the shear wave splitting parameters vis-à-vis the thicknesses of the continental lithosphere beneath the two permanent seismic broadband stations located at Dhanbad (DHN) and Bokaro (BOKR) in the Eastern Indian Shield region. Broadband seismic data of 146 and 131 teleseismic earthquake events recorded at DHN and BOKR stations during 2007–2014 were analyzed for the present measurements. The study is carried out using rotation-correlation and transverse component minimization methods. We retain our “Good”, “Fair” and “Null” measurements, and estimate the splitting parameters using 13 “Good” results for DHN and 10 “Good” results for BOKR stations. The average splitting parameters (ϕ, δt) for DHN and BOKR stations are found to be 50.76°±5.46° and 0.82 ± 0.2 s and 56.30°±5.07° and 0.95 ± 0.17 s, and the estimated average thicknesses of the anisotropic layers beneath these two stations are ∼ 94 and ∼109 km, respectively. The measured deviation of azimuth of the fast axis direction (ϕ) from the absolute motion of the Indian plate ranges from ∼8° to 14°. The measured deviation of azimuth of the fast axis direction (ϕ) from the absolute motion of the Indian plate ranges from ∼8° to 14°. The eastward deviation of the fast axis azimuths from absolute plate motion direction is interpreted to be caused by induced outflow from the asthenosphere. Further, the delay time found in the present analysis is close to the global average for continental shield areas, and also coherent with other studies for Indian shield regions. The five “Null” results and the lower delay time of ∼0.5–0.6 s might be indicating multilayer anisotropy existing in the mantle lithosphere beneath the study area.

M‐Split: A Graphical User Interface to Analyze Multilayered Anisotropy from Shear‐Wave Splitting

ABSTRACT Shear‐wave splitting analysis is commonly used to infer deep anisotropic structure. For simple cases, obtained delay times and fast‐axis orientations are averaged from reliable results to define anisotropy beneath recording seismic stations. However, splitting parameters show systematic variations with back azimuth in the presence of complex anisotropy and cannot be represented by average time delay and fast‐axis orientation. Previous researchers had identified anisotropic complexities at different tectonic settings and applied various approaches to model them. Most commonly, such complexities are modeled using multiple anisotropic layers with priori constraints from geologic data. In this study, a graphical user interface called M‐Split is developed to easily process and model multilayered anisotropy with capabilities to properly address the inherited nonuniqueness. M‐Split program runs user‐defined grid searches through the model parameter space for two‐layered anisotropy using formulation of Silver and Savage (1994) and creates sensitivity contour plots to locate the local maxima and analyze all possible models with parameter trade‐offs. To minimize model ambiguity and identify the robust model parameters, various misfit calculation procedures are also developed and embedded in M‐Split and can be used depending on the quality of the observations and their back azimuthal coverage. M‐Split is an open‐source program and can be extended by users for additional capabilities or for other applications.

On the improvement of SKS splitting measurements by the Simultaneous Inversion of Multiple Waveforms (SIMW)

The birefringence of core-refracted shear waves (e.g. SKS or SKKS) is often used to study seismic anisotropy in the Earth. However, depth resolution and multilayer anisotropy is generally poor for many regions on Earth. This is primarily due to SKS or SKKS phases that are not observable for different backazimuths either because of missing seismicity at the required distance range or because of a too low signal-to-noise ratio (SNR). We propose a new method called Simultaneous Inversion of Multiple Waveforms (SIMW), which allows the joint inversion of multiple core-refracted shear waves from different earthquakes within the same source region, observed by either the same seismic station or by a seismic network. The waveforms are concatenated into a combined signal, which is then inverted with the Silver & Chan method to determine the two splitting parameters: time delay δt, and fast polarization direction Φ. We apply our method to recordings at the large aperture Norwegian NORSAR Array and the German Grafenberg array (GRF). Our results demonstrate that SIMW allows a stable determination of splitting results for low-amplitude or noisy SKS signals. Splitting parameter uncertainties can be reduced and reliable results are obtained for both arrays. Moreover, new backazimuth directions can be explored, enabling a more accurate derivation of two-layer anisotropy models. Our new methodology is particularly helpful for temporary station deployments with limited recording times in order to utilize as many as possible signals including such with low-amplitude and small SNR.

Optimum Multilayer Anisotropy Distribution for Microwave-Assisted Magnetic Recording [CoX/Pt]<italic>n</italic>Media

The optimum multilayered anisotropy distribution in [CoX/Pt]4 media is studied for microwave-assisted magnetic recording by micromagnetic simulations. The model of the magnetic CoX layer was built upon a regular mesh, including a Voronoi-tessellation polycrystalline structure. If each CoX layer has the same anisotropy, it is found that the moments of grains at the same in-plane position but in different layers may be in the opposite directions, which damages the signal-to-noise ratio (SNR) performance and the thermal stability of recording media. In the optimum media design, the anisotropy field constant $K_{n}$ decreases from the top layer to the bottom layer, where the magnetic moments of grains in each bits are switched nearly uniformly. A medium SNR gain of $1.5\sim 6.8$ dB assisted by spin-transfer oscillator field is confirmed for media with average $\bar H_{k}$ in a range of 2–2.4 T. A best SNR performance of 15.8 dB is found in media with an average $\bar H_{k} =2$ T, with ( $K_{1}-K_{4})$ of $2\times 10^{6}$ erg/cc and an interlayer exchange field constant of 2 kOe.

Upper mantle anisotropy beneath Peru from SKS splitting: Constraints on flat slab dynamics and interaction with the Nazca Ridge

The Peruvian flat slab is by far the largest region of flat subduction in the world today, but aspects of its structure and dynamics remain poorly understood. In particular, questions remain over whether the relatively narrow Nazca Ridge subducting beneath southern Peru provides dynamic support for the flat slab or it is just a passive feature. We investigate the dynamics and interaction of the Nazca Ridge and the flat slab system by studying upper mantle seismic anisotropy across southern Peru. We analyze shear wave splitting of SKS, sSKS, and PKS phases at 49 stations distributed across the area, primarily from the PerU Lithosphere and Slab Experiment (PULSE). We observe distinct spatial variations in anisotropic structure along strike, most notably a sharp transition from coherent splitting in the north to pervasive null (non-split) arrivals in the south, with the transition coinciding with the northern limit of the Nazca Ridge. For both anisotropic domains there is evidence for complex and multi-layered anisotropy. To the north of the ridge our KS⁎ splitting measurements likely reflect trench-normal mantle flow beneath the flat slab. This signal is then modified by shallower anisotropic layers, most likely in the supra-slab mantle, but also potentially from within the slab. To the south the sub-slab mantle is similarly anisotropic, with a trench-oblique fast direction, but widespread nulls appear to reflect dramatic heterogeneity in anisotropic structure above the flat slab. Overall the regional anisotropic structure, and thus the pattern of deformation, appears to be closely tied to the location of the Nazca Ridge, which further suggests that the ridge plays a key role in the mantle dynamics of the Peruvian flat slab system.

In-plane uniaxial magnetic anisotropy induced by anisotropic strain relaxation in high lattice-mismatched Dy/Sc superlattices

We report on the magnetic and structural characterization of high lattice-mismatched [Dy${}_{2nm}$/Sc${}_{{t}_{\text{Sc}}}$] superlattices, with variable Sc thickness ${t}_{\text{Sc}}$= 2--6 nm. We find that the characteristic in-plane effective hexagonal magnetic anisotropy K${}_{6}^{6,ef}$ reverses sign and undergoes a dramatic reduction, attaining values of $\ensuremath{\approx}$13--24 kJm${}^{\ensuremath{-}3}$, when compared to K${}_{6}^{6}=\ensuremath{-}0.76$ MJm${}^{\ensuremath{-}3}$ in bulk Dy. As a result, the basal plane magnetic anisotropy is dominated by a uniaxial magnetic anisotropy (UMA) unfound in bulk Dy, which amounts to $\ensuremath{\approx}$175--142 kJm${}^{\ensuremath{-}3}$. We attribute the large downsizing in K${}_{6}^{6,ef}$ to the compression epitaxial strain, which generates a competing sixfold magnetoelastic (MEL) contribution to the magnetocrystalline (strain-free) magnetic anisotropy. Our study proves that the in-plane UMA is caused by the coupling between a giant symmetry-breaking MEL constant M${}_{\ensuremath{\gamma},2}^{2}\ensuremath{\approx}1$ GPa and a morphic orthorhombiclike strain ${\ensuremath{\varepsilon}}_{\ensuremath{\gamma},1}\ensuremath{\approx}{10}^{\ensuremath{-}4}$, whose origin resides on the arising of an in-plane anisotropic strain relaxation process of the pseudoepitaxial registry between the nonmagnetic bottom layers in the superstructure. This investigation shows a broader perspective on the crucial role played by epitaxial strains at engineering the magnetic anisotropy in multilayers.

Magnetic Properties of Fe/WRe (001) Multilayers

Following recent theoretical predictions on perpendicular uniaxial magnetocrystalline anisotropy in multilayers of 5 monolayers of Fe and 2 monolayers of W x Re100–x with tetragonally strained components, we have measured the magnetization and detailed atomic structure in a series of such samples with x = 20–80 at%. Although the achieved strain in the WRe layers is smaller than in the modelled case, we do observe a positive uniaxial magnetocrystalline anisotropy of around 0.6 MJ/m3 at 40 K, after accounting for the in-plane shape anisotropy. This causes the out-of-plane saturation field to be reduced considerably compared to a pure Fe thin film, while the saturation magnetization is maintained at a value which is enhanced by at least 30% from what would be expected from the multilayer composition alone. The coercivity is low, on the order of 6 kA/m in all samples.

A new analysis of electromagnetic transmission characteristics of anisotropic honeycomb sandwiches

Honeycomb sandwiches are widely used as electromagnetic transparent materials for radomes. However, the electric anisotropy has a significant influence on the transmission performance. This work aims to investigate the electromagnetic transmission characteristics of the anisotropic sandwich panel. First, we deduce the effective permittivity of multilayered anisotropic sandwich material in the respect of the horizontal polarization and the perpendicular polarization components of the incident wave. Second, the transmission line network method related to the multilayered homogeneous medium is improved to simulate the electromagnetic transmission through honeycomb sandwiches and to calculate the transmission ratio. As the proposed method takes into account the three-dimensional anisotropy of each slab, it can simulate the transmission of plane wave with arbitrary incident direction in multilayered anisotropy sandwich panels, moreover, it can reveal the influence of material orientation on the transmission characteristics. Since the multilayer configuration is simulated by transmission line network, the proposed method is far more efficient than the finite element method. Numerical experiments indicate that the influence of the electric anisotropy on the transmission performance of honeycomb sandwich materials can be well revealed. In an incident angle range between 0 and 80 degrees, the simulation results fit well to the results obtained by the finite element method.

Shear wave splitting at the Hawaiian hot spot from the PLUME land and ocean bottom seismometer deployments

We examine upper mantle anisotropy across the Hawaiian Swell by analyzing shear wave splitting of teleseismic SKSwaves recorded by the PLUME broadband land and ocean bottom seismometer deployments. Mantle anisotropy beneath the oceans is often attributed to flow‐induced lattice‐preferred orientation of olivine. Splitting observations may reflect a combination of both fossil lithospheric anisotropy and anisotropy due to present‐day asthenospheric flow, and here we address the question whether splitting provides diagnostic information on possible asthenospheric plume flow at Hawaii. We find that the splitting fast directions are coherent and predominantly parallel to the fossil spreading direction, suggesting that shear wave splitting dominantly reflects fossil lithospheric anisotropy. The signature of anisotropy from asthenospheric flow is more subtle, although it could add some perturbation to lithospheric splitting. The measured delay times are typically 1 s or less, although a few stations display larger splitting delays of 1–2 s. The variability in the delay times across the different stations indicates differences in the degree of anisotropy or in the thickness of the anisotropic layer or in the effect of multilayer anisotropy. Regions with smaller splitting times may have experienced processes that modified the lithosphere and partially erased the fossil anisotropy; alternatively, asthenospheric splitting may either constructively add to or destructively subtract from lithospheric splitting to produce the observed variability in delay times.

No direct correlation of mantle flow beneath the North China Craton to the India-Eurasia collision: constraints from new SKS wave splitting measurements

SUMMARY The long-range effects of the ongoing India–Eurasia collision and their role in the Cenozoic tectonic evolution of eastern China are not well understood. Here, we investigate deformation beneath the North China Craton and the Qilian Mountain region between the Haiyuan and Kunlun faults, both of which are located immediately to the northeast of the Tibetan Plateau. We present teleseismic shear wave splitting parameters from 299 broad-band stations. These stations provided good spatial coverage in mapping upper-mantle deformation beneath the North China Craton and the Qilian Mountain. The backazimuth dependence of fast directions fromstationslocatedwithintheQilianMountainregionindicatesacomplexanisotropypattern. A multilayer anisotropy and/or anisotropy with a dipping symmetry axis may exist beneath the Qilian Mountain. The overlapping or underthrusting of material evident in anisotropy from this region may be a long-range effect of the India–Eurasia collision. The central North China Craton showed spatially coherent fast directions and the shear wave velocity anomalies within the upper mantle. These patterns suggest horizontal deflection of a regional mantle upwelling, possibly originating from the mantle transition zone. Anisotropy with large delay times was observed in the vicinity of Shanxi rift system. High ∂lnVS/∂lnVP values for the upper mantle indicate that anisotropy in this area is enhanced by vertically aligned melt-filled fissures. Our results suggest that the dynamic processes beneath the central North China Craton are not directly related to the India–Eurasia collision.

Frequency-dependent shear-wave splitting and multilayer anisotropy in northeast Japan

[1] We analyzed carefully shear-wave splitting on 320 intermediate-depth earthquakes occurring in the subducting Pacific slab in different frequency bands to investigate the S-wave anisotropy and subduction dynamics under Northeast (NE) Japan. Our results show that the differential time between the fast and slow shear waves (δt) is definitely smaller (<0.2 s) in the high-frequency band than that (0.3–0.4 s) in the low-frequency band, and so the splitting parameters, especially δt, are strongly frequency-dependent. Although the δt is indubitably smaller under NE Japan than the other subduction zones regardless of the frequency band, nine large δt values (0.5–0.7 s) are detected, which indicates that the anisotropy is potentially strong in NE Japan. Both the P and S wave anisotropy results in NE Japan are consistent with a model of subduction-driven back-arc spreading and convection in the mantle wedge causing trench-normal fast orientations in the wedge and aligned faults and cracks in the subducting Pacific slab causing trench-parallel fast orientations in the slab. When an S wave travels through the area with the multilayer orthogonal anisotropies, some of its splitting would be cancelled and thus small δt is observed.