Wave Splitting Research Articles

Impacts of zonal winds on planetary oscillations and Saturn ring seismology

ABSTRACT The excitation of density and bending waves in Saturn’s C ring by planetary oscillation modes presents a unique opportunity to learn about gas giant interiors and rotation. However, theoretical complications related to Saturn’s rapid and differential rotation pose a barrier to the full utilization of ring wave detections. We calculate oscillation modes using a complete, non-perturbative treatment of differential rotation modelled after Saturn’s zonal winds in self-consistently computed, polytropic equilibria. We find that previous, approximate treatments of the effects of differential rotation in Saturn overestimate shifts in the frequencies of fundamental modes (f modes) thought to be responsible for the majority of the waves detected in the C ring, due to an omitted modification of the equilibrium shape and structure of the planet by differential rotation. The bias introduced by these frequency overestimates is small, but significant relative to the uncertainties afforded by Cassini data. We additionally consider the non-perturbative effects of Saturn-like differential rotation on the rotational mixing of f modes and internal gravity modes (g modes), which is relevant to detections of multiple density waves with very closely split pattern speeds. We find that higher-order rotational effects can produce orders-of-magnitude enhancements in the surface gravitational perturbations of g modes dominated by large spherical harmonic degrees ℓ, regardless of frequency separation from the sectoral f mode. Despite this enhancement, we find that the observed fine splitting of density waves is unlikely to involve g modes dominated by ℓ ≳ 10. This restriction may aid in the inference of possible internal structures for Saturn.

Varying Shear Wave Splitting Parameters Suggest Interaction Between Lithosphere and Asthenosphere in Arxan‐Chaihe Volcanic Field, NE China

Over 2 years of broadband seismic data from 27 temporary stations deployed in the Arxan-Chaihe Volcanic Field (ACVF) provides us a valued opportunity to study mantle deformation and related geodynamic processes in NE China intraplate region. We estimated 153 pairs of high-quality teleseismic shear wave splitting parameters at 22 stations and 98 non-splitting measurements at 19 stations. Most of the stations in ACVF are characterized by NNW–SSE trending fast directions with variable splitting delay times, suggesting the fossilized extensional deformation within the lithosphere is locally thermal erased by the hot mantle upwellings associated with the volcanism. In the region south of the 46.5°N, a sharp swing of fast polarization occurs, and both nearly NW-SE and W-E oriented fast directions are observed, which could be explained by small local mantle convection caused by the lithospheric delamination.

Wave Equation Numerical Simulation and RTM With Mixed Staggered-Grid Finite-Difference Schemes

For the conventional staggered-grid finite-difference scheme (C-SFD), although the spatial finite-difference (FD) operator can reach 2Mth-order accuracy, the FD discrete wave equation is the only second-order accuracy, leading to low modeling accuracy and poor stability. We proposed a new mixed staggered-grid finite-difference scheme (M-SFD) by constructing the spatial FD operator using axial and off-axial grid points jointly to approximate the first-order spatial partial derivative. This scheme is suitable for modeling the stress–velocity acoustic and elastic wave equation. Then, based on the time–space domain dispersion relation and the Taylor series expansion, we derived the analytical expression of the FD coefficients. Theoretically, the FD discrete acoustic wave equation and P- or S-wave in the FD discrete elastic wave equation given by M-SFD can reach the arbitrary even-order accuracy. For acoustic wave modeling, with almost identical computational costs, M-SFD can achieve higher modeling accuracy than C-SFD. Moreover, with a larger time step used in M-SFD than that used in C-SFD, M-SFD can achieve higher computational efficiency and reach higher modeling accuracy. For elastic wave simulation, compared to C-SFD, M-SFD can obtain higher modeling accuracy with almost the same computational efficiency when the FD coefficients are calculated based on the S-wave time–space domain dispersion relation. Solving the split elastic wave equation with M-SFD can further improve the modeling accuracy but will decrease the efficiency and increase the memory usage as well. Stability analysis shows that M-SFD has better stability than C-SFD for both acoustic and elastic wave simulations. Applying M-SFD to reverse time migration (RTM), the imaging artifacts caused by the numerical dispersion are effectively eliminated, which improves the imaging accuracy and resolution of deep formation.

Splitting of waves in rotor-in-rotor nonlocal metamaterials by internal rotor coupling

Mechanical metamaterials can be used to control wave propagation because they are flexibly adjustable. In this paper, a one-dimensional rotor-in-rotor metamaterial with internal rotor coupling is proposed. This metamaterial introduces a new connection between lattices into a rotor-in-rotor system to produce a nonlocal metamaterial. Meanwhile, a zero-frequency bandgap with negative mass can be obtained. The vibration reduction efficiency of bandgaps is analysed based on the recursive method and numerically verified. Furthermore, wave splitting is found in the rotor-in-rotor metamaterial due to strong equivalent nonlocal interactions. Its dispersion relation shows a “dual wavevector” phenomenon: there are frequency ranges where a positive slope and a negative slope coexist. Numerical simulations using finite cell analysis are performed to validate the wave splitting by calculating the phase velocity and group velocity. The finding of wave splitting in the dual wavevector region is expected to provide new insights to control wave propagation.

Depletion-induced seismicity in NW-Germany: lessons from comprehensive investigations

Evaluating various investigations for north-German gas fields, we discuss past and actual evolutions of the rock fabric in the light of dilatant driven and spontaneous contractant critical phenomena. Features of the latter were discovered by multi-stage triaxial tests with water-saturated sandstone samples and were similarly observed around the gas fields. A Mohr–Coulomb condition with quasi-local stress components ({hat{sigma }}'_1 and {hat{sigma }}'_3), and variable parameters phi ’ and {{hat{c}}}', can capture successive critical states of the solid fabric. The implied driven dilatation up to a collapse with contraction is captured by a stress-dilatancy relation. Fractal patterns of shear bands (faults) dominate if the smallest principal stress {hat{sigma }}'_3 exceeds {{hat{c}}}', otherwise cracks dominate and can lead to a rockburst. Triaxial tests with X-ray attenuation, seismometry including the splitting of shear waves and/or neutron beam diffraction contribute to clarification and validation. Seismic early warning and calculation models for various geotechnical operations with dominating faults can thus be improved, but the task is more difficult for rockbursts.

The splitting mechanism of the second-order rogue wave—Interaction between two component first-order Akhmediev breathers

Based on the nonlinear Schrödinger equation, it's illustrated that the peak intensity of Akhmediev breather is negatively correlated with the modulation frequency firstly. Then, two splitting modes of the second-order rogue wave have been studied in detail through the second-order rogue wave train formed by the nonlinear superposition of two first-order Akhmediev breathers whose modulation frequency ratio is 1:2. It is revealed that the process of the mode B splitting can be divided into three stages: the restructuring stage where the intensity of the latter sub-peaks of the main peak decreases first and then increases, the competition stage where the fine structure forms and the relaxation stage. The process of the mode A splitting can be divided into two stages: the restructuring stage and the relaxation stage. And both are due to the interaction between the two first-order component Akhmediev breathers. The results can be used to explain the different characteristics of the two splitting modes of the second-order rogue wave. In addition, the inevitability of the fine structure appearance in the process of the mode B splitting of the second-order rogue wave, which causes the transformation point on the peak intensity curve of the second order rogue wave, is also revealed. We anticipate that it will be a meaningful and novel way to study the characteristics of the high-order rogue wave through interaction between breathers with certain modulation frequency ratio.

Stable reference wave generation using a repeater laser for remote digital holography

We experimentally evaluate the tolerance of a repeater laser (RL) method, which generates a reference wave using a phase-locked laser. The tolerance to the optical power variation of a plane wave component in a split object wave is limited by the dynamic range of an imager to record holograms in a conventional method, whereas it is limited by that of a photodetector for phase-locking in the RL method. The RL method using a commercial imager and photodetector achieved constant high measurement accuracy under the varying range of 50 dB and 100 times higher tolerance than the conventional method.

Classification of Teleseismic Shear Wave Splitting Measurements: A Convolutional Neural Network Approach

AbstractShear wave splitting (SWS) analysis is widely used to provide critical constraints on crustal and mantle structure and dynamic models. In order to obtain reliable splitting measurements, an essential step is to visually verify all the measurements to reject problematic measurements, a task that is increasingly time consuming due to the exponential increase in the amount of data. In this study, we utilized a convolutional neural network (CNN) based method to automatically select reliable SWS measurements. The CNN was trained by human‐verified teleseismic SWS measurements and tested using synthetic SWS measurements. Application of the trained CNN to broadband seismic data recorded in south central Alaska reveals that CNN classifies 97.0% of human selected measurements as acceptable, and revealed ∼30% additional measurements. To our knowledge, this is the first study to systematically explore the potential of a machine‐learning based technique to assist with SWS analysis.

Slab Induced Mantle Upwelling Beneath the Anatolian Plateau

AbstractThe subducting African Plate in the easternmost Mediterranean is actively tearing and deforming beneath the Anatolian Plate as the margin transitions from long‐lived subduction to collision. In central Anatolia, the subducting slab is characterized by both lateral and vertical slab tears. We investigate patterns of mantle flow around the edges of a contorting and fragmenting African slab segment, called the Cyprean slab, using measurements of shear wave splitting. We observe three distinct regions of coherent shear wave splitting that correlate with the segmentation boundaries of the Cyprean slab. Regionally coherent mantle flow occurs near both the eastern and western the edges of the slab. These regions of coherent splitting are separated by an area of null splitting that encompasses the Central Anatolian Volcanic Province near the easternmost edge of the slab. The null measurements likely result from mantle upwelling due to the displacement of asthenosphere from the vertical Cyprean slab.

Constraints on fluid flow pathways from shear wave splitting in and around an active fluid-escape structure: Scanner Pockmark, North Sea

SUMMARYVertical fluid-escape structures observed in seismic reflection data represent an important class of potentially active fluid flow pathways. An understanding of the mechanism of fluid flow in these types of structures is needed to assess the risk of natural gas venting from potential subsurface carbon dioxide storage operations. The Scanner Pockmark Complex is a 22 m deep, 900 × 450 m seabed depression in the North Sea, which actively vents methane, and is underlain by a seismic chimney structure with horizontal dimensions of ∼300 × 600 m. Gas accumulation is evidenced by the presence of bright reflectors at the top of this seismic chimney, at a depth of ∼50 m below the seabed. Here, we analyse seismic anisotropy in these shallow sediments using shear wave splitting observed on ocean bottom seismographs (OBS). Anisotropy varies spatially, with a strength of ∼1–4 per cent, on several OBS located in and around the pockmark complex. By correlating these observations with calculated subsurface P- and S-wave velocities, we show that there is anisotropy present throughout the sediments through which the chimney passes, which are interpreted as relating to syn- and post-depositional glaciomarine processes. However, within the chimney itself the orientation of the fast direction is different to that outside the chimney and the degree of anisotropy is lower. We attribute this difference as indicating that the anisotropy observed within the chimney is associated with the formation and continued presence of the gas migration system, which overprints the background depositional anisotropy.

Detection and differentiation of ataxic and hypokinetic dysarthria in cerebellar ataxia and parkinsonian disorders via wave splitting and integrating neural networks.

Dysarthria may present during the natural course of many degenerative neurological conditions. Hypokinetic and ataxic dysarthria are common in movement disorders and represent the underlying neuropathology. We developed an artificial intelligence (AI) model to distinguish ataxic dysarthria and hypokinetic dysarthria from normal speech and differentiate ataxic and hypokinetic speech in parkinsonian diseases and cerebellar ataxia. We screened 804 perceptual speech analyses performed in the Samsung Medical Center Neurology Department between January 2017 and December 2020. The data of patients diagnosed with parkinsonian disorders or cerebellar ataxia were included. Two speech tasks (numbering from 1 to 50 and reading nine sentences) were analyzed. We adopted convolutional neural networks and developed a patch-wise wave splitting and integrating AI system for audio classification (PWSI-AI-AC) to differentiate between ataxic and hypokinetic speech. Of the 395 speech recordings for the reading task, 76, 112, and 207 were from normal, ataxic dysarthria, and hypokinetic dysarthria subjects, respectively. Of the 409 recordings of the numbering task, 82, 111, and 216 were from normal, ataxic dysarthria, and hypokinetic dysarthria subjects, respectively. The reading and numbering task recordings were classified with 5-fold cross-validation using PWSI-AI-AC as follows: hypokinetic dysarthria vs. others (area under the curve: 0.92 ± 0.01 and 0.92 ± 0.02), ataxia vs. others (0.93 ± 0.04 and 0.89 ± 0.02), hypokinetic dysarthria vs. ataxia (0.96 ± 0.02 and 0.95 ± 0.01), hypokinetic dysarthria vs. none (0.86 ± 0.03 and 0.87 ± 0.05), and ataxia vs. none (0.87 ± 0.07 and 0.87 ± 0.09), respectively. PWSI-AI-AC showed reliable performance in differentiating ataxic and hypokinetic dysarthria and effectively augmented data to classify the types even with limited training samples. The proposed fully automatic AI system outperforms neurology residents. Our model can provide effective guidelines for screening related diseases and differential diagnosis of neurodegenerative diseases.

Upper crustal anisotropy in the Southeastern Sichuan Basin, China from shear-wave splitting measurements

The Sichuan basin which embodies salt and shale gas resources, is characterized by low seismicity. Seismicity in the southeastern Sichuan basin increased dramatically in recent years, including a series of moderate to strong earthquakes. We investigate seismic anisotropy in the upper crust by performing shear wave splitting analyses on local earthquakes recorded at seventeen stations around the Changning-Xingwen area, southeastern Sichuan basin between January 2016–February 2021. We perform an automatic shear wave splitting analysis and obtain 2926 high-quality splitting measurements by examining 5174 local earthquakes. The delay times range between 0.08 and 0.20 s, and the anisotropy magnitude ranges between 1.9% and 5.5% with a mean of 4.2%, indicating that the study area has significant strength of anisotropy. The fast polarization directions are highly variable reflecting a combined effect of complex local structure and stress field in this area. In the southeastern part of the study area, fast directions are aligned parallel to NW- and NE-oriented surface structures, suggesting structural anisotropy. In the northwestern part of the study area, fast-polarization directions align parallel to the strike of folds and the direction of maximum horizontal compressive stress, probably reflecting the combination of both stress- and structural-controlled anisotropy. Whereas at these stations of central part, fast directions tend to align parallel to the direction of maximum horizontal compressive stress, suggesting stress-induced anisotropy. We examined the temporal variation of seismic anisotropy associated with the 2019 MS 6.0 Changning earthquake. The significant changes in splitting parameters during the Changning earthquake period may only reflect the spatial changes in seismic anisotropy.

Temporal variation in crustal shear wave splitting associated with a moderate earthquake around Nias Island, Indonesia

We investigate temporal variation in shear-wave splitting parameters of local earthquakes around Nias Island, the northern part of Sumatran forearc. We analyse a total of 79 high-quality splitting measurements recorded at a GEOFON seismic station (GSI) from 2009-2015. We observe that there is no clear time variation in the fast polarization of the splitting waves. On the other hand, an increase in the delay time of the splitting waves is found prior to a medium earthquake that occurred around the island in August 2010. Therefore, we suggest that this temporal change may be related to the change in crack geometry of rocks following the accumulation of the stress in the crust before the earthquake occurred.

Mode splitting of spin waves in magnetic nanotubes with discrete symmetries

We investigate how geometry influences spin dynamics in polygonal magnetic nanotubes. We find that lowering the rotational symmetry of nanotubes by decreasing the number of planar facets, splits an increasing number of spin-wave modes, which are doubly degenerate in cylindrical tubes. This symmetry-governed splitting is distinct form the topological one recently observed in cylindrical nanotubes. Doublet modes with half-integer or integer multiple azimuthal periods of the number of facets, split to singlet pairs with lateral standing-wave profiles of opposing mirror-plane symmetries. Furthermore, the polygonal geometry facilitates the hybridization of modes with different azimuthal periods but the same symmetry, manifested in avoided level crossings. These phenomena, unimaginable in cylindrical geometry, provide new tools to control spin dynamics on nanoscale. The presented concept can be generalized to nano-objects of versatile geometries and order parameters, offering new routes to engineer dynamic response in nanoscale devices.

Crustal anisotropy and deformation of the southeastern Tibetan Plateau revealed by seismic anisotropy of mylonitic amphibolites

The composition and deformation patterns in the deep crust of southeastern Tibet are remain controversial. In this study, we present microstructures, fabric and seismic properties of amphibolites from the Ailao Shan-Red River shear zone, western Yunnan, China. The amphibolites display a distinct mylonitic foliation, and the amphibole grains show low intracrystalline deformation, slight compositional zoning but strong crystallographic preferred orientations (CPOs). We explain these CPOs could have formed by rigid body rotation during deformation accommodated by diffusion creep and grain boundary sliding. The amphibolites show strong seismic anisotropies (AVp = 4.6–9.3% and Max. AVs = 3.2–9.0%). Rock recipe modelling results indicate the seismic anisotropies of the mylonitic amphibolites are dominated by the content of biotite and amphibole. The delay time (0.28–1.01 s, average of 0.58 s) calculated for a 40 km-thick amphibolite layer is consistent with the observed delay time (0.52 s) of Pms wave splitting in this region, which indicates a thick amphibolitic layer in the crust of the southeastern Tibetan Plateau. The observed Pms wave splitting, extensive fold deformation and low velocity zones (LVZs) in the Diancang Shan (DCS) massif indicate two anisotropic layers in the crust. The upper layer related to frozen fabrics of amphibolites, which deformed during compressional folding and strike-slip shear. The lower layer related to the alignment of amphibole and melt pocket induced by present-day channel flow. The observed Pms wave splitting in the Ailao Shan (ALS) massif is attributed to amphibolites with vertical foliation and horizontal lineation which are generated by compressional folding and strike-slip shearing.

Distinct Lithospheric Structure in the Xing'an‐Mongolian Orogenic Belt

AbstractDetailed knowledge of the lithospheric structure is essential for a better understanding of the tectonic evolution of the Xing'an‐Mongolian Orogenic Belt (XMOB). We constrained the lithospheric structure across the XMOB by receiver function imaging and shear wave splitting analysis from a dense seismic array. The lithosphere‐asthenosphere boundary (LAB) is coherently imaged in different tectonic blocks. The mid‐lithospheric discontinuity is also identified at ∼75–100 km depth over the deeper LAB (∼110–130 km) beneath the western side of the North‐South Gravity Lineament (NSGL), roughly at the same depth as the LAB beneath the eastern side of the NSGL. The distinct variations in both the lithospheric structure and fast shear‐wave polarization direction across the NSGL are likely caused by different tectonic processes in the two sides, indicating that the NSGL might represent the western boundary influenced by the subduction of the (Paleo‐)Pacific Plate. Lithosphere thinning in the XMOB is evidenced to have been limited mainly to the east of the NSGL.

Seismic Anisotropy Within the Subducting Northern Philippine Sea Plate, SW Japan, Using DONET Seafloor Observation Network

AbstractAnisotropy within subducted slabs carries important information about the deformation history of the slabs. Seismic waves propagating a long distance within a subducted slab can be utilized to invert for the slab anisotropy. The Dense Ocean Floor Network System (DONET) deployed near the Nankai trough are located in a suitable position to record seismic waves guided along the subducted northern Philippine Sea plate (nPSP). In this study, we perform the shear wave splitting analysis on local event data recorded by ocean bottom seismometers from DONET to measure the anisotropy within the subducting nPSP. The obtained fast axis is sub‐trench‐parallel with inferred strength of 1%–3% anisotropy, arising likely from the fossilized fabric formed during the seafloor spreading period and strengthened by trench‐parallel serpentinized hydrated faults. This result shows that the fossilized anisotropy of nPSP is preserved during the subduction.

D″ anisotropy inverted from shear wave splitting intensity

The D″ layer, located at the bottom of the mantle, is an active thermochemical boundary layer. The upwelling of mantle plumes, as well as possible plate subduction in the D″ layer, could lead to large-scale material transformation and mineral deformation, which could result in significant seismic anisotropy. However, owing to limited observations and immense computational cost, the anisotropic structures and geodynamic mechanisms in the D″ layer remain poorly understood. In this study, we proposed a new inversion method for the seismic anisotropy in the D″ layer quantitatively with shear wave splitting intensities. We first proved the linearity of the splitting intensities under the ray-theory assumption. The synthetic tests showed that, with horizontal axes of symmetry and ray incidences lower than 30º in the D″ layer (typical SKS phase), the anisotropy is well resolved. We applied the method to the measured dataset in Africa and Western Europe, and obtained strong D″ anisotropy in the margins of the large low shear-wave velocity provinces and subducting slabs. The new method makes it possible to obtain D″ anisotropy, which provides essential constraints on the geodynamical processes at the base of the mantle.

Mantle Wedge Seismic Anisotropy and Shear Wave Splitting: Effects of Oblique Subduction

AbstractWe investigate the evolution of olivine crystal preferred orientation (CPO) and its effect on local shear wave splitting (SWS) in the mantle wedge of oblique subduction zones. Based on model‐predicted 3‐D mantle wedge flow fields, we compute the A‐type and E‐type olivine CPO distribution for a range of subduction obliquity. The results show that the seismically fast axis does not necessarily align with the flow direction. To model the local SWS parameter distribution for oblique subduction zones, we apply a full range of initial polarization to multilayer models that approximate the model‐predicted CPO distributions. These models result in a bimodal SWS parameter distribution, which relaxes as subduction obliquity increases. Unlike non‐oblique subduction models, these models indicate considerable variations in the SWS parameters with subduction obliquity and initial polarization and also among the forearc, arc, and backarc regions. Because of this variability, a single SWS measurement cannot constrain the CPO distribution, and shear waves with a range of initial polarization are required to interpret the SWS parameters in oblique subduction zones. Our results indicate that 3‐D mantle wedge flow due to oblique subduction cannot explain commonly observed margin‐parallel fast direction in the forearc region but can explain margin‐normal fast directions that are observed in the arc and backarc regions of oblique subduction zones.

Analysis of crustal seismic anisotropy of the Eastern Himalayan collision zone and its adjoining regions

The crustal anisotropy of the Eastern Himalaya collision zone, Brahmaputra Valley and Shillong Plateau of Northeast India Region, is investigated by measuring the shear wave splitting of the PmS phase of teleseismic earthquakes recorded by 17 seismic stations. The anisotropic parameters are nearly stable for each tectonic regime. The mean value of delay time for the Eastern Himalaya stations is 0.45 s which indicates significant crustal anisotropy originated mostly from mid to lower crustal depth. Some stations near deep-seated fault zones exhibit fault-parallel fast polarization directions, indicating structure-induced crustal anisotropy. The fast polarization directions are mostly parallel or sub-parallel to the Himalayan arc suggesting the effect of extension tectonics predominant in the lower crustal depth owing to NW-SE compression tectonics. The maximum shearing of anisotropic minerals presents in the lower crust along the extension direction is the primary causing factor of observed anisotropy. Comparision of the PmS phase splitting with fast polarization directions from shear wave splitting of local earthquakes and stress field analysis demonstrates large angle differences at most stations, imply contrasting anisotropy properties between upper and mid-lower crust.