XKSj剪切波分裂

We investigate seismic anisotropy in the lowermost mantle in the vicinity of the African large low shear velocity province (LLSVP) using observations of differential SKS‐SKKS shear‐wave splitting. We use data from 375 permanent and temporary stations in Africa which enable us to map the spatial distribution of the anisotropic regions of the lowermost mantle in unprecedented detail. Our results corroborate previous findings that anisotropy is most clearly observed at the margins of the LLSVP, indicating strong deformation at its border, and they are generally consistent with a mostly isotropic LLSVP interior. We find that most discrepant SKS‐SKKS measurements sample the lowermost mantle close to what is inferred to be the root of the Afar plume. We also identify strongly discrepant splitting in the vicinity of a previously mapped ultralow velocity zone (ULVZ) at the base of the LLSVP, beneath Central Africa. This represents an unusual observation of lowermost mantle anisotropy that is spatially coincident with a ULVZ and may reflect a unique anisotropic mechanism such as alignment of partial melt or the presence of strongly anisotropic magnesiowüstite. We interpret discrepant measurements outside of the LLSVP as likely reflecting a change in flow direction from the horizontal plane to a more vertical direction, which may be caused by deflection at the steep LLSVP border. We propose that our observations of D″ anisotropy associated with the African LLSVP can be explained by a mantle flow regime that maintains passive thermochemical piles with slab‐driven flow and allows for the formation of upwellings at their edges.

… of seismic anisotropy in the lowermost mantle are abundant, the mechanism which generates anisotropy in … Here I present splitting measurements for SKS and SKKS waves recorded at …

… discrepancy in splitting between SKS and SKKS, we measured (ϕ, δt) with two types of anisotropic models: an isotropic or anisotropic lower mantle plus an anisotropic upper mantle. …

Widespread rotations of maximum compressive stress in the upper crust are revealed in eastern Tibet by the focal mechanism stress inversion technique. Because of the good correlation with locations of folds, anticlines, and mountains in the region, the upper crustal stress variation is related to the lateral displacement of upper crust. To compare deformations in upper crust and upper mantle, we use SKS/SKKS splitting measurements to investigate the seismic anisotropy in eastern Tibet. The fast polarization directions subparallel to the strikes of faults in Chuandian fragment and southern boundary tectonic belt. This may indicate that the induced anisotropy mainly resulted from alignment of fault fabrics by strong shearing along strike‐slip faults, while the widespread stress rotations and systemic angular difference between upper crustal stress field and seismic anisotropy suggest that the decoupling of upper crustal deformation from middle‐lower crust by a series of detachments in the process of the crust thickening resulted from the obstruction of eastward expansion by Sichuan Basin.

Observations of seismic anisotropy at the base of the mantle are abundant. Given recent progress in understanding how deformation relates to anisotropy in lowermost mantle minerals at the relevant pressure and temperature conditions, these observations can be used to test specific geodynamic scenarios, and have the potential to reveal patterns of flow at the base of the mantle. For example, several recent studies have sought to reproduce measurements of shear wave splitting due to D″ anisotropy using models that invoke specific flow and texture development geometries. A major limitation in such studies, however, is that the forward modelling is nearly always carried out using a ray theoretical framework, and finite-frequency wave propagation effects are not considered. Here we present a series of numerical wave propagation simulation experiments that explore the finite-frequency sensitivity of SKS, SKKS and ScS phases to laterally varying anisotropy at the base of the mantle. We build on previous work that developed forward modelling capabilities for anisotropic lowermost mantle models using the AxiSEM3D spectral element solver, which can handle arbitrary anisotropic geometries. This approach enables us to compute seismograms for relatively short periods (∼4 s) for models that include fully 3-D anisotropy at moderate computational cost. We generate synthetic waveforms for a suite of anisotropic models with increasing complexity. We first test a variety of candidate elastic tensors in laterally homogeneous models to understand how different lowermost mantle elasticity scenarios express themselves in shear wave splitting measurements. We then consider a series of laterally heterogeneous models of increasing complexity, exploring how splitting behaviour varies across the edges of anisotropic blocks and investigating the minimum sizes of anisotropic heterogeneities that can be reliably detected using SKS, SKKS and ScS splitting. Finally, we apply our modelling strategy to a previously published observational study of anisotropy at the base of the mantle beneath Iceland. Our results show that while ray theory is often a suitable approximation for predicting splitting, particularly for SK(K)S phases, full-wave effects on splitting due to lowermost mantle anisotropy can be considerable in some circumstances. Our simulations illuminate some of the challenges inherent in reliably detecting deep mantle anisotropy using body wave phases, and point to new strategies for interpreting SKS, SKKS and ScS waveforms that take full advantage of newly available computational techniques in seismology.

… Using a northeast–southwest trending seismic array deployed in Songpan–… SKS/SKKS wave splitting parameters at 15 broadband seismic stations to study the variation in anisotropy …

… Anisotropy in the lowermost few 100 km of mantle, or D″ region, is indicative of deformation… anisotropy. We investigate the effects of lower-mantle seismic anisotropy on SKS and SKKS …

Since the Mesozoic, central and eastern European tectonics have been dominated by the closure of the Tethyan Ocean as the African and European plates collided. In the Miocene, the edge of the East European Craton and Moesian Platform were reworked in collision during the Carpathian orogeny and lithospheric extension formed the Pannonian Basin. To investigate the mantle deformation signatures associated with this complex collisional-extensional system, we carry out SKS splitting analysis at 123 broad-band seismic stations in the region. We compare our measurements with estimates of lithospheric thickness and recent seismic tomography models to test for correlation with mantle heterogeneities. Reviewing splitting delay times in light of xenolith measurements of anisotropy yields estimates of anisotropic layer thickness. Fast polarization directions are mostly NW–SE oriented across the seismically slow West Carpathians and Pannonian Basin and are independent of geological boundaries, absolute plate motion direction or an expected palaeo-slab roll-back path. Instead, they are systematically orthogonal to maximum stress directions, implying that the indenting Adria Plate, the leading deformational force in Central Europe, reset the upper-mantle mineral fabric in the past 5 Ma beneath the Pannonian Basin, overprinting the anisotropic signature of earlier tectonic events. Towards the east, fast polarization directions are perpendicular to steep gradients of lithospheric thickness and align along the edges of fast seismic anomalies beneath the Precambrian-aged Moesian Platform in the South Carpathians and the East European Craton, supporting the idea that craton roots exert a strong influence on the surrounding mantle flow. Within the Moesian Platform, SKS measurements become more variable with Fresnel zone arguments indicating a shallow fossil lithospheric source of anisotropy likely caused by older tectonic deformation frozen in the Precambrian. In the Southeast Carpathian corner, in the Vrancea Seismic Zone, a lithospheric fragment that sinks into the mantle is sandwiched between two slow anomalies, but smaller SKS delay times reveal weaker anisotropy occurs mainly to the NW side, consistent with asymmetric upwelling adjacent to a slab, slower mantle velocities and recent volcanism.

… We report a fast direction of seismic anisotropy oriented roughly NE–SW, perpendicular to the structures exposed at the surface. This would seem to indicate that the anisotropy is not …

… SKKS and SKS splitting parameters exhibit an azimuthal variation given by ϕ = ϕ 0 + d 1 … Seismic Network stations in southern California. We use other estimates of seismic anisotropy …

Alaska provides an ideal tectonic setting for investigating the interaction between subduction and asthenospheric flow. Within the span of a few hundred kilometers along strike, the geometry of the subducting Pacific plate varies significantly and terminates in a sharp edge. Furthermore, the region documents a transition from subduction along the Aleutian Arc to strike‐slip faulting along the Pacific Northwest. To better understand mantle interactions within this subduction zone, we conduct an SKS shear‐wave splitting analysis on passive‐source seismic data collected between 2011 and 2018 at 239 broadband seismometers, including those from the Transportable Array. Anisotropic fast directions in the east of our study area parallel the Queen Charlotte and Fairweather transform faults, suggesting that the ongoing development of lithospheric anisotropy dominates the results there. However, our observed delay times (δt = 1–1.5 s) obtained across the study region may also imply an asthenospheric contribution to the splitting pattern. Our splitting observations exhibit slab‐parallel fast directions northwest of the trench and a rotation of fast directions around the northeastern slab edge. These observations suggest the presence of toroidal asthenospheric flow around the edge of the downgoing Pacific plate. We suggest that Wrangell Volcanic Field volcanism might be caused by mantle upwelling associated with this flow. Splitting observations closer to the trench can be explained by fossil anisotropy within the downgoing Pacific‐Yakutat plate combined with entrained subslab mantle. The geometry of the slab, including its variable dip and its abrupt eastern edge, thus plays an important role in governing mantle flow beneath Alaska.

… of the upper mantle beneath eastern China. We present seismic shear wave splitting … The splitting parameters exhibit complex regional patterns but are relatively coherent within …

Shear wave splitting is often assumed to be caused by mantle flow or preexisting lithospheric fabrics. We present 2,389 new SKS shear wave splitting observations from 384 broadband stations deployed in Alaska from January 2010 to August 2017. In Alaska, splitting appears to be controlled by the absolute plate motion (APM) of the North American and Pacific plates, the interaction between the two plates, and the geometry of the subducting Pacific‐Yakutat plate. Outside of the subduction zone's influence, the fast directions in northern Alaska parallel the North American APM direction. Fast directions near the Queen Charlotte‐Fairweather transform margin are parallel to the faults and are likely caused by the strike‐slip deformation extending throughout the lithosphere. In the mantle wedge, fast directions are oriented along the strike of the slab with large splitting times and are caused by along‐strike flow in the mantle wedge as the slab provides a barrier to flow. South of the Alaska Peninsula, the fast directions are parallel to the trench regardless of sea floor fabric, indicating along strike flow under the Pacific plate. Under the Kenai Peninsula, the complex flat slab geometry may cause subslab flow to be parallel to Pacific APM direction or to the North America‐Pacific relative motion.

… mantle plays an important role in our understanding of the Earth’s internal dynamics, and shear wave splitting … To date the interpretation of shear wave splitting in terms of anisotropy has …

Abstract Observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D layer), based on differences in the arrival times of vertically ( S V ) and horizontally ( S H ) polarized shear waves. By computing waveforms at a period > 6 s for a wide range of 1‐D and 3‐D Earth structures, we illustrate that a time shift (i.e., apparent splitting) between S V and S H may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near‐surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection s S . For deep earthquakes, apparent splitting can be due to the S wave triplication in D , reflections off discontinuities in the upper mantle, and 3‐D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations.

… The anisotropy on the east side of the gravity lineament primarily results from mantle flows in the big mantle wedge above the flat Pacific slab in the mantle transition zone, and the SKS …

… , we lack the events with opposite azimuths for shear wave splitting. In this study, we employ the spatial coherence method of shear wave splitting parameters to search for the depth of …

… the shear-wave splitting analysis in this particular region (will be discussed below), which hampers our understanding of the detailed upper mantle deformation beneath eastern China. …

We 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.

We investigate shear‐wave splitting of SKS and SKKS phases from teleseismic earthquakes to study the anisotropic structure of the lithosphere and upper mantle beneath the Colombian Andes. This area is shaped by complex interactions between the subducting Nazca and Caribbean plates beneath the South American plate. A recent broadband deployment across the Colombian Andes (MUSICA) provides insight into the dynamic response of the mantle to these adjacent slabs, especially across an offset in the Wadati‐Benioff Zones known as the Caldas Tear. We interpret observed splitting measurements as the result of three interacting flow components within the mantle: (a) entrained trench‐perpendicular corner flow in the mantle wedge, (b) mantle flow through the Caldas Tear, and (c) trench‐parallel flow well east of both subducting plates. While our results are complex, our observed anisotropic patterns in this study are consistent with recent modeling efforts to understand how mantle flow might pass through flat‐slab tears. These findings provide new insight into the heterogeneous anisotropic structure beneath the Colombian Andes at high spatial resolution, and further our understanding of the linkage between complex subduction geometry and surrounding geodynamics.

… We analyze shear wave splitting in seismograms recorded during the seven-month operation of the Eifel Network, and we compare the observed fast polarization azimuths to those …

… of measurements of shear-wave splitting due to azimuthal anisotropy in the upper mantle of … which is caused by deformations in the upper mantle. Directions of polarization of the fast …

… Since teleseismic shear wave splitting is a marker of upper mantle tectonic fabric, our aim is to give some insights into the deep structures of the Pyrenees and to investigate the …

We present shear wave splitting … shear wave splitting corrected for sub-station splitting and anisotropy. In order to carry out these corrections we used published shear wave splitting …

… The upwelling of mantle plumes, as well as possible plate subduction in the D″ layer, … with shear wave splitting intensities. We first proved the linearity of the splitting intensities under …

… Map of Hawaii and surrounding region showing our corrected shear-wave splitting station estimates for H2O and JOHN. Triangles indicate stations analyzed in Walker et al. [2001]. …

… is investigated using observations of shear-wave splitting in teleseismic and local shear phases. … records for analysis. Analysis of shear-wave splitting in teleseismic core phases (eg, …

… SUMMARY We examine seismic anisotropy beneath eastern China by analysing shear wave splitting of teleseismic SKS, SKKS and PKS phases recorded at 402 permanent and …

… SKS and PKS splitting parameters were determined in the broader Greek region … teleseismic events that occurred between 2010 and 2017. Data were processed for shear-wave splitting …

… teleseismic earthquakes, using the seismic analysis code (SAC 2000). After that, we perform SKS/SKKS analysis … period of the SKS/SKKS phases (*5–20 s) before shear wave splitting …

Abstract We have investigated the anisotropy strength and fast-axis orientation using an SKS/SKKS splitting technique of seismic phases at Sikkim Himalaya, which is a seismically active zone situated in the central portion of the Great Himalyan Arc in the Indian region. This region lies between two major plate boundary faults, the Main Central Thrust (MCT) and the Main Boundary Thrust (MBT) at its north and south respectively, along with a few regional lineaments. In this study we deployed eight broadband seismic stations and acquired two years of tele-seismic earthquake data, from which we derived 66 good quality anisotropic measurements. In general, the splitting results from both the SKS and SKKS phases show a complex pattern of fast-axis orientation along the northern periphery of the MCT. However, at the central part of the Sikkim between the MBT and the MCT, both results are consistent with the upper mantle deformation of the Indian Plate. We also observed that the anisotropic strength varies between 0.6 s to 3 s and is skewed towards higher anisotropy with orthogonal polarization, which indicate the presence of a two-layer anisotropy. Results of the modelling of 66 anisotropic measurements indicate that the bottom-layer fast-axis orientations are towards N180E with higher anisotropic strength of ∂t = 1.3 s, which elucidates the pristine nature of the upper mantle deformation as a result of asthenospheric flow. But the tectonic deformation of the upper mantle within the lithosphere is prominently observed in the top layer, where the fast axis orientations are towards N480E with lower anisotropic strength of ∂t = 0.6 s.

… 120 from our seismographic stations, to collect SKS phases of sufficient energy. The collected … Analysis and Discussion The analysis performed here is done on teleseismic SKS phases, …

Shear wave splitting of teleseismic core phases such as SKS is commonly used to constrain mantle seismic anisotropy, a proxy for convective deformation. In plate boundaries, sharp lateral variations of splitting measurements near transform faults are often linked to deformation within a lithospheric shear zone below, but potential seismic waveform effects from heterogeneous structure on small scales may influence the interpretation. Here, we explore possible finite frequency effects on shear wave splitting near fault zones in a fully three‐dimensional anisotropic setting. We find that shear zones wider than ∼ ${\sim} $ 80 km, a scale set by the Fresnel zone, can be clearly detected, but narrower zones are less distinguishable. Near the edge of the shear zone, the combined effect of anisotropy and scattering generates false splitting measurements with large delay times and fast axis orientation approaching the back‐azimuth, a bias which can only be identified when records from different back‐azimuths are analyzed together. This substantiates that back‐azimuthal variations of splitting can arise not just from vertical layering but also lateral changes of anisotropic media. We also test the effects of shear zone edge geometry, epicentral distance, filtering frequency, crustal thickness, and sediment cover. Our study delineates the ability of shear wave splitting to resolve and investigate fault zones, and emphasizes the importance of good azimuthal coverage to correctly interpret observed anisotropy. Based on revisiting previous shear wave splitting and lithospheric deformation studies, we infer that many crustal fault zones are underlain by lithospheric shear zones at least 20 km wide.

… We observed upper mantle shear wave splitting using teleseismic SKS waves in LHB, East Antarctica. We propose that the anisotropy is caused by the LPO of mantle minerals in …

The present study investigates azimuthal anisotropy and its relation to the geodynamical processes beneath the back‐arc of the Hellenic subduction zone in the eastern Aegean and western Anatolia where surface tectonics is dominated by the right‐lateral strike‐slip North Anatolian Fault Zone (NAFZ) in the north and E‐W oriented normal fault systems. We obtained apparent SKS splitting parameters from 1,660 good quality and 137 null measurements extracted from 542 events recorded at 40 permanent broadband seismic stations. Overall, the station‐averaged splitting parameters indicate NNE‐SSW oriented fast directions (∼N20°E) and splitting delays around ∼1.5 s. The large splitting delays, particularly observed beneath the northern Aegean can be explained by either an enlarged mantle wedge thickness or increased strength of upper mantle anisotropy. We constrain complex anisotropy structures within two layer models from notable backazimuthal variations in individual splitting measurements observed beneath a few stations at the north located in a close proximity to the NAFZ and central‐western Anatolia. At the western end of the NAFZ, our estimated upper layer anisotropy direction (at ∼120 km) is rather parallel to the NAFZ reflecting the imprint of a lithospheric petrofabric formed by recent deformation while in central‐western Anatolia they correlate well with maximum shear directions and small splitting delays (∼0.6 s) appear to further support relatively thin lithosphere (∼90 km). An overall pattern of extension‐parallel fast directions (N10°E) within lower layer can be attributed to the slab rollback‐induced mantle flow that is highly oblique with respect to the WSW‐ward motion of the Anatolian lithosphere.

… Analyzing teleseismic shear-wave splitting has become a widely adopted technique for detecting such anisotropic structures in the Earth’s crust and mantle. Two complementary types …

Seismic anisotropy provides constraints on the lithospheric deformation and mantle flow. SKS splitting analysis is a common approach to investigate anisotropy, but tends to suffer from potentially insufficient back‐azimuthal coverage. Teleseismic direct S waves provide complementary constraints; however, they cannot be directly used in conventional splitting analysis due to source‐side anisotropy and phase contamination. In this study, we propose a new method based on splitting intensity (SI) to measure receiver‐side seismic anisotropy under stations. The method calculates differential SI between adjacent stations in which the similar contribution of source‐side anisotropy and phase contamination may be eliminated properly. Data with polarization directions within the ranges of 0°–30°, 75°–105°, and 150°–180° referred from the back azimuths and epicentral distances of 30°–80° are considered useful. Splitting parameters obtained from our new method are comparable to those from SKS splitting measurements. We further apply this method to a data set from the southeastern Tibetan Plateau. Using records at 95 permanent stations from 2,783 events, we obtain 36,185 high‐quality SI measurements, which yield 11,185 differential SI measurements. Our results present a similar anisotropy pattern to those revealed by previous research, that is, dominant transition of fast polarizations from N‐S to the north and E‐W to the south of 26°N. The new method largely expands the application of shear wave splitting analysis, which increases the data coverage in many regions worldwide. Absolute SI measurements appear to be necessary for homogeneous anisotropic structures, although this still requires further investigation.

Abstract Seismic azimuthal anisotropy beneath the transitional region between the southeastern Tibetan Plateau and the Indochina Peninsula, an area in which the fast orientation of mantle anisotropy changes to dominantly E-W from mostly N-S on the plateau at the north, is quantified using splitting of the SKS, SKKS, and PKS phases. Among the 50 stations with one or more splitting measurements, 22 possess an azimuthal coverage that is adequate for the identification and characterization of complex anisotropy. The resulting splitting parameters from 15 such stations exhibit systematic back azimuthal variations with a 90° periodicity, which is consistent with a two-layered anisotropy model. The upper layer parameters are consistent with crustal anisotropy measurements obtained independently based on the sinusoidal moveout of P-to-S conversions from the Moho, with the fast orientations being mostly parallel to major shear zones. The lower layer fast orientations and the fast orientations at stations with azimuthally invariant splitting parameters are mostly E-W, which is significantly different from the dominantly N-S trend of the surface expression of major structural fabrics in the area. They are also inconsistent with the absolute plate motion directions. When combined with results from seismic tomography and focal mechanism solutions, the observed azimuthal anisotropy can be adequately explained by the movement of the lithosphere relative to the underlain asthenosphere, most likely associated with the westward rollback of the subducted Indian Plate.

… [1] We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in … We infer that anisotropy sampled by teleseismic phases is localized within or below the …

Observations of seismic anisotropy can provide direct constraints on the character of mantle flow in subduction zones, critical for our broader understanding of subduction dynamics. Here we present over 750 new SKS splitting measurements in the vicinity of Mount St. Helens in the Cascadia subduction zone using a combination of stations from the iMUSH broadband array and Cascades Volcano Observatory network. This provides the highest density of splitting measurements yet available in Cascadia, acting as a focused “telescope” for seismic anisotropy in the subduction zone. We retrieve spatially consistent splitting parameters (mean fast direction Φ: 74°, mean delay time ∂t: 1.0 s) with the azimuthal occurrence of nulls in agreement with the fast direction of splitting. When averaged across the array, a 90° periodicity in splitting parameters as a function of back azimuth is revealed, which has not been recovered previously with single‐station observations. The periodicity is characterized by a sawtooth pattern in Φ with a clearly defined 45° trend. We present new equations that reproduce this behavior based upon known systematic errors when calculating shear wave splitting from data with realistic seismic noise. The corrected results suggest a single layer of anisotropy with an ENE‐WSW fast axis parallel to the motion of the subducting Juan de Fuca plate; in agreement with predictions for entrained subslab mantle flow. The splitting pattern is consistent with that seen throughout Cascadia, suggesting that entrainment of the underlying asthenosphere with the subducting slab is coherent and widespread.

Over the years, seismic anisotropy characterization has become one of the most popular methods to study and understand the Earth’s deep structures. Starting from more than 20 years ago, considerable progress has been made to map the anisotropic structure beneath Italy and the Central Mediterranean area. In particular, several past and current international projects (such as RETREAT, CAT/SCAN, CIFALPS, CIFALPS-2, AlpArray) focused on retrieving the anisotropic structure beneath Italy and surrounding regions, promoting advances in the knowledge of geological and geodynamical setting of this intriguing area. All of these studies aimed at a better understanding the complex and active geodynamic evolution of both the active and remnant subduction systems characterising this region and the associated Apennines, Alps and Dinaric belts, together with the Adriatic and Tyrrhenian basins. The presence of dense high-quality seismic networks, permanently run by INGV and other institutions, and temporary seismic stations deployed in the framework of international projects, the improvements in data processing and the use of several and even more sophisticated methods proposed to quantify the anisotropy, allowed to collect a huge amount of anisotropic parameters. Here a collection of all measurements done on core refracted phases are shown and used as a measure of mantle deformation and interpreted into geodynamic models. Images of anisotropy identify well-developed mantle flows around the sinking European and Adriatic slabs, recognised by tomographic studies. Slab retreat and related mantle flow are interpreted as the main driving mechanism of the Central Mediterranean geodynamics.

… to explain previously reported teleseismic SKS delay times (St… the lithosphere, then the SKS delay times represent variations … rocks to the shear wave splitting of teleseismic SKS waves. …

The region around the Celebes Sea, SE Asia, is evolving within a convergent tectonic environment involving the Pacific plate to the east and the Indian‐Australian plate to the south. It is arguably one of the most tectonically complex regions in the world and serves as an ideal setting to study dynamic interactions between the Pacific and Tethys tectonic domains. The issue of which subducting plate plays a leading role in governing the regional mantle flow is not well understood. Mantle flow can be characterized by seismic anisotropy observations, providing clues for understanding regional tectonics. We conducted SKS‐wave splitting analysis by using data from seven seismic stations located around the Celebes Sea. Our results, when combined with previous observations, suggest the presence of various types of mantle flow in this area, including (a) corner flow in the mantle wedge above the westward‐subducting Molucca Sea (Sangihe) slab, (b) two‐layer anisotropy related to the eastward‐subducting Molucca Sea (Halmahera) slab, (c) deflected flow due to proximity to the Sangihe slab's edges, (d) trench‐normal mantle flow beneath southeastern Borneo due to the subduction of the Indian‐Australian plate, and (e) Northwest Borneo‐Palawan trough‐parallel mantle flow beneath northeastern Borneo. Various types of mantle flow indicate that the dynamic interactions of adjacent subduction zones played crucial roles in influencing the regional upper mantle dynamics between the Tethys and western Pacific domains since the breakup of the Gondwana supercontinent in the Mesozoic.

… The teleseismic XKS data set used in this study was obtained from broadband seismic stations installed in the TSOB and adjacent areas with data archived in the IRIS (Incorporated …

Physical mechanisms of seismic anisotropy in the D” layer are examined based on seismological and mineral physics observations. The results of body-wave seismology on the fine structure of the D” layer and of mineral physics studies on the elastic constants and the lattice preferred orientation in lower mantle minerals as well as the shape preferred orientation of melt pockets are taken into account. Evidence of large but depth (pressure)-dependent elastic anisotropy of lower mantle minerals, particularly (Mg,Fe)O, and of tilted shape preferred orientation of sheared partial melts is summarized. It is shown that both shape preferred orientation of partial melts (or iron-rich secondary phases) and lattice preferred orientation of minerals with well-documented slip systems are difficult to reconcile with seismological observations. However, lattice preferred orientation of highly anisotropic mineral, (Mg,Fe)O, is consistent with most of the seismic observations if the dominant glide plane under the D” layer conditions is 100 rather than 110 as observed at lower pressures. Such a change in glide plane in MgO (or (Mg,Fe)O) is likely to occur as a result of pressure-induced change in elastic anisotropy and/or in the nature of chemical bonding (and possibly due to high temperatures). Both solid-state and partial melt mechanisms of anisotropy imply that the VSH > VSV (VSV > VSH) polarization anisotropy means horizontal (vertical) flow. In the solid-state mechanism, significant VSH > VSV in the D” layer beneath the circum-Pacific (Alaska and the Caribbean) implies horizontal shear at high stress caused presumably by the collision of subducting materials with the core-mantle boundary. Highly variable anisotropy beneath the central-Pacific can be attributed to solid-state fabrics caused by a complicated three-dimensional flow presumably related to the upwelling of plumes, but anisotropy in this region could also be attributed to the shape preferred orientation of melt pockets the presence of which is suggested by very low average velocities.

… \SV anisotropy in the cold thick portions of the D¦ layer is likely … SH anisotropy in an ascending plume far from the D¦ layer but … anisotropy in the D¦ suggests that the regions of the D¦ layer …

… Various mechanisms for generating anisotropy in D" are … of anisotropic minerals in the D" boundary layer, anisotropic structures … D" dynamical properties from shear wave anisotropy has …

… We conclude that the seismic anisotropy observed in the D″ layer beneath both circum Pacific and the central Pacific regions mostly results from lattice-preferred orientation of mixture …

Abstract We use a forward multiscale model that couples atomistic modeling of intracrystalline plasticity mechanisms (dislocation glide ± twinning) in MgSiO3 post-perovskite (PPv) and periclase (MgO) at lower mantle pressures and temperatures to polycrystal plasticity simulations to predict crystal preferred orientations (CPO) development and seismic anisotropy in D″. We model the CPO evolution in aggregates of 70% PPv and 30% MgO submitted to simple shear, axial shortening, and along corner-flow streamlines, which simulate changes in flow orientation similar to those expected at the transition between a downwelling and flow parallel to the core–mantle boundary (CMB) within D″ or between CMB-parallel flow and upwelling at the borders of the large low shear wave velocity provinces (LLSVP) in the lowermost mantle. Axial shortening results in alignment of PPv [010] axes with the shortening direction. Simple shear produces PPv CPO with a monoclinic symmetry that rapidly rotates towards parallelism between the dominant [100](010) slip system and the macroscopic shear. These predictions differ from MgSiO3 post-perovskite textures formed in diamond-anvil cell experiments, but agree with those obtained in simple shear and compression experiments using CaIrO3 post-perovskite. Development of CPO in PPv and MgO results in seismic anisotropy in D″. For shear parallel to the CMB, at low strain, the inclination of ScS, Sdiff, and SKKS fast polarizations and delay times vary depending on the propagation direction. At moderate and high shear strains, all S-waves are polarized nearly horizontally. Downwelling flow produces Sdiff, ScS, and SKKS fast polarization directions and birefringence that vary gradually as a function of the back-azimuth from nearly parallel to inclined by up to 70° to CMB and from null to ∼5%. Change in the flow to shear parallel to the CMB results in dispersion of the CPO, weakening of the anisotropy, and strong azimuthal variation of the S-wave splitting up to 250 km from the corner. Transition from horizontal shear to upwelling also produces weakening of the CPO and complex seismic anisotropy patterns, with dominantly inclined fast ScS and SKKS polarizations, over most of the upwelling path. Models that take into account twinning in PPv explain most observations of seismic anisotropy in D″, but heterogeneity of the flow at scales

… of a sizable elastic anisotropy in a polycrystalline sample of … geometry, we observed the anisotropic lattice strain and {1 0 0… wave velocities, and the velocity anisotropy of the silicate ppv. …

We compile and make publicly available a global digital database of body wave observations of seismic anisotropy in the D′′ layer, grouped using the method used to analyze deep mantle anisotropy. Using this database, we examine the global distribution of seismic anisotropy in the D′′ layer, evaluating the question of whether seismic anisotropy is more likely to be located at the edges of the two large‐low velocity provinces (LLVPs) in Earth's mantle than elsewhere. We show that this hypothesis lacks statistical justification if we consider previously observed lowermost mantle anisotropy, although there are multiple factors that are difficult to account for quantitatively. One such factor is the global lowermost mantle ray coverage for different phases that are commonly used to detect deep mantle anisotropy in shear wave splitting studies. We find that the global ray coverage of the relevant seismic phases is highly uneven, with LLVP edges and their interiors less well‐sampled than the global average.

… Solutions to the wave equation for plane waves in unbounded, homogeneous, anisotropic … More general wave types occurring in bounded and/or layered anisotropic media are, on …

… and compare them with splitting parameters obtained from … for the inversion of the observed splitting parameters (1) a … direct interpretation of the splitting parameters in terms of medium …

… A method for inversion of splitting parameters of shear waves … to be determined by the inversion of these splitting parameters is a … velocities of seismic waves along the symmetry axis, …

… wave splitting have been used in many areas of seismology. … As demonstrated above, a splitting parameter can be … as a simple four-parameter inversion. To demonstrate this, I have …

… of single waveform inversions. We are able to determine stable splitting parameters even from … The aim is to improve the SNR to allow a more stable splitting analysis for seismic source …

… estimates of features of the seismic waves to the elastic … inversion, and reduce the accuracy of the splitting estimates. Although these results are valid for the particular model parameters …

… splitting, and shear wave splitting measurements are now reasonably routine. Interpreting the splitting parameters … in inversions for velocity structure. Interpretations by Plomerovg e! al. …

… deep earthquakes are examined for shear-wave splitting diagnostic of seismic velocity anisotropy in the upper-mantle wedge between the subducting Tonga slab and stations in the Fiji …

… shear wave splitting of SKS phases at 26 permanent broadband stations in western North America to constrain regional trends in anisotropy at the Cascadia subduction zone … subduction…

In this study, we utilize data from 64 broadband seismic stations of the Japanese F-net network to investigate the three-dimensional pattern of anisotropy in the subduction system …

… mantle around subducting plates from surface measurements of shear wave splitting patterns … -like subduction. This computational strategy accounts for the non-steady-state evolution of …

Abstract Observations of shear wave splitting have provided constraints on patterns of deformation in the mantle in a variety of tectonic settings, including those with complex dynamics such as subduction zones. The South American subduction system is characterized by the longest laterally continuous subducting slab present today. As such, it represents an excellent natural laboratory to study variability in anisotropy along thousands of kilometers of a subduction zone. We perform shear wave splitting analyses at 59 stations along the Chilean coast spanning over 2500 km of the subducting Nazca slab. This dataset specifically targets stations that sample minimal amounts of thickened South American crust and mantle wedge material making it easier to attribute splitting to the sub-slab mantle. We observe a stark transition in splitting from the central Bolivian Orocline to southern Chile indicative of a transition in mantle deformation beneath the subducting slab from convergent flow beneath the Orocline to trench parallel flow farther south. This supports previous conceptual models of mantle dynamics in the region where a stagnation point exists beneath the Bolivian Orocline resulting in plate motion driven flow with trench parallel escape flow to both the north and south. Our results as well as previous measurements largely agree with a recent numerical modeling study that has proposed similar large-scale deformation patterns as well as local deviations due to slab structures like tears. The agreement between model predictions and our observations, however, is not universal along strike. We see several regions where measured splitting deviates significantly from predictions. These areas warrant further study and will aid in refining future geodynamic modeling efforts.

… Patterns of SKS shear wave splitting (SWS), defined via fast … We confirm earlier findings that the amount of splitting … high δt in the southern Cascadia Subduction Zone (SCSZ) back-arc …

… [1] We perform shear-wave splitting measurements to determine seismic anisotropy in the … Region, more precisely the New Hebrides subduction zone. We obtained 29 good and 35 fair …

The traditional mantle plume model fails to explain late Cenozoic intraplate volcanism in Northeast China (NEC). We constrained the pattern of upper mantle deformation to study the origin of intraplate volcanism by quantifying the shear wave splitting parameters captured by three NW‐SE linear seismic arrays in NEC. The dense station spacing (10 km) allowed us to image the small‐scale variations in anisotropic structures in unprecedented detail. The WNW‐ESE oriented subduction‐parallel anisotropy is likely induced by the asthenospheric return flow in the big mantle wedge convection associated with Pacific subduction. Therein the most notable feature is a toroidal pattern of anisotropy beneath the Songliao Basin, which is distinguished by a larger splitting time (approximately 1.0 s) relative to weak anisotropy in the center. The toroidal anisotropy pattern coincides with a high‐velocity anomaly extending from uppermost mantle down to ∼300 km depth, presumably indicating early stage foundering lithosphere. The asthenospheric return flow then moved around the foundering lithosphere and induced the corresponding toroidal anisotropy. The decompression partial melting of upwelling asthenospheric materials erupted along weak zones generating late Cenozoic intraplate basalts on both flanks of the Songliao Basin. The high‐density seismic array reveals that the Pacific subduction and early stage lithospheric foundering processes controlled the genesis of late Cenozoic volcanism in NEC.

Measurements of the splitting or birefringence of seismic shear waves that have passed through the Earth’s mantle yield constraints on the strength and geometry of elastic anisotropy in various regions, including the upper mantle, the transition zone, and the D″ layer. In turn, information about the occurrence and character of seismic anisotropy allows us to make inferences about the style and geometry of mantle flow because anisotropy is a direct consequence of deformational processes. While shear wave splitting is an unambiguous indicator of anisotropy, the fact that it is typically a near-vertical path-integrated measurement means that splitting measurements generally lack depth resolution. Because shear wave splitting yields some of the most direct constraints we have on mantle flow, however, understanding how to make and interpret splitting measurements correctly and how to relate them properly to mantle flow is of paramount importance to the study of mantle dynamics. In this paper, we review the state of the art and recent developments in the measurement and interpretation of shear wave splitting—including new measurement methodologies and forward and inverse modeling techniques,—provide an overview of data sets from different tectonic settings, show how they help us relate mantle flow to surface tectonics, and discuss new directions that should help to advance the shear wave splitting field.

… In this report we present shear wave splitting measurements from all presently available three-component broadband stations from the GDSN network, the NARS array, and the Chihese …

We compare sensitivity and reliability of three basic techniques of shear-wave splitting analysis – cross-correlation of wave components, minimization of covariance matrix eigenvalues …