The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Stuart Crampin - One of the best experts on this subject based on the ideXlab platform.
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Shear-Wave Splitting Indicates Non-Linear Dynamic Deformation in the Crust and Upper Mantle
Advances in Nonlinear Geosciences, 2017Co-Authors: Stuart Crampin, Yuan Gao, Gulten Polat, David B. Taylor, Nurcan Meral OzelAbstract:We demonstrate that non-linear dynamic deformation exists throughout the crust and upper mantle of the Earth. Stress-aligned Shear-Wave Splitting, seismic birefringence, is widely observed in the Earth’s upper crust, lower-crust, and uppermost ∼400 km of the mantle. Attributed to the effects of pervasive distributions of stress-aligned fluid-saturated microcracks in the crust (and controversially intergranular films of hydrated melt in the mantle), the degree Splitting indicates that ‘microcracks’ are so closely spaced that they verge on failure in fracturing and earthquakes if there is any disturbance. Phenomena that verge on failure are critical systems with non-linear dynamics that impose a range of new properties on conventional sub-critical geophysics that we suggest is a New Geophysics. Consequently, Shear-Wave Splitting provides directly interpretable information about the progress of non-linear dynamic deformation in the deep otherwise-inaccessible interior of the microcracked Earth. Possibly uniquely for non-linear dynamic phenomena, observation of Shear-Wave Splitting allows the progress towards singularities to be monitored in deep in situ rock, so that earthquakes and volcanic eruptions can be predicted (we prefer stress-forecast). The response to other processes, such as hydraulic fracking, can be monitored, and in some cases calculated and effects predicted. Here, we review Shear-Wave Splitting and demonstrate the prevalence of non-linear dynamic deformation of the New Geophysics in the crust and uppermost ∼400 km of the mantle.
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An Expert System for measuring Shear-Wave Splitting above small earthquakes
Computers & Geosciences, 2008Co-Authors: Ping Hao, Yuan Gao, Stuart CrampinAbstract:As part of the development of a system for routinely measuring Shear-Wave Splitting, this paper introduces an Expert System (ES) to measure the polarisations and time-delays of seismic Shear-Wave Splitting in three-component seismograms above small earthquakes. Expert Systems are rule-based computer techniques designed to provide expertise in particular topics, where the rules are algorithms developed from previous knowledge and experience. The technique is tested on data recorded by the seismic network in Iceland. The statistics suggests that the ES is reasonably successful and provides appropriate initial input parameters for a more precise analysis, which leads to the success of the comprehensive Shear-Wave Analysis System (SWAS) for measuring Shear-Wave Splitting.
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A review of the current understanding of seismic Shear-Wave Splitting in the Earth’s crust and common fallacies in interpretation
Wave Motion, 2008Co-Authors: Stuart Crampin, Sheila PeacockAbstract:Azimuthally-aligned Shear-Wave Splitting is widely observed in the Earth’s crust. The Splitting is diagnostic of some form of seismic anisotropy, although the cause of this anisotropy has been sometimes disputed. The evidence in this review unquestionably indicates cracks, specifically stress-aligned fluid-saturated microcracks, as the predominant cause of the azimuthally-aligned Shear-Wave Splitting in the crust. Although, in principle, Shear-Wave Splitting is simple in concept and easy to interpret in terms of systems of anisotropic symmetry, in practice there are subtle differences from isotropic propagation that make it easy to make errors in interpretation. Unless authors are aware of these differences, misinterpretations are likely which has led to incorrect conclusions and charges of controversy where only misinterpretations exist. As a consequence, stress-aligned fluid-saturated microcracks as the cause of azimuthally-aligned Shear-Wave Splitting in the crust is still not universally accepted despite there being distinguishing features that directly indicate crack-induced anisotropy. This paper reviews observations and interpretations of crack-induced Shear-Wave Splitting and demonstrates that claims for aligned crystals and other sources of Shear-Wave Splitting are due to fallacies in interpretation. This review shows how previous contrary interpretations are resolved and discusses common fallacies and misinterpretations. It is suggested that this new interpretation of Shear-Wave Splitting has such fundamental implications for almost all solid-earth geoscience that it amounts to a New Geophysics with applications to particularly exploration and earthquake geoscience but also to almost to all other branches of solid Earth geoscience.
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A review of techniques for measuring Shear-Wave Splitting above small earthquakes
Physics of the Earth and Planetary Interiors, 2006Co-Authors: Stuart Crampin, Yuan GaoAbstract:Seismic Shear-Wave Splitting is difficult to measure accurately because of the complexity of the Shear-Wave signal. A variety of techniques have been developed for measuring time-delays and polarisations of Shear-Wave Splitting above small earthquakes. These range from ‘display’ techniques, where measurements depend on visual examination of rotated seismograms and polarisation diagrams, through a range of increasingly automatic techniques, to what are almost fully automatic processes. All techniques have disadvantages. Visual techniques are subjective and, although arguably the most accurate, are tedious and time-consuming. More automated techniques work well on noise-free impulsive near-classic examples of Shear-Wave Splitting, but on typical records either require visual checking or need to pass stringent selection criteria which may severely limit the data and bias the results. The accompanying paper presents a combination of visual and automatic techniques to provide a user-friendly semi-automatic measurement technique. Such techniques are important because the new understanding of fluid-rock deformation suggests that Shear-Wave Splitting monitors the low-level deformation of fluid-saturated microcracks in hydrocarbon production processes, as well as the accumulation of stress before earthquakes, and other applications.
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A review of Shear-Wave Splitting in the compliant crack-critical anisotropic Earth
Wave Motion, 2005Co-Authors: Stuart Crampin, Sheila PeacockAbstract:Shear-Wave Splitting due to stress-aligned anisotropy is widely observed in the Earth’s crust and upper mantle. The anisotropy is the result of stress-aligned fluid-saturated grain-boundary cracks and pore throats in almost all crustal rocks, and we suggest by stress-aligned grain-boundary films of liquid melt in the uppermost 400 km of the mantle. The evolution of such fluid-saturated microcracks under changing conditions can be modelled by anisotropic poro-elasticity (APE). Numerical modelling with APE approximately matches a huge range of phenomena, including the evolution of Shear-Wave Splitting during earthquake preparation, and enhanced oil recovery operations. APE assumes, and recent observations of Shear-Wave Splitting confirm, that the fluid-saturated cracks in the crust and (probably) upper mantle are so closely spaced that the cracked rocks are highly compliant critical systems with self-organised criticality. Several observations of Shear-Wave Splitting show temporal variation displaying extreme sensitivity to small stress changes, confirming the crack-critical system. Criticality has severe implications for many Solid Earth applications, including the repeatability of seismic determinations of fluid flow regimes in time-lapse monitoring of hydrocarbon production. Analysis of anisotropy-induced Shear-Wave Splitting is thus providing otherwise unobtainable information about deformation of the inaccessible deep interior of the Earth.
Yuan Gao - One of the best experts on this subject based on the ideXlab platform.
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Shear-Wave Splitting Indicates Non-Linear Dynamic Deformation in the Crust and Upper Mantle
Advances in Nonlinear Geosciences, 2017Co-Authors: Stuart Crampin, Yuan Gao, Gulten Polat, David B. Taylor, Nurcan Meral OzelAbstract:We demonstrate that non-linear dynamic deformation exists throughout the crust and upper mantle of the Earth. Stress-aligned Shear-Wave Splitting, seismic birefringence, is widely observed in the Earth’s upper crust, lower-crust, and uppermost ∼400 km of the mantle. Attributed to the effects of pervasive distributions of stress-aligned fluid-saturated microcracks in the crust (and controversially intergranular films of hydrated melt in the mantle), the degree Splitting indicates that ‘microcracks’ are so closely spaced that they verge on failure in fracturing and earthquakes if there is any disturbance. Phenomena that verge on failure are critical systems with non-linear dynamics that impose a range of new properties on conventional sub-critical geophysics that we suggest is a New Geophysics. Consequently, Shear-Wave Splitting provides directly interpretable information about the progress of non-linear dynamic deformation in the deep otherwise-inaccessible interior of the microcracked Earth. Possibly uniquely for non-linear dynamic phenomena, observation of Shear-Wave Splitting allows the progress towards singularities to be monitored in deep in situ rock, so that earthquakes and volcanic eruptions can be predicted (we prefer stress-forecast). The response to other processes, such as hydraulic fracking, can be monitored, and in some cases calculated and effects predicted. Here, we review Shear-Wave Splitting and demonstrate the prevalence of non-linear dynamic deformation of the New Geophysics in the crust and uppermost ∼400 km of the mantle.
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Crustal anisotropy in north Taiwan from Shear-Wave Splitting
Chinese Journal of Geophysics, 2011Co-Authors: Ling‐xue Tai, Yuan Gao, En‐tzu Lee, Yutao Shi, Hsin‐i LinAbstract:Using the seismic data recorded by 13 seismic stations in north Taiwan from July 1991 to December 2002, this study analyzes the feature of Shear-Wave Splitting in north Taiwan by SAM method of Shear-Wave Splitting. The results show that predominant polarization directions of fast Shear-Waves at Yilan basin strikes to nearly E-W, while polarization directions at mountain ranges (Western Foothill, HsuCeshan Range and Central Range) are in NNE or NE direction. Polarizations are scattered if the stations are on seashore or on island, and often have two predominant polarizations, which may be caused by irregular topography or complicated local tectonics. From the spatial distribution of time delays, we also find that taking station TWE as a boundary, time delays at station TWE and north of the station are longer than those at south of the station. It possibly suggests that anisotropy at station TWE and north of the station are stronger than anisotropy at south of the station.
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Shear-Wave Splitting in the crust beneath the southeast Capital area of North China
Journal of Seismology, 2008Co-Authors: Yuan Gao, Yun-tai ChenAbstract:This study focuses on the southeast Capital area of North China (38.5–39.85° N, 115.5–118.5° E). Shear-Wave Splitting parameters at 20 seismic stations are obtained by a systematic analysis method applied to data recorded by the Capital Area Seismograph Network (CASN) between the years 2002 and 2005. Although some differences in the results are observed, the average fast-wave polarization is N88.2° W ± 40.7° and the average normalized slow wave time delay is 3.55 ± 2.93 ms/km. The average polarization is consistent with the regional maximum horizontal compressive stress and also with the maximum principal strain derived from global positioning system measurements in North China. In spite of the uneven distribution of faults around the array stations that likely introduce some amount of scatter in the Shear-Wave Splitting measurements, site-dependent polarizations of fast shear wave are clearly observed: in the northern half of the study area, the polarizations at CASN stations show E–W direction, whereas in the southern half the polarizations exhibit a variety of possible azimuths, thus suggesting dissimilar stress field and tectonic frame in both areas. Comparing the Splitting results with those previously obtained in the northwest part of the region, we find a difference in polarization of about 20° between the southeast and northwest parts of the Capital area; also, in the southeast Capital area the average time delay is smaller than in the northwest Capital area, thus making clear that the magnitude of crustal seismic anisotropy is not the same in the two zones. Being the Shear-Wave Splitting polarizations in the southeast Capital area, which lies on the basin, clearly different from the observed polarizations in the northwest Capital area, where uplifts and basin converge, it is quite evident that the Shear-Wave Splitting results are consequence of the tectonics and stress field affecting the two regions.
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An Expert System for measuring Shear-Wave Splitting above small earthquakes
Computers & Geosciences, 2008Co-Authors: Ping Hao, Yuan Gao, Stuart CrampinAbstract:As part of the development of a system for routinely measuring Shear-Wave Splitting, this paper introduces an Expert System (ES) to measure the polarisations and time-delays of seismic Shear-Wave Splitting in three-component seismograms above small earthquakes. Expert Systems are rule-based computer techniques designed to provide expertise in particular topics, where the rules are algorithms developed from previous knowledge and experience. The technique is tested on data recorded by the seismic network in Iceland. The statistics suggests that the ES is reasonably successful and provides appropriate initial input parameters for a more precise analysis, which leads to the success of the comprehensive Shear-Wave Analysis System (SWAS) for measuring Shear-Wave Splitting.
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A review of techniques for measuring Shear-Wave Splitting above small earthquakes
Physics of the Earth and Planetary Interiors, 2006Co-Authors: Stuart Crampin, Yuan GaoAbstract:Seismic Shear-Wave Splitting is difficult to measure accurately because of the complexity of the Shear-Wave signal. A variety of techniques have been developed for measuring time-delays and polarisations of Shear-Wave Splitting above small earthquakes. These range from ‘display’ techniques, where measurements depend on visual examination of rotated seismograms and polarisation diagrams, through a range of increasingly automatic techniques, to what are almost fully automatic processes. All techniques have disadvantages. Visual techniques are subjective and, although arguably the most accurate, are tedious and time-consuming. More automated techniques work well on noise-free impulsive near-classic examples of Shear-Wave Splitting, but on typical records either require visual checking or need to pass stringent selection criteria which may severely limit the data and bias the results. The accompanying paper presents a combination of visual and automatic techniques to provide a user-friendly semi-automatic measurement technique. Such techniques are important because the new understanding of fluid-rock deformation suggests that Shear-Wave Splitting monitors the low-level deformation of fluid-saturated microcracks in hydrocarbon production processes, as well as the accumulation of stress before earthquakes, and other applications.
Steven W. Roecker - One of the best experts on this subject based on the ideXlab platform.
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Three‐dimensional shear‐wave Splitting tomography in the Parkfield, California, region
Geophysical Research Letters, 2007Co-Authors: Haijiang Zhang, Y. Liu, Clifford H. Thurber, Steven W. RoeckerAbstract:[1] We developed a three-dimensional (3D) Shear-Wave Splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave Splitting delay times along ray paths derived from a 3D shear velocity model, assuming the delay times are accumulated along the ray paths. The local strength of the anisotropy is indicated by a parameter of anisotropy percentage, K. Using the Shear-Wave Splitting delay times for 575 earthquakes measured at PASO and HRSN stations, we imaged a detailed 3D anisotropy percentage model around the San Andreas Fault Observatory at Depth (SAFOD). The anisotropy percentage model shows strong heterogeneities, consistent with the strong spatial variations in both measured delay times and fast polarization directions. The San Andreas Fault (SAF) zone is highly anisotropic down to a depth of ∼4 km and then becomes less anisotropic at greater depths. Outside the fault zone, the highly anisotropic zone extends as deep as ∼7 km, consistent with the systematic depth dependence of the average time delays. To the southwest of the SAF, the Salinian granitic block shows relatively strong anisotropic anomalies that are presumably caused by aligned microcracks consistent with the direction of the regional maximum compressive horizontal stress. To the northeast of the fault zone, a strong anisotropic anomaly between depths ∼2 and ∼4 km corresponds to a serpentinite body sandwiched between Franciscan rocks.
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Shear-Wave Splitting and small-scale convection in the continental upper mantle
Nature, 1992Co-Authors: L. I. Makeyeva, Lev Vinnik, Steven W. RoeckerAbstract:THE deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt, but this deformation is difficult to observe. Seismic azimuthal anisotropy manifested by Shear-Wave Splitting provides the best geophysical evidence of deformation in the upper mantle. Here we use observations of Shear-Wave Splitting to investigate deformation in the upper mantle beneath the Tien Shan in Central Asia. We find that underneath most of the Tien Shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt, but it deviates by nearly 90° where the upper mantle is anomalously hot. The former relationship, previously observed in other collisional belts, seems to be usual for this environment. The anomalous directions, however, provide evidence of a rising plume beneath the range, suggesting more generally that the along-strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection.
Paul G. Silver - One of the best experts on this subject based on the ideXlab platform.
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The non-commutivity of shear wave Splitting operators at low frequencies and implications for anisotropy tomography
Geophysical Journal International, 2011Co-Authors: Paul G. Silver, Maureen D. LongAbstract:SUMMARY Measurements of the Splitting or birefringence of seismic shear waves constitute a powerful and popular technique for characterizing azimuthal anisotropy in the upper mantle. The increasing availability of data sets from dense broad-band seismic arrays has driven interest in the development of techniques for the tomographic inversion of shear wave Splitting data and in comparing Splitting measurements with anisotropic upper-mantle models obtained from other constraints, such as surface wave analysis. Two different theoretical approaches have been developed for predicting apparent shear wave Splitting parameters (fast direction and delay time) for models that include multiple layers of anisotropy at depth, which is useful for comparing azimuthally anisotropic surface wave models with shear wave Splitting measurements. These approaches differ in one key aspect, which is whether or not the shear wave Splitting operator can be treated as commutative. In this paper, we investigate the theoretical source of this discrepancy, and show that at frequencies relevant to most studies of upper-mantle anisotropy, the term that results in the non-commutivity of the shear wave Splitting operator in the expressions for multiple-layer Splitting must be retained. In contrast, the quantity known as the Splitting intensity, which is closely related to the apparent fast direction and delay time, does commute at these frequencies. We illustrate these inferences with forward modelling examples and discuss their implications for the tomographic inversion of shear wave Splitting measurements, the comparison of surface wave models with shear wave Splitting observations and the joint inversion of surface wave and shear wave Splitting observations for upper-mantle anisotropic models.
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Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions
Surveys in Geophysics, 2009Co-Authors: Maureen D. Long, Paul G. SilverAbstract: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.
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Shear‐wave Splitting beneath the Galápagos archipelago
Geophysical Research Letters, 2005Co-Authors: Fabrice R. Fontaine, Emilie E. E. Hooft, Peter Burkett, Douglas R. Toomey, Sean C. Solomon, Paul G. SilverAbstract:Shear-Wave Splitting measurements in the Galapagos archipelago show a rapid change from consistently oriented anisotropy to no measurable anisotropy. At the western edge of the archipelago delay times are 0.4– 0.9 s and fast polarization directions are 81– 109°E. These directions are consistent with anisotropy resulting from shear of the asthenosphere by the overlying plate; there is no indication of fossil lithospheric anisotropy in the plate spreading direction. In contrast, beneath the center of the archipelago the upper mantle is isotropic or weakly anisotropic. The isotropic region coincides approximately with a volume of anomalously low upper mantle velocities, suggesting that the presence of melt may weaken the effects of fabric on anisotropy or that melt preferred orientation generates a vertical fast polarization direction. Alternatively, the complex flow field associated with a near-ridge hotspot may result in apparent isotropy.
Maureen D. Long - One of the best experts on this subject based on the ideXlab platform.
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The non-commutivity of shear wave Splitting operators at low frequencies and implications for anisotropy tomography
Geophysical Journal International, 2011Co-Authors: Paul G. Silver, Maureen D. LongAbstract:SUMMARY Measurements of the Splitting or birefringence of seismic shear waves constitute a powerful and popular technique for characterizing azimuthal anisotropy in the upper mantle. The increasing availability of data sets from dense broad-band seismic arrays has driven interest in the development of techniques for the tomographic inversion of shear wave Splitting data and in comparing Splitting measurements with anisotropic upper-mantle models obtained from other constraints, such as surface wave analysis. Two different theoretical approaches have been developed for predicting apparent shear wave Splitting parameters (fast direction and delay time) for models that include multiple layers of anisotropy at depth, which is useful for comparing azimuthally anisotropic surface wave models with shear wave Splitting measurements. These approaches differ in one key aspect, which is whether or not the shear wave Splitting operator can be treated as commutative. In this paper, we investigate the theoretical source of this discrepancy, and show that at frequencies relevant to most studies of upper-mantle anisotropy, the term that results in the non-commutivity of the shear wave Splitting operator in the expressions for multiple-layer Splitting must be retained. In contrast, the quantity known as the Splitting intensity, which is closely related to the apparent fast direction and delay time, does commute at these frequencies. We illustrate these inferences with forward modelling examples and discuss their implications for the tomographic inversion of shear wave Splitting measurements, the comparison of surface wave models with shear wave Splitting observations and the joint inversion of surface wave and shear wave Splitting observations for upper-mantle anisotropic models.
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Frequency-dependent shear wave Splitting beneath the Japan and Izu-Bonin subduction zones
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Erin A. Wirth, Maureen D. LongAbstract:Abstract Despite its importance for our understanding of physical processes associated with subduction, the geometry of mantle flow in subduction zones remains poorly understood, particularly in the mantle wedge above subducting slabs. Constraints on mantle flow and deformation can be obtained by measurements of shear wave Splitting, a valuable tool used to characterize the geometry and strength of seismic anisotropy. A complete characterization of shear wave Splitting is particularly important for understanding the mantle wedge beneath Japan, which overlies multiple subduction zones with complex slab morphologies; previous studies indicate that the upper mantle beneath Japan exhibits complicated anisotropy that manifests itself in complex Splitting patterns. To characterize better the geometry of mantle anisotropy beneath Japan, we analyzed direct S waves from local earthquakes originating in the subducting slabs for evidence of shear wave Splitting using data from 54 broadband seismic stations in Japan's F-net array. In addition, both local S and teleseismic SKS phases were examined using data from four F-net stations in the Izu-Bonin arc. In order to characterize any frequency dependence of Splitting parameters that may indicate the presence of complex anisotropy, we carried out our Splitting analysis in two different frequency bands (0.02–0.125 Hz and 0.125–0.5 Hz). Our measurements indicate that shear wave Splitting due to upper mantle anisotropy beneath Japan is highly complex and exhibits both dramatic spatial variations and a strong dependence on frequency.
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Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions
Surveys in Geophysics, 2009Co-Authors: Maureen D. Long, Paul G. SilverAbstract: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.
