Shear Wave Anisotropy Research Articles

Structure and anisotropy of the Mexico subduction zone based on Rayleigh‐wave analysis and implications for the geometry of the Trans‐Mexican Volcanic Belt

We develop a three‐dimensional model of shear wave velocity and anisotropy for the Mexico subduction zone using Rayleigh wave phase velocity dispersion measurements. This region is characterized by both steep and flat subduction and a volcanic arc that appears to be oblique to the trench. We give a new interpretation of the volcanic arc obliqueness and the location of the Tzitzio gap in volcanism based on the subduction morphology. We employ the two‐station method to measure Rayleigh phase velocity dispersion curves between periods of 16 s to 171 s. The results are then inverted to obtain azimuthally anisotropic phase velocity maps and to model 3‐D variations in upper mantle velocity and anisotropy. Our maps reveal lateral variations in phase velocity at all periods, consistent with the presence of flat and steep subduction. We also find that the data are consistent with two layers of anisotropy beneath Mexico: a crustal layer, with the fast directions parallel to the North American absolute plate motion, and a deeper layer that includes the mantle lithosphere and the asthenosphere, with the fast direction interpreted in terms of toroidal mantle flow around the slab edges. Our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure. Our anisotropy results favor a tear, which is also consistent with the geometry of the volcanic belt.

Lithological control on shear-wave velocity anisotropy in core samples from the Taiwan Chelungpu Fault Drilling Project

The present study provides laboratory constrains on the in situ source of shear wave birefringence in siltstone and sandstone lithologies from the Taiwan Chelungpu Fault Drilling Project (TCDP). Bench top measurements at ambient pressure and temperature are performed across the end faces of vertically drilled samples while the polarization angle of the source and receiver is rotated from 0 to 360° in azimuth. Shear wave birefringence in siltstone is low (∼3.5%) and controlled by low angle (∼30°) bedding dip. The sandstone samples exhibit moderate (∼15%) shear wave anisotropy due to a network of vertical microcracks with normal about the bedding strike direction. As related studies infer that the microcracks responsible for shear wave birefringence in sandstone are also open at depth, our results suggest that anisotropy in borehole measurements can simply be accounted for by the observed siltstone and sandstone microstructures, i.e., without invoking larger scale objects, nor randomly oriented cracks/fractures subjected to triaxial stress state.

Direct probabilistic inversion of shear wave data for seismic anisotropy

SUMMARY Shear wave splitting is perhaps the most unambiguous signature of the effect of anisotropic materials on the propagation of seismic waves. It has been used extensively to study anisotropy in the Earth, at global scales from the inner core to the tectonics of the uppermost mantle and crust, and at smaller scales for imaging deformation in hydrocarbon reservoirs. Wellestablished techniques exist for measuring shear wave splitting in a single (three-component) seismogram and more recently these have been extended to treat shear wave splitting in a tomographic fashion: determining non-uniform anisotropic models using large data sets of splitting measurements. Here, I propose an extension to a recent shear wave splitting tomography methodology which incorporates the data analysis into the inversion itself. This methodology uses a non-linear neighbourhood algorithm inversion to explore the parameter space defined by an anisotropic model consisting of a number of uniform domains. Each candidate model is assessed by applying the splitting it predicts to the entire data set. This approach is computationally expensive, but is highly amenable to parallelization. I apply the methodology to three simple synthetic cases to demonstrate the utility of the method. Finally, I apply the approach to the problem of inferring two-layer anisotropy from SKS splitting, which is a commonly attempted problem in global seismology. This uses data from the seismic station EKTN, where two-layer splitting has been previously inferred. This highlights some of the inherent trade-offs with such studies, and emphasizes the need to incorporate extra information to resolve these. This method is applicable to shear wave anisotropy analysis in a broad range of settings from global to reservoir scale.

Fracture and 3D seismic interpretations of the Fruitland Formation and cover strata: Implications for CO2 retention and tracer movement, San Juan Basin Pilot test

Abstract This study presents data on faults and fractures in the Fruitland Formation and its cover strata at a carbon sequestration pilot site in the high-rate Fruitland coal fairway in the northeastern San Juan Basin. The study incorporates satellite imagery, fracture detection and shear wave anisotropy logs and 3D seismic to identify potential issues that could impact long term reservoir integrity, CO2 flow and retention. The properties of fracture networks within the reservoir and sealing strata are important to understand in carbon sequestration and enhanced recovery efforts. Fracture networks exert significant control on the ability of a reservoir to retain CO2 for long (thousand year) time frames with minimal leakage. Local structural complexity at the pilot site resulted in tracer migration patterns that differed from predictions based on regional descriptions of fracture and cleat orientations measured in areas several miles from the site. Perfluorocarbon (PFC) tracer breakthrough in the production stream of nearby producing wells, for example, occurred first in a well to the east, in a direction close to the presumed butt cleat trend rather than in a well at equal distance along what had been assumed to be the more permeable face cleat trend. The study reveals local tectonic complexity not suggested by well log derived structure maps of the area and suggests that fractures and faults disrupt the local Fruitland coal reservoir and the overlying Kirtland Shale primary seal.

Distinguishing between stress-induced and structural anisotropy at Mount Ruapehu volcano, New Zealand

[1] We have created a benchmark of spatial variations in shear wave anisotropy around Mount Ruapehu, New Zealand, against which to measure future temporal changes. Anisotropy in the crust is often assumed to be caused by stress-aligned microcracks, and the polarization of the fast quasi-shear wave (ϕ) is thus interpreted to indicate the direction of maximum horizontal stress, but can also be due to aligned minerals or macroscopic fractures. Changes in seismic anisotropy have been observed following a major eruption in 1995/96 and were attributed to changes in stress from the depressurization of the magmatic system. Three-component broadband seismometers have been deployed to complement the permanent stations that surround Ruapehu, creating a combined network of 34 three-component seismometers. This denser observational network improves the resolution with which spatial variations in seismic anisotropy can be examined. Using an automated shear wave splitting analysis, we examine local earthquakes in 2008. We observe a strong azimuthal dependence of ϕ and so introduce a spatial averaging technique and two-dimensional tomography of recorded delay times. The anisotropy can be divided into regions in which ϕ agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which ϕ is aligned with structural features such as faults, suggesting structural anisotropy. The pattern of anisotropy that is inferred to be stress related cannot be modeled adequately using Coulomb modeling with a dike-like inflation source. We suggest that the stress-induced anisotropy is affected by loading of the volcano and a lithospheric discontinuity.

Shear wave anisotropy from cross-correlation of seismic noise in the Parkfield pilot hole

SUMMARY We use cross-correlation of seismic noise recorded at stations in the San Andreas Fault Observatory at Depth (SAFOD) pilot hole to extract P and S waves and measure S-wave anisotropy on the horizontal components. The data are recorded at seven three-component stations at depths from 1857 to 2097 m in the pilot hole. In late September and early 2004 October drilling noise underneath the stations generated propagating waves, which were absent in the rest of October. Estimates of the P- and S-wave velocities from the cross-correlations, on the vertical and horizontal components, respectively, are consistent with velocity measurements taken directly in the borehole. We observe polarization of the S wave, with a fast polarization direction of 120°–130° that is 4 per cent faster than the slow direction. Cross-correlation of the seismic noise can accurately determine S-wave anisotropy.

The effect of pressure on the elastic properties and seismic anisotropy of diopside and jadeite from atomic scale simulation

The full elastic constants tensors of diopside (CaMgSi2O6) and jadeite (NaAlSi2O6) have been calculated using a planewave and pseudopotentials based implementation of density functional theory within the generalised gradient approximation at pressures between 0 and 20GPa. Both minerals stiffen over this pressure range with the isotropic average bulk moduli increasing by ∼50% and the shear moduli by ∼20%. However, in detail the behaviour of the individual elastic constants varies and this drives changes in the anisotropy. Overall, and in contrast to predictions based on the extrapolation of calculations based on inter-atomic potential models, the elastic anisotropy of diopside decreases with increasing pressure. The elastic anisotropy of jadeite increases slightly at low pressure, exhibits a maximum at around 10GPa and then begins to slowly decrease. Despite the small changes in the total and maximum anisotropy, the shear-wave anisotropy for certain propagation directions vary dramatically with pressure. For example, the anisotropy experienced by a shear-wave propagating in the [010] direction in diopside doubles between 5 and 15GPa.

Shear wave splitting in the crust in North China: stress, faults and tectonic implications

SUMMARY This paper uses data from the Capital Area Seismograph Network in North China (NC), currently the largest regional seismograph network in China to investigate the compressional stress orientations from the polarizations of faster shear waves (PFS), a parameter of shear wave velocity anisotropy. Data are restricted to earthquakes deeper than 5 km to reduce the influence of the heterogeneous uppermost few kilometres of the crust. The results show that the regional compressional stress direction is ENE–WSW to nearly EW in NC, which agrees with seismic mechanisms and drilling and GPS data. Predominantly, PFS are also nearly EW in NC and are influenced by faults, geology and tectonics. PFS orientations at stations on seismically active faults are consistent with the strike of strike-slip faults and indicate large seismically active faults breaking, or nearly breaking, the free surface. PFS orientations are influenced by both regional compressional stress and by nearby faults. It is an effective way to study detailed spatial distribution of crustal compressional stress by shear wave anisotropy with dense seismograph network. Shear wave splitting may also be used to indicate hitherto unknown faults.

Estimation of azimuthal anisotropy from VSP data using multicomponent S-wave velocity analysis

Observation of azimuthal shear wave anisotropy can be useful for characterization of fractures or stress fields. Shear wave anisotropy is often estimated by measuring splitting of individual shear wave events in vertical seismic profile (VSP) data. However, this method may become unreliable for zero-offset (marine) VSP where the seismogram often contains no strong individual shear events, such as direct downgoing shear wave, but often contains many low-amplitude PS mode converted waves. We have developed a new approach for estimation of the fast and slow shear wave velocities and orientation of polarization planes based on the multicomponent linear traveltime moveout velocity analysis. This technique is applicable to zero-offset VSP data, and should take advantage of the presence of a large number of shear wave events with the same apparent velocity (which, for a horizontally layered medium, should be close to the interval velocity). The approach assumes that the VSP data are acquired in a vertical well drilled in an orthorhombic medium with a horizontal symmetry plane (including horizontal transverse isotropy). The main idea is to estimate the dominant apparent velocity for a given polarization direction by measuring the coherency of the seismic signal of a large number of events as a function of the apparent velocity. The algorithm was tested on marine three-component (3C) VSP acquired in the North West Shelf of Australia, and on land 3C VSP acquired with different sources in the same borehole located in Otway Basin, Victoria. These tests show good agreement between anisotropy parameters (magnitude and orientation) derived from the VSP and cross-dipole sonic log data.

Fabric development in the mantle section of a paleotransform fault and its effect on ophiolite obduction, New Caledonia

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. New, detailed field measurements document the rotation of foliation, lineation, and pyroxenite dikes across a 50-km-wide region. Deformation intensity recorded by folding and boudinage of dikes increases toward a central, 3-km-wide mylonitic zone. We used several additional methods to characterize fabric patterns across the shear zone. The shape-preferred orientation of orthopyroxene grains, computed from outcrop tracings, closely parallels field fabrics, with increased alignment and flattening near the center of the shear zone. The lattice-preferred orientations of olivine are consistent with high-temperature fabrics; the a axes within the mylonitic core were used to constrain the orientation of shear zone boundaries. Seismic anisotropy calculations, based on the lattice-preferred orientation of olivine, indicate 5%–11% shear-wave anisotropies, with increased values in the center of the shear zone. The magnetic silicate fabric in the rocks, determined from anisotropy of magnetic susceptibility techniques, broadly matches field fabrics but provides less consistent information across the shear zone than other fabric methods. This suite of field and laboratory data provides a unique and detailed view of strain and fabric patterns across a shear zone in oceanic mantle lithosphere. Because the primary mantle fabrics seem to be related to the present distribution of ophiolitic rocks in New Caledonia, we propose that ophiolite obduction and Neogene extension may have been controlled by preexisting fabrics and structures in the oceanic lithosphere.

Algebraic processing technique for extracting frequency-dependent shear-wave splitting parameters in an anisotropic medium

Based on the dual source cumulative rotation technique in the time-domain proposed by Zeng and MacBeth (1993), a new algebraic processing technique for extracting shear-wave splitting parameters from multi-component VSP data in frequency-dependent medium has been developed. By using this dual source cumulative rotation technique in the frequency-domain (DCTF), anisotropic parameters, including polarization direction of the shear-waves and timedelay between the fast and slow shear-waves, can be estimated for each frequency component in the frequency domain. It avoids the possible error which comes from using a narrow-band filter in the current commonly used method. By using synthetic seismograms, the feasibility and validity of the technique was tested and a comparison with the currently used method was also given. The results demonstrate that the shear-wave splitting parameters frequency dependence can be extracted directly from four-component seismic data using the DCTF. In the presence of larger scale fractures, substantial frequency dependence would be found in the seismic frequency range, which implies that dispersion would occur at seismic frequencies. Our study shows that shear-wave anisotropy decreases as frequency increases.

Crustal anisotropy structure related to lateral and down‐dip variations in interplate coupling beneath the Kii Peninsula, SW Japan

Anisotropy is an important feature of elastic wave propagation in the Earth, and can arise from a variety of ordered architectures such as fractures with preferential alignments or lattice‐preferred orientations. We studied regional variations in shear wave anisotropy beneath southern Kii Peninsula, which is located above the subducting Philippine Sea plate, using waveforms of local earthquakes observed with a dense seismic network. The fast directions of polarization are normal to the strike of the plate margin in the proximity of the locked portion of the underlying mega‐thrust fault, but they are subparallel to the strike of the margin along the fault's down‐dip extension. The distribution of the fast directions roughly coincides with that of the maximum horizontal compressive stress axis directions as inferred from earthquake focal mechanisms and borehole stress measurements. This indicates that the observed anisotropy is likely controlled by crustal stresses. Furthermore, the directions of the stress‐induced anisotropy correlate well with variations of the plate coupling ratio along the plate boundary as inferred from geodetic measurements. These results indicate the possibility that crustal anisotropy within an overlying plate can be used as an effective measure to monitor temporal variations of the plate coupling strength during the interseismic period.

Shear wave anisotropy beneath the Aegean inferred from SKS splitting observations

[1] SKS splitting parameters are measured in the Aegean region using events recorded at a dense temporary network in the south Aegean and the operating permanent networks, especially focusing in the back-arc and the near-trench areas of the Hellenic arc. In general, fast anisotropy directions are trench perpendicular in the back-arc area and trench parallel near the trench. Anisotropy measurements near the volcanic arc mark the transition between these two regions. In the back arc, a gradual increase is observed in delay times from south to north, with a prevailing NE-SW direction. In Cyclades, this pattern is correlated with GPS velocities and stretching lineations of metamorphic core complexes. Our preferred source of anisotropy in the back-arc region is the mantle wedge flow, induced by the retreating descending slab. The westernmost termination of the trench reveals directions parallel with the Kefalonia Transform Fault and perpendicular to the convergence boundary. Beneath Peloponnese, the trench-parallel flow is probably located beneath the shallow-dipping slab, although scattered measurements may also reflect fossil anisotropy from a past NW-SE strike of the trench. In western Crete, which may be entering a stage of continental collision, the anisotropy pattern changes to trench perpendicular, with a possible subslab source. Good nulls in central east Crete indicate a change in the anisotropy origin toward the east. At the easternmost side of the trench, fast directions are trench parallel. This reflects a similar subslab flow that may become toroidal around the slab edge beneath western Turkey. This may also produce a trench-parallel flow within the mantle wedge.

Fracture characterization using frequency-dependent shear wave anisotropy analysis of microseismic data

SUMMARY The presence of fractures in hydrocarbon reservoirs can enhance porosity and permeability, and consequently increase production. The use of seismic anisotropy to characterize fracture systems has gained much interest in the last two decades. However, estimating fracture sizes from observations of seismic anisotropy has not been possible. Recent work has shown that frequency-dependent anisotropy (FDA) is very sensitive to the length-scale of the causative mechanism for the anisotropy. In this study, we observe FDA in a microseismic data set acquired from a carbonate gas field in Oman. The frequency-dependent shear wave anisotropy observations are modelled using a poroelastic model, which considers fluid communication between grain size pore spaces and larger scale fractures. A grid search is performed over fracture parameters (radius, density and strike) to find the model that best fits the real data. The results show that fracture size varies from the microscale within the shale cap rocks, to the metre-scale within the gas reservoir, to the centimetre-scale within the non-producing part of the carbonate formation. The lateral variation in fracture density agrees with previous conclusions from ordinary shear wave splitting (SWS) analysis. Cumulatively, the results show the potential for characterizing fracture systems using observations of FDA.

Reply to `A Second Opinion on "Operational Earthquake Forecasting: Some Thoughts on Why and How," by Thomas H. Jordan and Lucile M. Jones,' by Stuart Crampin

In folklore, a “silver bullet” is an effective weapon against were-wolves and witches. In earthquake prediction, a silver bullet is a diagnostic precursor —a signal observed before an earthquake that indicates with high probability the location, time, and magnitude of the impending event (Jordan 2006). In his comment, Crampin (2010) claims that shear-wave splitting (SWS) observations provide a silver bullet. He asserts that seismology is thus capable of raising earthquake forecasting out of the low-probability environment to which we assigned it in our recent opinion piece (Jordan and Jones 2010). We find his arguments unconvincing. His conceptual basis is the notion that changes in shear-wave anisotropy are sensitive to changes in tectonic stress—a respectable hypothesis. But he burdens this concept with a thick pile of more dubious assumptions to somehow conclude that SWS observations can yield diagnostic precursors of large earthquakes. This is not the appropriate forum for a critical analysis of the prediction methodology he has advocated for several decades ( e.g. , Crampin et al. 1984). Suffice it to note that Crampin's …

Shear wave anisotropy in the crust, mantle wedge, and subducting Pacific slab under northeast Japan

To study the anisotropic structure beneath northeast (NE) Japan, we made 4366 shear wave splitting measurements using high-quality seismograms of many earthquakes occurring in the crust and the subducting Pacific slab. Our results provide important new information on the S wave anisotropy in the upper crust, lower crust, mantle wedge, and subducting Pacific slab. In the upper crust, the anisotropy is mainly caused by the stress-aligned fluid-saturated microcracks. The measured delay times (DTs) increase to 0.10 s at 10–11 km depth; the fast velocity directions (FVDs) are parallel to either the tectonic stress or the strike of active faults. The maximum DTs for the low-frequency earthquakes near the Moho are 0.15–0.17 s, suggesting strong anisotropy at the base of the crust or in the uppermost mantle. The measurements for the intermediate-depth earthquakes in the Pacific slab show dominant E-W (trench-normal) FVDs in the back-arc area and N-S (trench-parallel) FVDs in the fore-arc area. The trench-normal FVDs in the back-arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the Pacific plate. The maximum DTs for the slab earthquakes reach 0.30–0.32 s at 100 km depth, but only half of the total DTs are produced in the mantle wedge. The small DTs in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge. In the fore arc, the dominant trench-parallel FVDs for the slab earthquakes are consistent with those for the upper crust earthquakes, and ∼80% of the total DTs can be accounted for by the anisotropy in the crust. In the subducting Pacific slab, the trench-parallel FVDs may reflect either the original fossil anisotropy in the Pacific plate when the plate was produced in the mid-ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab.

Shear wave anisotropy of textured hcp-Fe in the Earth's inner core

Many seismological studies have confirmed that Vp travels 3–4% faster along the rotation axis of the Earth than along the equatorial plane in the inner core, indicating that the inner core is elastically anisotropic. However, seismic and mineral physics observations of the polarized Vs are still emerging. Thus far, the Vs anisotropy of the constitute iron crystals at relevant pressures of the Earth's core has remained mostly theoretical mainly because of the technical difficulties involved in measuring reliable Vs velocities of iron crystals. Here we have measured azimuthal Vs anisotropy of highly textured hcp-Fe at high pressures using nuclear resonant inelastic X-ray scattering, a technique sensitive to Vs, in a diamond anvil cell. Our results show that the azimuthal Vs is 2–4% faster along the crystallographic c axis than along the a axis at 158 GPa and 172 GPa. If one describes the Vp anisotropy of the inner core as a result of the textured hcp-Fe crystals, it is conceivable that azimuthal and polarized Vs anisotropies with a magnitude of a few percent also exist in the region. Since Vp and Vs of candidate iron phases behave quite differently in theoretical predictions, our results here indicate that future seismic observations of the Vs and Vp anisotropies of the inner core thus hold the key to deciphering the causes for the seismic and dynamic signatures as well as constitute iron phase(s) of the region.

Shear wave anisotropy of the Godavari rift in the south Indian shield: Rift signature or APM related strain?

Shear wave splitting analysis using SKS, SKKS and S waveforms recorded at eight broadband seismic stations within and in the vicinity of the Proterozoic Godavari rift system in the south Indian shield yielded 104 new measurements of azimuthal anisotropy. Our results reveal that the delay times lie between 0.70 and 1.20 s, similar to the previous measurements from the Indian shield. The orientations of fast polarization directions at most of the stations within and beyond the rift are generally aligned orthogonal to the rift axis, consistent with asthenospheric flow in the extension direction. Absence of clear evidences for a preferentially thinner lithosphere beneath the Godavari rift or anomalously high heat flow values coupled with the fact that the NE oriented fast polarization directions are also close to the APM direction of the Indian plate, result in an ambiguity in interpreting the results as being due to strain related to plate motion or signatures of rifting. A thin lithosphere beneath the rift suggests that the source of anisotropy is likely to be in the asthenosphere. In such a scenario, either the mechanism of (1) active rifting where the asthenospheric flow drives the rifting or (2) passive rifting where mantle flow responds to lithospheric thinning caused by far field stresses or (3) present day APM related strain appear to be the plausible mechanisms for forging the observed anisotropy.

On the robustness of global radially anisotropic surface wave tomography

A number of recent global tomographic studies have modeled three dimensional variations in the parameters of radial anisotropy. As yet there is limited agreement among such studies, suggesting significant uncertainties in the models, which could lead to divergent geodynamical interpretations. In this study we assess the robustness of lateral variations in radial anisotropy globally in the upper mantle and in the transition zone to determine the extent to which anisotropic parameters are constrained by a data set of over 10,000,000 fundamental and higher mode surface wave dispersion measurements. We carry out inversions for isotropic and radially anisotropic shear wave velocity, systematically changing regularization and using three different crustal models to remove the effects of the crust on the data. Using crustal corrections from different crustal models has an impact on the data fit comparable or larger than that obtained by including lateral variations of radial anisotropy in the modeling. Moreover, the use of crustal corrections from different a priori crustal models may lead to different images of radial anisotropy suggesting divergent geodynamical interpretations. This work suggests that the three‐dimensional determination of global radial anisotropy in the Earth's mantle using surface wave dispersion data is still an ongoing experiment.

3D upper mantle structure beneath Japan and its surrounding region from inter-station dispersion measurements of surface waves

Three-dimensional shear wave speed structure of the upper mantle beneath the Japanese Islands and the surrounding regions is constructed from the measurements of inter-station phase speeds of surface waves. We employ the dense broad-band seismic network (F-net), deployed throughout Japan, as well as permanent stations of global seismic network in East Asia. A temporary broad-band seismic network in Far-East Russia, which has been constructed as a part of the Stagnant Slab Project since 2004, is also used in combination with the permanent stations. Using seismic events from 2005 to 2007 with moment magnitude greater than 6.0 and depth shallower than 100km, we measure the phase speeds of the fundamental-mode Rayleigh and Love waves, working with the conventional two-station technique. Phase speed maps of the Japanese Islands and the Sea of Japan are obtained from the inversions of inter-station phase speeds that are measured for a large number of station-pairs. We have collected over 5300 paths for Rayleigh waves and 3800 paths for Love waves. The phase speed maps of Rayleigh waves, in the period range from 25 to 140s, are inverted to obtain an isotropic 3D shear wave speed model in the depth range from 40 to 200km. The isotropic S wave model represents prominent fast wave speed anomalies in northeastern Japan, associated with the subducting Pacific Plate. Fast anomalies of the lithosphere beneath the Sea of Japan are also imaged, implying that the average thickness of lithosphere beneath the Yamato Rise and Japan Basin is about 60±10km. Immediately below this oceanic lithosphere, slow anomalies are found in the asthenosphere beneath the entire region of the Sea of Japan in the depth down to about 200km. In southwestern Japan, the low-angle subduction of the Philippine Sea Plate is mapped clearly, indicating that the northern end of the plate extends beyond the northern coast of the Chugoku region, reaching beneath the Oki Islands in the Sea of Japan. A strong low velocity anomaly off the west coast of Kyushu is found in the depth down to about 130km, which supports the existence of an upwelling flow beneath this area. We also invert the phase speed maps of Rayleigh and Love waves in the period range from 30 to 80s simultaneously, to construct the radially anisotropic shear wave speed model. A remarkable anisotropy with SH>SV is found in the mantle wedge beneath northeastern Japan, while only weak anisotropy is seen beneath the Sea of Japan. Such anisotropic properties, which cannot readily be obtained from body wave studies only, will be an important key to understanding the dynamic processes in this tectonically actively region.