Wave Polarization Anisotropy Research Articles

Microstructures, hydrogen concentrations, and seismic properties of a tectonically exhumed sliver of oceanic mantle lithosphere, Moa Island, Timor-Tanimbar outer-arc, eastern Indonesia

We characterize and quantify the microstructure, hydrogen concentrations, and seismic properties of a tectonically exhumed sliver of oceanic lithospheric mantle outcropping in the Moa Island (Leti archipelago, Timor-Tanimbar outer-arc). The 18 spinel peridotites (lherzolites and harzburgites) have coarse-porphyroclastic microstructures and olivine crystal-preferred orientations (CPO) with axial-[010] (also known as AG-type) or [100](010) (A-type) patterns, similar to those observed in peridotitic xenoliths from oceanic mantle lithosphere. These coarse-porphyroclastic microstructures are variably overprinted by growth of strain-free olivine neoblasts and crystallization of secondary pyroxenes. Recrystallized fractions vary from 6.9 up to 31.3%. The interstitial (cuspate) shapes and CPOs of clinopyroxene, uncorrelated with the olivine CPOs, indicate that refertilization by a reactive melt percolation post-dated deformation. Seismic properties are calculated based on the modal compositions and CPOs of all samples. Increase in the recrystallized olivine fraction decreased the seismic anisotropy, since static recrystallization produced some dispersion of the CPO, but did not change drastically the texture acquired during deformation. Mean seismic velocities (mean Vp = 7.9 km.s−1; mean Vs = 4.5 km.s−1) and anisotropy (mean maximum S wave polarization anisotropy = 4.5%), estimated by considering coherent orientation of the foliation and lineation of all samples, are within the range of typical values for the uppermost mantle. The nominally anhydrous minerals contain small amounts of hydrogen (olivine: 13–18 ppm H2O by weight; orthopyroxene: 58–175 wt ppm H2O and clinopyroxenes: 244–288 wt ppm H2O). A bulk water content of 50 wt ppm H2O is estimated based on nomminally anhydrous minerals for the Moa peridotites, in agreement with previous estimates for the oceanic mantle lithosphere based on peridotitic xenoliths. This is the first direct measurement of hydrogen concentrations in peridotites from an oceanic mantle lithosphere which experienced melt extraction.

Influence of Melt‐Peridotite Interactions on Deformation and Seismic Properties of the Upper Mantle Beneath a Destroyed Craton: A Case Study of the Damaping Peridotites From the North China Craton

AbstractExtensive melt‐peridotite interactions had been documented in the lithospheric mantle beneath the North China Craton (NCC), a prime example of destroyed cratons in the world. Yet the impacts of melt‐peridotite interactions on the deformation and seismic anisotropy of the NCC upper mantle remain unclear. Here we studied in detail the microstructure, crystallographic preferred orientation (CPO) of minerals, and seismic properties of 26 peridotite xenoliths from the Damaping area of the NCC. The studied samples can be classified into two groups: weakly to nonfoliated and strongly foliated. Petrographic and microstructural observations suggest that multiple melt‐peridotite interactions and at least two stages of deformation had influenced samples from both microstructural groups. Dislocation creep in response to a transpression deformation led to the [010]‐fiber type olivine CPOs in most samples. Variable degrees of annealing followed the last stage of deformation. Due to a higher degree of melt‐peridotite interactions, which had promoted nondislocation creep, and more extensive annealing, olivine and pyroxene in the strongly foliated samples developed weaker CPOs. This in turn leads to weaker maximum P wave propagation anisotropy and S wave polarization anisotropy for this microstructural group. Our data, therefore, cast light on a strong control of intensity of melt‐peridotite interactions on deformation and seismic properties of the upper mantle beneath the NCC. If foliation and lineation are vertical and horizontal, respectively, the measured SKS splitting parameters can be well explained by the “fossil” anisotropy frozen in the lithospheric mantle, with no need to invoke asthenospheric flow as a source of the anisotropy.

A novel full-polarization SAR image ship detector based on scattering mechanisms and wave polarization anisotropy

Synthetic aperture radar (SAR) is suitable for earth observation due to its unique advantages. In this study, we constructed an adaptive ship detector using full-polarization SAR images. First, we thoroughly investigated the differences in scattering characteristics between ships, their background, and the wave polarization anisotropy. We then constructed a novel ship detector that incorporates scattering and anisotropy (known as joint scattering–anisotropy [joint-SA]). We found that joint-SA is an effective physical quantity representing the difference between the ship and its background; thus, joint-SA can be used for ship detection using full-polarization image data. Second, we used generalized gamma distribution to characterize joint-SA statistics of sea clutter with a large homogeneity range. Third, an adaptive constant false alarm rate (CFAR) method was implemented based on joint-SA. Finally, RADARSAT-2 and GF-3 data in the C-band and ALOS data in the L-band were used for verification. We tested five datasets, and the experimental results verified the correctness and superiority of the CFAR method based on joint-SA. The results show that the signal–clutter ratio of the proposed ship detector (33.17 dB, 35.98 dB, and 57.25 dB) was higher than that of DBSP (8.92 dB, 3.43 dB, and 25.40 dB) and RsDVH (17.28 dB, 11.17 dB, and 54.55 dB). Furthermore, the proposed detector has a higher detection accuracy and lower false alarm rate than those of the other two other methods.

Shallow Temporal Changes in S Wave Velocity and Polarization Anisotropy Associated With the 2016 Kumamoto Earthquake Sequence, Japan

AbstractApplying deconvolution interferometry technique to surface‐borehole (down to 300 m) seismogram pairs of KiK‐net, we retrieved temporal changes in S wave velocities and polarization anisotropies associated with the 2016 Kumamoto earthquakes, Japan. The average S wave velocity decreased after the largest foreshock (Mw 6.2) and again after the Mw 7.0 mainshock, in the latter case, by as much as 6.4%. The seismic velocity susceptibility (SVS), defined as the ratio of the relative S wave velocity change to the maximum dynamic strain, ranged from −170 to −13. The absolute value of the SVS was larger at the sites near the ocean or covered by an embankment. Systematic changes were not observed for SVSs obtained after the largest foreshock and mainshock. At most sites, the average S wave velocity continued to recover for at least 1 year, proportional to the logarithm of the lapse time after the mainshock. The speed of the log‐time recovery (m‐value) appears to be independent of the maximum dynamic strain of the mainshock. Large m values were observed at the sites of large absolute SVS values, indicating that a more susceptible medium is more resilient. The strength of the S wave polarization anisotropy (anisotropy coefficient) increased by up to 0.8% after the mainshock, which was much smaller than the relative change in the average S wave velocity and was below the background change due to phenomena other than earthquakes. These observations can be explained qualitatively by considering fracture theory in a medium that contains many vertical cracks with randomly oriented polarizations.

Temporal Variations of Near‐Surface Anisotropy Induced by Hydraulic Fracturing at a Shale Play Site in Southwest China

AbstractKnowledge of the geometric properties of fractures and cracks in a petroleum reservoir is important to reservoir exploitation. When aligned and partially connected, fractures and cracks can act as conduits for fluid flow and thus can significantly increase the permeability of the reservoir. The aligned fractures and cracks, on the other hand, are an effective means to generate seismic anisotropy. In this study, we utilize the seismic data recorded by a vertical array installed in a shallow borehole at a shale play site in southwest China. By applying seismic interferometry to the ambient noise data recorded by 12 three‐component geophones, we extractPandSwaves propagating vertically along the borehole. TheSwaves show up to 20% velocity variations with respect to their polarization directions. Such largeSwave anisotropy can be explained by the horizontal transverse isotropic model and is likely caused by natural fractures that are widely present in the area and align approximately in the NE‐SW direction. During the 13‐day period of hydraulic fracking treatment, we also observe large and systematic temporal variations inSwave velocity, degree ofSwave polarization anisotropy, and fast polarization direction. By comparing our observations with normal strain changes calculated with a half‐space elastic model, we speculate that strain changes induced by hydraulic injection and fracturing are likely to be responsible for the observed temporal variations in seismic anisotropy. As such, seismic interferometry with shallow borehole acquisition might provide an alternative means to monitor hydraulic fracturing and wastewater injection in the future.

Elasticity of ε‐FeOOH: Seismic implications for Earth's lower mantle

AbstractWe have calculated the structure and elasticity of low‐spin ferromagnetic ε‐FeOOH to 140 GPa using density functional theory calculations with a Coulombic self‐interaction term (U). Using these data, the elastic moduli and sound velocities of ε‐FeOOH were calculated across the pressure stability of the hydrogen bond symmetrized structure (30 to 140 GPa). The obtained values were compared with previously published values for phase H (MgSiH2O4) and δ‐AlOOH, which likely form a solid solution with ε‐FeOOH. In contrast to these Mg and Al end‐members, ε‐FeOOH has smaller diagonal and larger off‐diagonal elastic constants, leading to an eventual negative pressure dependence of its shear wave velocity. Because of this behavior, iron‐enriched solid solutions from this system have smaller shear wave velocities than surrounding mantle and therefore are a plausible contributor to large low‐shear velocity provinces (LLSVPs) which exhibit similar seismic properties. Additionally, ε‐FeOOH has substantial shear wave polarization anisotropy. Consequently, if iron‐rich solid solutions from the FeOOH–AlOOH–MgSiH2O4 system at the core‐mantle boundary exhibit significant lattice‐preferred orientation due to the strong shear stresses which occur there, it may help explain the seismically observed SH > SV anisotropy in this region.

Stress state in the upper crust around the source region of the 1891 Nobi earthquake through shear wave polarization anisotropy

We investigate shear wave polarization anisotropy in the upper crust around the source region of the 1891 Nobi earthquake, central Japan. At most stations, the orientation of the faster polarized shear wave is parallel to the axes of the maximum horizontal compressional strain rate and stress, indicating that stress-induced anisotropy is dominant in the analyzed region. Furthermore, near the source faults, the orientation of the faster polarized shear wave is oblique to the strike of the source faults. This suggests that microcracks parallel to the strike of the source fault, which would be produced by the fault movement of the Nobi earthquake, have healed with the healing of the faults. For stress-induced anisotropy, time delays normalized by path length in the anisotropic upper crust as a function of the differential strain rate are coincident with those in the inland high strain rate zone, Japan. These data, together with those of a previous study, show that the variation in the stressing rate, estimated from shear wave splitting, is close to that estimated from geodetic observation. This implies that the variation in the stressing rate in the brittle upper crust is linked to that in the strain rate on the ground surface.

Deformation, hydration, and anisotropy of the lithospheric mantle in an active rift: Constraints from mantle xenoliths from the North Tanzanian Divergence of the East African Rift

We have analyzed the microstructures and crystal preferred orientations (CPO), and calculated the seismic properties of 53 mantle xenoliths from four localities within the North Tanzanian Divergence of the East African rift: two within the rift axis and two in the transverse volcanic belt. Olivine OH concentrations were measured in 15 xenoliths.Most samples have harzburgitic to dunitic compositions and high olivine Mg#. Microstructures and olivine CPO patterns vary strongly depending on the location. In-axis peridotites display mylonitic to porphyroclastic microstructures, which record recent deformation by dislocation creep. Highly stretched orthopyroxenes in mylonites indicate that the deformation was initiated under high stress and probably low temperature. Orthopyroxene replacement by olivine in mylonitic and porphyroclastic peridotites suggest syn-kinematic melt–rock reactions and further deformation under near-solidus conditions. Exsolutions in orthopyroxene imply significant cooling between melt-assisted deformation and xenolith extraction. Late metasomatism is evidenced by the occurrence of veins crosscutting the microstructure and interstitial clinopyroxene and phlogopite. Axial-[100] olivine CPOs predominate, suggesting activation of the high temperature, low pressure [100] {0kl} slip systems and, probably, transtensional deformation. In the volcanic belt, Lashaine peridotites display very coarse-granular textures, indicating deformation by dislocation creep under low deviatoric stress conditions followed by annealing. Axial-[010] olivine CPOs are consistent with transpressional deformation or simultaneous activation of the [100](010) and [001](010) slip systems. Intermediate microstructures and CPOs in Olmani suggests heterogeneous deformation within the volcanic belt. Olivine OH concentrations range between 2 and 12ppmwt. H2O. No systematic variations are observed between in- and off-axis samples. Maximum P wave azimuthal anisotropy (AVp) ranges between 3.3 and 18.4%, and the maximum S wave polarization anisotropy (AVs) between 2.3 and 13.2%. Comparison between seismic properties of in-axis peridotites and SKS splitting data suggests transtensional deformation in the lithospheric mantle beneath the rift.

Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

Abstract. We calculated the seismic properties of 47 mantle xenoliths from 9 kimberlitic pipes in the Kaapvaal craton based on their modal composition, the crystal-preferred orientations (CPO) of olivine, ortho- and clinopyroxene, and garnet, the Fe content of olivine, and the pressures and temperatures at which the rocks were equilibrated. These data allow constraining the variation of seismic anisotropy and velocities within the cratonic mantle. The fastest P and S2 wave propagation directions and the polarization of fast split shear waves (S1) are always subparallel to olivine [100] axes of maximum concentration, which marks the lineation (fossil flow direction). Seismic anisotropy is higher for high olivine contents and stronger CPO. Maximum P wave azimuthal anisotropy (AVp) ranges between 2.5 and 10.2% and the maximum S wave polarization anisotropy (AVs), between 2.7 and 8%. Changes in olivine CPO symmetry result in minor variations in the seismic anisotropy patterns, mainly in the apparent isotropy directions for shear wave splitting. Seismic properties averaged over 20 km-thick depth sections are, therefore, very homogeneous. Based on these data, we predict the anisotropy that would be measured by SKS, Rayleigh (SV) and Love (SH) waves for five endmember orientations of the foliation and lineation. Comparison to seismic anisotropy data from the Kaapvaal shows that the coherent fast directions, but low delay times imaged by SKS studies, and the low azimuthal anisotropy with with the horizontally polarized S waves (SH) faster than the vertically polarized S wave (SV) measured using surface waves are best explained by homogeneously dipping (45°) foliations and lineations in the cratonic mantle lithosphere. Laterally or vertically varying foliation and lineation orientations with a dominantly NW–SE trend might also explain the low measured anisotropies, but this model should also result in backazimuthal variability of the SKS splitting data, not reported in the seismological data. The strong compositional heterogeneity of the Kaapvaal peridotite xenoliths results in up to 3% variation in density and in up to 2.3% variation of Vp, Vs, and Vp / Vs ratio. Fe depletion by melt extraction increases Vp and Vs, but decreases the Vp / Vs ratio and density. Orthopyroxene enrichment due to metasomatism decreases the density and Vp, strongly reducing the Vp / Vs ratio. Garnet enrichment, which was also attributed to metasomatism, increases the density, and in a lesser extent Vp and the Vp / Vs ratio. Comparison of density and seismic velocity profiles calculated using the xenoliths' compositions and equilibration conditions to seismological data in the Kaapvaal highlights that (i) the thickness of the craton is underestimated in some seismic studies and reaches at least 180 km, (ii) the deep sheared peridotites represent very local modifications caused and oversampled by kimberlites, and (iii) seismological models probably underestimate the compositional heterogeneity in the Kaapvaal mantle root, which occurs at a scale much smaller than the one that may be sampled seismologically.

Shear wave anisotropy beneath the volcanic front in South Kyushu area, Japan: Development of C-type olivine CPO under H2O-rich conditions

[1] Shear wave splitting from local intermediate-depth earthquakes is investigated to detect the anisotropic structure in the mantle wedge beneath the South Kyushu area, Japan. We observed shear wave splitting with NEE-SWW to NWW-SEE polarization directions and delay times of 0.04–0.63 s. Trench-normal shear wave polarization anisotropy with a delay time greater than 0.3 s probably overlaps in the high VP/VS region at a depth of 100–150 km beneath the volcanic front. Model calculations for the cause of the anisotropy suggest that the development of “C-type” olivine crystallographic preferred orientation (CPO) with a trench-parallel b axis concentration and a trench-normal a axis concentration best reproduces observations compared to A-, B-, or E-type olivine CPOs and antigorite CPO. We conclude that a thick anisotropic layer, approximately 50 km, is formed by concentration of interstitial fluid in peridotite. We briefly discuss the mechanism carrying water to the deep mantle wedge. A possible explanation is that interstitial fluids in the mantle wedge are effectively released because of a decrease in dihedral angles of olivine-fluid interfaces at 150 km depth. Our results imply a close relation between the high density of explosive volcanoes in the South Kyushu area and the underlying water-rich mantle.

Elastic, vibrational, and thermodynamic properties of MgGeO3 postperovskite investigated by first principles simulation

Elastic, vibrational and thermodynamic properties of MgGeO3 perovskite (Pv) and postperovskite (PPv) were calculated based on the density functional first principles methods. We found that the calculated properties of MgGeO3 are quite similar to those of MgSiO3 in particular for anomalously large c66 and small c55 of PPv, but not fully comparable in particular for the velocity contrasts across the phase change, which are all negative in compressional, shear and bulk velocities at the transition pressure unlike the typical features of the D″ seismic discontinuity. MgGeO3 PPv has larger elastic anisotropy than MgSiO3, but their style is quite similar. Significant orientational preference is needed for PPv polycrystalline aggregate to reproduce seismic shear wave polarization anisotropy observed in D″ except for a case with the c direction aligned vertically. Lattice dynamics calculations indicate that the both phases are vibrationally stable under high pressure, and quasi‐harmonic thermodynamics provides the Clapeyron slope of PPv phase boundary of ∼+7.8 MPa/K, which is quite comparable to that reported for MgSiO3. While the calculated Raman frequencies are in excellent agreement with experimental values in Pv, those are found less consistent in PPv.

Upper mantle anisotropy beneath central and southwest Japan: An insight into subduction-induced mantle flow

Analysis of seismic anisotropy in the crust and mantle wedge above subduction zones gives much information about the dynamic processes inside the Earth. For this reason, we measure shear wave polarization anisotropy in the crust and upper mantle beneath central and southwestern Japan from local shallow, intermediate, and deep earthquakes occurring in the subducting Pacific slab. We analyze S phases from 198 earthquakes recorded at 42 Japanese F-net broadband seismic stations. This data set yields a total of 980 splitting parameter pairs for central and southwestern Japan. Dominant fast polarization directions of shear waves obtained at most stations in the Kanto–Izu–Tokai areas are oriented WNW–ESE, which are sub-parallel to the subduction direction of the Pacific plate. However, minor fast polarization directions are oriented in NNE–SSW directions being parallel to the strike of the Japan Trench, especially in the north of Izu Peninsula and the northern Tokai district. Generally, fast directions obtained at stations located in Kii Peninsula and the Chubu district are oriented ENE–WSW, almost parallel to the Nankai Trough, although some fast directions have NW–SE trends. The fast directions obtained at stations in northern central Honshu are oriented N–S. Delay times vary considerably and range from 0.1 to 1.25 s depending on the source depth and the degree of anisotropy along the ray path. These lateral variations in splitting character suggest that the nature of anisotropy is quite different between the studied areas. Beneath Kanto–Tokai, the observed WNW–ESE fast directions are probably caused by the olivine A-fabric induced by the corner flow. However, the slab morphology in this region is relatively complicated as the Philippine Sea slab is overriding the Pacific slab. This complex tectonic setting may induce lateral heterogeneity in the flow and stress state of the mantle wedge, and may have produced NNE–SSW orientations of fast directions. The ENE–WSW fast directions in Kii Peninsula and the Chubu district are more coherent and may be partly induced by the subduction of the Philippine Sea plate. The N–S fast directions in northern central Honshu might be produced by the trench-parallel stretching of the wedge due to the curved slab at the arc–arc junction.

1997年鹿児島県北西部地震（<i>M</i> 6.6, <i>M</i> 6.4）の余震域におけるS波偏向異方性

1997年鹿児島県北西部地震（<i>M</i> 6.6, <i>M</i> 6.4）の余震域におけるS波偏向異方性

Shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake

Abstract Crustal shear wave polarization anisotropy is caused by the alignment of vertical microcracks. Leading shear wave polarization directions (LSPDs) are presumed to be consistent with the maximum horizontal compressional axis in many cases. We analyzed shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake. Almost all of the LSPDs are oriented in the E-W direction, which is consistent with the maximum horizontal compressional axis inferred from the mechanism of the main shock. These E-to W-oriented LSPDs are caused by the alignment of stress-induced microcracks. Crack densities at most stations are estimated to be 0.02. Little spacial stress variation around focal region is suspected

Ship detection and characterization using polarimetric SAR

Polarimetric information is investigated for ship detection and characterization at operational satellite synthetic aperture radar (SAR) incidence angles (20°‐60°). It is shown that among the conventional single-channel polarizations (HH, VV, or HV), HV provides the best ship‐sea contrast at incidence angles smaller than 50°. Furthermore, HH polarization permits the best ship‐sea contrast at near-grazing incidence angles. The wave polarization anisotropy is used for optimal information extraction from polarimetric SAR data. It is shown that fully polarimetric information permits a significant improvement in the ship‐sea contrast for relatively calm wind conditions, in comparison with conventional (i.e., scalar) single-channel polarizations (i.e., HH, VV, or HV). For rougher sea conditions, the effectiveness of polarimetric tools may be significantly degraded. Ship characterization is also investigated using the symmetric scattering characterization method (SSCM). Identification of ship targets with significant symmetric scattering can provide a useful ship pitch angle estimate under certain conditions.

S wave splitting observed in southwest Japan

Shear wave polarization anisotropy was investigated for S and ScS waves recorded at seismological station SBK in the Chugoku district, southwest Japan. Polarization direction of the fast component and time lag of shear wave which was split by elastic anisotropy were measured by a waveform cross-correlation method. Comparison between splittings of the S and ScS waves reveals that the shear wave anisotropy is higher in the upper mantle above the depth of 400 km than in the lower mantle below it. The fast components of the split ScS waves were polarized nearly in the NW–SE direction, irrespective of azimuth angles of earthquake epicenters. This polarization direction agrees with that of the fast components of the direct S waves, but the S waves radiated from deep earthquakes in the Izu–Ogasawara arc region show the polarization anisotropy of the NE–SW direction which is different from that of ScS waves. The NW–SE polarization direction is attributed to the velocity anisotropy in the upper mantle caused by subduction of the Philippine Sea plate beneath southwest Japan and the NE–SW polarization direction is interpreted as being due to the anisotropy in the back arc mantle under the Izu–Ogasawara region which is a subduction zone of the Pacific plate. The measured polarization anisotropy is due to deformation-induced lattice preferred orientation which is caused to olivine crystals in the upper mantle by subduction of oceanic plate.

Shear Wave Splitting Observed in the Southwestern Part of Fukushima Prefecture, Notheastern Japan.

We have investigated shear wave polarization anisotropy in the southwestern part of Fukushima Prefecture, northeastern Japan. Waveform data we analyzed are obtained by a local seismic networks of Tohoku University and Utsunomiya University. We used a cross correlation method to detect the shear wave splitting. For the shallow events beneath the southwestern part of Fukushima Prefecture, observed that fast shear wave oscillation directions (FSODs) are oriented to the NW-SE. The observed delay times are less than 0.1 s and tend to have large values in high seismicity regions. For the intermediate-depth events, observed FSODs are oriented parallel to the dip direction of the subducted slab and most of the observed delay times are less than 1 s. We estimated the location and the cause of anisotropy based on these observations. In the upper crust beneath the southwestern part of Fukushima Prefecture anisotropy is estimated to be caused mainly by preferredly oriented cracks controlled by the tectonic stress.

Shear wave polarization anisotropy observed in a rift zone in Japan

We have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in Central Kyushu, Japan. We have recognized two different directions in the polarization of the faster shear waves: about E-W directions at three stations located in the rift zone, and about N-S at two stations possibly located outside and close to the edge of the rift zone. The former case (E-W) is consistent with a hypothesis that shear-wave splitting is caused by vertical cracks or fractures striking in the E-W direction which have formed under the present tectonic stress inside a rift zone extending in the N-S direction, with the maximum horizontal compressive stress striking in the direction about E-W and the minimum compressive stress vertical. In contrast, the latter case (N-S) is inconsistent with the above hypothesis. This inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone. Alternatively, it may result from a complicated stress regime within the crust close to the edge of the rift zone. Arrival-time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0.12 s, while those from the upper mantle events are less than about 0.25 s. The relatively small arrival-time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust.

38B. Shear wave polarization anisotropy in Unzen volcanic region.(Abstracts of Papers Presented at the Fall Meeting of the Society)

38B. Shear wave polarization anisotropy in Unzen volcanic region.(Abstracts of Papers Presented at the Fall Meeting of the Society)

Surface wave polarization anisotropy near the East Pacific Rise as revealed from group velocities and synthetic waves : Yoshida, Mitsuru, 1984. Bull. Earthq. Res. Inst., Univ. Tokyo, 59(3):383–398. Earthquake Res. Inst., Tokyo, Japan

Surface wave polarization anisotropy near the East Pacific Rise as revealed from group velocities and synthetic waves : Yoshida, Mitsuru, 1984. Bull. Earthq. Res. Inst., Univ. Tokyo, 59(3):383–398. Earthquake Res. Inst., Tokyo, Japan