Shear-wave Polarization Anisotropy Research Articles

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.

Shear-wave anisotropy of the upper and lower crusts estimated by stripping analysis of Ps-converted waves

The anisotropic structure of the crust is investigated by measuring the shear-wave polarization anisotropy of the Ps phases converted at the Conrad and Moho discontinuities. The Ps-converted phases are identified on the P-wave receiver functions, which are constructed from teleseismic records at the F-net and Hi-net stations in the Kinki, Chugoku, Shikoku, and Kyushu districts in southwest Japan. The Ps phase traveling through an anisotropic layer undergoes shear-wave splitting, which is described by splitting parameters [fast polarization direction (FPD) and split time]. The upper and lower crustal anisotropies are estimated from the splitting parameters of the Conrad and Moho Ps phases, respectively, which are determined by a waveform cross-correlation method. To reliably estimate the lower crustal anisotropy, the Moho Ps phase is corrected for the shear-wave splitting effect in the upper crust by a stripping method because the upper crustal anisotropy masks the anisotropic behavior of the Ps phase in the lower crust. The split times estimated for the upper and lower crusts are less than 0.2s, but their FPD characteristics differ from each other. The FPDs in the upper crust are predominantly in the E–W direction. The FPDs in the lower crust range from NW–SE to NE–SW in the region with the Philippine Sea (PHS) slab subduction, but from NE–SW to SE–NW in the region without slab subduction. Thus, seismic anisotropy exists in both the lower and upper crusts, and slab subduction significantly influences the lower crustal anisotropy. The most likely cause for the upper crustal anisotropy is the alignment of vertical cracks induced in the upper crust by the tectonic stress, while the lower crustal anisotropy is attributed to the lattice preferred orientation of rock-forming minerals along with ductile deformation of the lower crust.

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.

Regional variations of the shear-wave polarization anisotropy in the crust and mantle wedge beneath the Tohoku district

We investigated the regional variations of the shear-wave polarization anisotropy in the upper crust, lower crust, and mantle wedge beneath the Tohoku district in the northeast Japan arc. The shear-wave splitting parameters (fast polarization direction (FPD) and split time) for the upper and lower crusts are estimated by shear-wave splitting analysis of the Ps phases, which are generated by the conversion of a P wave into an S wave at the Conrad and Moho discontinuity interfaces. The splitting parameters for the mantle wedge are determined by splitting analysis of the slab Ps phase converted at the oceanic crust on the subducting Pacific slab and the direct S waves from deep focus earthquakes located just below seismic stations. The Ps phases are identified on the P-wave receiver functions constructed from the teleseismic records. To accurately determine the splitting parameters for the lower crust from the Moho Ps phase, the Ps phase should be corrected for the shear-wave splitting effect in the upper crust. Similarly, to estimate the mantle wedge anisotropy, the direct S wave and slab Ps phase must be corrected for the splitting effect in the crust. The anisotropies of the crust and mantle wedge are estimated from the corrected Ps phases and direct S waves. The anisotropy of the upper crust exhibits a regional variation where FPD of the split shear wave is predominant in the N–S direction in the Pacific coast area and in the E–W direction in the rest of the Tohoku district. The split time is less than 0.2 s. The upper crustal anisotropy is attributed to the alignment of vertical cracks induced in the upper crust by tectonic stress. In the lower crust, FPD is predominant in the E–W direction with a split time similar to that in the upper crust, and the anisotropy of the lower crust is due to the lattice preferred orientation of rock-forming minerals. In the mantle wedge, FPD is predominant in the N–S direction (trench-parallel) in the fore-arc side of the volcanic front, but in the E–W direction (trench-perpendicular) in the back-arc side. The split time in the mantle wedge is similar to that of the upper crust. The most likely cause for the trench-perpendicular anisotropy in the back-arc mantle wedge is the lattice preferred orientation of A-type olivine along flows of the secondary mantle convection. For trench-parallel anisotropy in the fore-arc mantle wedge, we considered two possible causes: the preferred orientation of B-type olivine along the secondary mantle convection and that of dry olivine along the trench-parallel flow produced by the along-strike dip variation of the Pacific slab. However, we could not determine which one is the origin.

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.

Regional variation in shear-wave polarization anisotropy of the crust in southwest Japan as estimated by splitting analysis of Ps-converted waves on receiver functions

Regional variation in shear-wave polarization anisotropy of the crust is investigated in the Kyushu, Chugoku, Shikoku and Kinki districts, southwest Japan, using Ps-converted phases generated at the Moho discontinuity. The Ps phases on radial and transverse receiver functions are analyzed to determine shear-wave splitting parameters of fast polarization direction and time lag by waveform cross-correction analysis. As a result, the fast polarization directions are shown to be obviously different between the Chugoku district and the Kyushu and Shikoku districts, which are separated from each other by the Seto Inland Sea. In the Kyushu, Shikoku and south Kinki districts, the Ps phases exhibit fast polarization directions ranging from E–W to NW–SE. The polarization directions are not only consistent with those of S waves from shallow crustal earthquakes but also parallel to the direction of maximum horizontal compression acting on southwest Japan and the strike directions of metamorphic belts, accretionary zones and rift zones. Thus the polarization anisotropy is understood to reflect mainly the upper crustal anisotropy that is attributable to stress-induced vertical cracks and geological lineament structure. On the other hand, in the Chugoku and north Kinki districts, fast polarization directions are in a range from N–S to NE–SW. They are not consistent with the tectonic stress field and the S-wave splitting observed for shallow crustal earthquakes. The disagreement between the fast polarization directions estimated from the S waves and the Ps-converted phases is interpreted as arising because the lower crustal anisotropy attributed to mineral preferred orientation has a significant influence on the Ps-phase splitting. The regional variation in Ps-phase polarization anisotropy of the crust in southwest Japan is dependent on which anisotropy of the upper and lower crust is prominently reflected in the Moho Ps-phase splitting.

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.

Sound velocities and elasticity of DHMS phase A to high pressure and implications for seismic velocities and anisotropy in subducted slabs

Dense hydrous magnesium silicate (DHMS) phase A forms in cold subducted slabs after the breakdown of antigorite serpentine, and may play an important role in the transport of water within the upper mantle. In this paper we present acoustic velocities and the single-crystal elastic properties of Fe-bearing phase A, (Mg 0.981Fe 0.019) 7Si 2O 8(OH) 6, measured by Brillouin spectroscopy on a sample compressed to 12.4(2) GPa in a diamond anvil cell. A fit to the acoustic data using a 3rd order finite-strain EOS yields the following adiabatic bulk ( K S) and shear ( μ) moduli and their pressure derivatives: K S = 106(1) GPa, (∂ K S/∂ P) T0 = 5.8(3), μ = 61(1) GPa, (∂ μ/∂ P) T0 = 1.8(1). Within the experimental resolution, the pure longitudinal elastic constants, C 11 and C 33, and the off-diagonal C 12 constants exhibit positive linear pressure dependence, whereas C 44, C 66 and C 13 increase with a quadratic dependence on pressure. The axial compressibility of phase A remains highly anisotropic in the investigated pressure range, with the a-axis being 15% more compressible than the c-axis at 12.4(2) GPa. Compared to forsterite, the aggregate compressional ( V P) and shear ( V S) acoustic velocities of phase A are 7% slower at room pressure. Although the velocity contrast diminishes to 3.5% for V P, it is maintained for V S over the investigated pressure range. Phase A has high shear wave anisotropy ( A S) and shear-wave polarization anisotropy ( A S P 0 ) of A S = 20% and A S P 0 = 18 % , and a more moderate compressional wave anisotropy A P = 12% at room pressure. The A P of phase A decreases to 8% at 12.4(2) GPa, remaining significantly lower than that of forsterite, whereas the shear anisotropy is nearly constant at ∼20% over the same pressure range and exceeds that of forsterite by 12% at 12.4(2) GPa. At upper mantle pressures, the shear wave splitting in phase A ( A S P 0 ) is 20% higher than in forsterite. The results of this study were used with thermoelastic data for other relevant minerals to compute the density, seismic velocities and V P/ V S ratios of subducted garnet–harzbugite and moderately depleted harzburgite assemblages with various degrees of hydration as a function of pressure along a slab isotherm at 1073 K. The results suggest that the seismic velocities of dry and water-saturated harzburgites (44.5 vol% phase A) may be indistinguishable at upper mantle P–T conditions because of the increasing concentration of high-pressure orthopyroxene upon hydration. This phase displays high seismic velocities that offset the decrease in velocities due to phase A, rendering hydration difficult to detect (anelastic attenuation is not considered). Combined observations from the analysis of seismic parameters indicate that significant shear wave anisotropy, accompanied by high V P/ V S and Poisson's ratios and pronounced shear wave splitting, could be major diagnostic features for identifying phase A-bearing assemblages at depth (180–350 km) in cold subducted slabs.

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

Shear-wave polarization anisotropy and subduction-induced flow in the mantle wedge of northeastern Japan

Shear-wave polarization anisotropy in the central part of northeastern (NE) Japan was investigated. We analyzed S phases of 293 intermediate-depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters. The obtained results show the systematic variation in splitting parameters across the arc. The leading shear-wave polarization directions (fast direction) obtained at stations in the western side of the study area are oriented nearly E–W, which is sub-parallel to the direction of relative plate motion between the Pacific plate and the North American/Okhotsk plate. In contrast, fast directions obtained at stations in the eastern side of the study area are oriented approximately N–S. Stations in the western side have a larger average delay time compared to those in the eastern side. These variations suggest that the nature of anisotropy is quite different between the eastern and the western sides. Both experimental studies on the deformation of olivine and numerical simulations of plate-driven flow predict that the lattice-preferred orientation of olivine causes the fast direction sub-parallel to the flow direction for the conditions in the mantle wedge of NE Japan. We thus infer that the lattice-preferred orientation of olivine generated by flow-induced strain is the most likely candidate for anisotropy that produces splitting with E–W fast direction. Three possible causes of anisotropy in the eastern side are considered: deformation of water-rich olivine in the mantle wedge, trench-parallel flow in the mantle wedge due to along-strike dip variations in the slab and anisotropy in the crust and the slab. We could not, however, determine which causes are dominant. Further systematic study is required to reveal the cause of anisotropy in the eastern side.

First‐principles determination of elastic properties of CaSiO3 perovskite at lower mantle pressures

We investigate the equation of state and elasticity of cubic CaSiO3 perovskite up to 140 GPa using the plane wave pseudopotential method within the local density approximation. The calculated equation of state parameters of the cubic phase are in excellent agreement with those from recent quasi‐hydrostatic compression data and from all‐electron linearized augmented plane wave calculations. We determine the elastic constant tensor of the mineral from the calculated stress‐strain relations. The bulk modulus of CaSiO3 perovskite is similar to that of MgSiO3 perovskite, however, its shear modulus is much higher at pressures corresponding to the lower mantle. This suggests that CaSiO3 perovskite can no longer be considered as an invisible component in modelling the composition of the lower mantle, and even small amounts of the mineral may affect significantly the seismic properties, particularly shear wave velocity, of the generally accepted Mg‐rich silicate perovskite dominated composition of this region. Moreover, CaSiO3 perovskite exhibits strong anisotropy (about 30% shear‐wave polarization anisotropy) at pressures corresponding to the transition zone and the top of the lower mantle.

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 polarization anisotropy beneath the north-eastern part of Honshu, Japan

SUMMARY We have investigated the shear-wave polarization anisotropy in the north-eastern Japan arc by using waveforms from many local earthquakes at various depths. We used a cross-correlation method to detect the shear-wave splitting. For intermediate-depth and deep events, the observed fast shear-wave oscillation directions ( FSODs) are oriented parallel to the dip direction of the slab at the western stations, while they are oriented perpendicular to the dip direction at the eastern stations. These observations indicate that significant anisotropy exists in the mantle wedge, and that its nature is quite different between the eastern and the western parts.

Shear-wave polarization anisotropy in the mantle wedge above the subducting Pacific plate

Research of azimuthal anisotropy in the mantle wedge above subduction zones opens up a new source of information about rising magma in the region, because the physical property estimated from shear-wave anisotropy is related to dynamic processes inside the earth. Regional variation in shear-wave polarization is investigated from a teleseismic event and some local deep earthquakes using a spatially dense seismic array of the NIED located in the central part of Japan. A characteristic pattern is found on the spatial distribution of shear-wave polarization data obtained from ScS waves. Large and small values of shear-wave polarization anisotropy are observed at seismic stations located to the west and east of the volcanic front, respectively. The anisotropic zone with maximum velocity in N-S direction is localized in the wedge portion between the surface and subducting slab using the data of the local deep events. The location of the anisotropic zone corresponds with a low- Q zone obtained from seismic tomography data. We conclude that the azimuthal anisotropic zone in the mantle wedge is caused by melt-filled cracks, which is consistent with models obtained by petrological studies.

Shear-wave polarization anisotropy in the upper mantle from a deep earthquake

Shear-wave polarization anisotropy in the upper mantle is investigated at the spatially dense seismic stations of the National Research Institute for Earth Science and Disaster Prevention located in central Japan, by the use of the ScS wave of a deep earthquake which occurred beneath Sakhalin island on 12 May 1990. A characteristic pattern is found: small and large shear-wave anisotropy values are observed at seismic stations located on the east and west sides of the volcanic front, respectively.

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.

Comparison between natural fractures and fracture parameters derived from VSP : Geophys J V107, N3, Dec 1991, P475–483

SUMMARY The feasibility of resolving a subsurface fracture system using P-wave and cross-polarized shear-wave vertical seismic profiling (VSP) was examined. In situ distributions of fractures and microcracks in a test borehole are characterized by an ultrasonic borehole televiewer (BHTV) and by measurements of core sample velocities. The BHTV images show that the fracture frequencies in the shallower part of the borehole are much higher than those in the deeper part. Two sets of dominant parallel fractures are observed by BHTV in the fracture-rich depth interval of the shallower part; the orientations of fracture strikes in the two sets are almost the same directing to NW, whereas the average dip angles of the fractures in the two sets are 73 and 46, respectively. The fracture orientations in the fracturepoor depth interval of the deeper part are found to be random. The degree of anisotropy of the core sample velocity is so small that the present VSP measurement cannot resolve the expected anisotropy from the core sample measurement. P- and two shear-wave velocities (Vp, V,, and V,) are determined from the downgoing phases of three-component VSP with a zero-offset P-wave source and with zero-offset shear-wave sources polarized in two orthogonal directions, respectively. The VSP results are as follows. (1) The shear-wave polarization anisotropy is not detected within the accuracy of 4 per cent. (2) V, and V, (average of V,, and V,) for the fracture-rich interval are 94 per cent and 86 per cent, respectively, of those for the fracture-poor interval. The VSP results are consistent with expected velocity changes due to an isotropic distribution of cracks. However, a biplanar crack model cannot satisfy the VSP results, whereas the BHTV images show two such sets of parallel fractures. It is suggested that this discrepancy is due to the lack of depth resolution in the velocity determination in the present VSP experiment.

Comparison between natural fractures and fracture parameters derived from VSP

SUMMARY The feasibility of resolving a subsurface fracture system using P-wave and cross-polarized shear-wave vertical seismic profiling (VSP) was examined. In situ distributions of fractures and microcracks in a test borehole are characterized by an ultrasonic borehole televiewer (BHTV) and by measurements of core sample velocities. The BHTV images show that the fracture frequencies in the shallower part of the borehole are much higher than those in the deeper part. Two sets of dominant parallel fractures are observed by BHTV in the fracture-rich depth interval of the shallower part; the orientations of fracture strikes in the two sets are almost the same directing to NW, whereas the average dip angles of the fractures in the two sets are 73 and 46, respectively. The fracture orientations in the fracturepoor depth interval of the deeper part are found to be random. The degree of anisotropy of the core sample velocity is so small that the present VSP measurement cannot resolve the expected anisotropy from the core sample measurement. P- and two shear-wave velocities (Vp, V,, and V,) are determined from the downgoing phases of three-component VSP with a zero-offset P-wave source and with zero-offset shear-wave sources polarized in two orthogonal directions, respectively. The VSP results are as follows. (1) The shear-wave polarization anisotropy is not detected within the accuracy of 4 per cent. (2) V, and V, (average of V,, and V,) for the fracture-rich interval are 94 per cent and 86 per cent, respectively, of those for the fracture-poor interval. The VSP results are consistent with expected velocity changes due to an isotropic distribution of cracks. However, a biplanar crack model cannot satisfy the VSP results, whereas the BHTV images show two such sets of parallel fractures. It is suggested that this discrepancy is due to the lack of depth resolution in the velocity determination in the present VSP experiment.