Shear-wave Polarization Anisotropy Research Articles

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)

Seismic anisotropy of the Pacific Ocean inferred from long-period surface waves dispersion

The dispersion of surface (Rayleigh and Love) waves in the period range 40–300 s along a large number of paths, allows the estimation of both the azimuthal anisotropy and the shear-wave polarization anisotropy. The regional dispersion is determined, taking into account simultaneously its dependence with age and an azimuthal factor. The Pacific Ocean has been divided into 5 regions for Rayleigh waves and into 3 regions for Love waves. This partition discriminates the regions of extreme age which show a fast variation of dispersion with age, from the regions of intermediate age where the variation is weak. A variation of ∼ 2% of Rayleigh-wave group velocity with the azimuth of the path, measured with respect to the direction of spreading is displayed, up to very long-period. On the contrary, the azimuthal anisotropy for Love waves is difficult to resolve. For Rayleigh waves, the present-day direction of plate motion seems to agree best with the direction of maximum velocity. On the other hand, the isotropic inversion of the regional dispersion curves indicates, except for young regions, a discrepancy between Rayleigh-wave and Love-wave models. With this hypothesis, SH-velocities are higher than SV-velocities for the regions older than 23 Ma, down to a depth of 300 km, which is indicative of the presence of polarization anisotropy. The latter, very weak for the young part of the ocean, increases with age and reaches 7%, for the oldest region.

Shear wave polarization anisotropy in the upper mantle beneath Honshu, Japan

Shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth's surface is investigated using three‐component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in Honshu, Japan. Eighty nine high‐quality seismograms were selected from a period of 3 years. The data used in this study are restricted such that incidence angles are smaller than the critical angle of 30° to the earth's surface in order to avoid phase shifts in the shear wave train. To find directions of the maximum and minimum velocities in split shear waves, where shear waves are resolved into two phases with the maximum time separation, each set of the two horizontal component seismograms is rotated in the horizontal plane. The split shear waves thus obtained are again recombined after the correction of anisotropy, and the anisotropy‐corrected particle motion is compared with the focal mechanism for a cross‐check of the observed anisotropy. Directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station. The stereograms and the cross sections of seismic ray paths show that (1) the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations, and (2) the anisotropic region is separated into two parts: one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0° to 30° from the north (north anisotropy) and the other in the south with it trending 90° to 120° (south anisotropy). The maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions. The horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km. perhaps prevalent in west Honshu. However, the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles. If we assume the vertical extent of the anisotropic material to be 100 km, the maximum velocity contrast would be 4%. If we adopt a crack alignment model, for the observed anisotropy, cracks are inferred to be distributed vertically trending 0° to 30° in the north and 90° to 120° in the south. If we assume an olivine alignment model, olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0° to 30° and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90° to 120° and the slip plane being horizontal.

Characterization of high-frequency strong ground motion

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, (Mg0.981Fe0.019)7Si2O8(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 (KS) and shear (μ) moduli and their pressure derivatives: KS = 106(1) GPa, (∂KS/∂P)T0 = 5.8(3), μ = 61(1) GPa, (∂μ/∂P)T0 = 1.8(1). Within the experimental resolution, the pure longitudinal elastic constants, C11 and C33, and the off-diagonal C12 constants exhibit positive linear pressure dependence, whereas C44, C66 and C13 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 (VP) and shear (VS) acoustic velocities of phase A are 7% slower at room pressure. Although the velocity contrast diminishes to 3.5% for VP, it is maintained for VS over the investigated pressure range. Phase A has high shear wave anisotropy (AS) and shear-wave polarization anisotropy (ASP0) of AS = 20% and ASP0=18%, and a more moderate compressional wave anisotropy AP = 12% at room pressure. The AP 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 (ASP0) 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 VP/VS 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 VP/VS 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.

Shear-wave polarization anisotropy in the Pacific Basin

Summary. Inversion of 295. Love- and Rayleigh-wave phase travel times across the Pacific Basin has yielded a structure which has a channel that is anisotropic with respect to the polarization of shear waves. The velocity of SHwaves is approximately 4.24 h/s, and the velocity of SY waves is approximately 4.10 km/s in the low-velocity channel. The lid to the channel is isotropic with respect to the polarization of S waves and the velocity is approximately 4.60 km/s. The lid to the low-velocity channel increases in thickness with lithospheric age at the expense of the channel, and its thickness is apparently still increasing at a sea-floor age of 150 Myr. These results can be explained in terms of a model with both randomlyand preferentially-oriented, liquid-filled cracks in the channel. In the model, it is assumed that the liquid-filled cracks are due to partial melting in the channel, and that any preferred orientation is caused by a shear-flow gradient resulting from differential motion between the lid and the deeper parts of the mantle.