A micro-earthquake swarm was observed at the southern end of Satsuma Peninsula, Kagoshima Prefecture; foci were located at a depth of 5 km in the southeast part of Lake Ikeda, the biggest caldera lake in South Kyushu. The largest event was M=2.0. The focal area, which was adjacent to an area of a local high Bouguer anomaly, lay on a tectonic line geologically presumed. Polarity data of P waves gave a focal mechanism solution of a fault plane striking N60°W, which accords with the strike of the above-cited tectonic line. Although the swarm occurred in a so-called volcanic region, not only the seismograms but also the related phenomena themselves were very similar to those of earthquakes occurring in non-volcanic regions. Compression axes in an NE-SW direction were estimated from the focal mechanisms of the swarm, while a stress field of an E-W compression is widely admitted in Kyushu. They were concordant with the minor principal strain axes in the southwestern part of Kyushu. Compression axes in the NE-SW direction were also observed for the 1968 Ebino Earthquakes. There is a clear difference in trend of compression axis between the southwestern and other parts of Kyushu. A probable explanation is possible if the crust in the southwestern part is cut off from the northern part through the Usuki-Yatsushiro Tectonic Line and from the eastern part through a fault along the volcanic front. From the arrival time data of a clear reflected P phase, a reflection surface was estimated at a depth of about 7 to 9 km around the focal area.

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Following the method described by KIKUCHI and KANAMORI (1982), we develop an iterative deconvolution of complex body waves in which subevents may take two different fault mechanisms. The method is applied to teleseismic P waves of the Izu-Oshima earthquake of 1978. The result shows that the coseismic slip motion occurred not only on a submarine fault but also on an inland fault of the Izu-Peninsula. The seismic moment of the inland faulting is 4.3×10²⁵ dyn·cm, which amounts to about one third of that of the submarine aulting : 1.4×10²⁶ dyn·cm. Our result is consistent with the fact that aftershock activity increased around the inland and submarine faults almost simultaneously after the main shock.

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ScS polarization anisotropy is investigated using WWSSN short-period seismograms of intermediate-depth and deep earthquakes recorded at 24 stations around the Pacific Ocean. Forty-one seismograms are selected from a period between 1966 and 1978. The data used in this study are restricted to epicentral distances less than 30° to avoid phase distortion at the core-mantle boundary. Shear-wave splitting is observed indicating some form of anisotropy in the mantle. In order to find the polarizations of the faster and slower split shear waves, each of the horizontal component seismograms is rotated. The polarizations of the split shear-waves are approximately NNE-SSE under Australia and North America, E-W under South America, and NWW-SEE under Japan. The polarization of the faster wave is generally found to be parallel to plate motion near the receiver points. The arrival time difference is 1.0±0.4s, corresponding to a velocity difference of 4 % when the vertical extent of an anisotropic zone is assumed to be 100 km in the upper mantle.

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Inversion method for estimating the three-dimensional isotropic seismic velocity structure developed by Aki and his colleagues is extended to include the effect of velocity anisotropy. Assuming weakly inhomogeneous anisotropic media with some symmetry in anisotropy, we reduce the number of unknown parameters. We present two sets of linearized equations of quasi P-wave travel time residuals suitable for inversion. One set is for a weakly anisotropic medium with a hexagonal symmetry about a horizontal axis, and the other for a medium with a spheroidal P-wave velocity surface. Using these equations, we invert P-wave travel time residual data to estimate the isotropic and anisotropic velocity perturbations in the subdivided three-dimensional blocks within the modeling space. For local earthquakes, the hypocentral parameter perturbations should also be included in the inversion. This three-dimensional anisotropic velocity inversion is a natural extension of the previous isotropic velocity inversion. The three-dimensional anisotropic inversion method is applied to seismic data in Southwest Japan to determine velocity perturbations in the blocks which were well-determined in the previous isotropic study. The isotropic velocity perturbations are well determined, but the anisotropic velocity perturbations are less resolved. In spite of the rather poor resolution, our preliminary analysis shows some interesting features. Within the descending Pacific and Philippine Sea plates, the fast directions of P-wave velocity seem to be perpendicular to the magnetic lineations in the sea-floors near the trenches, suggesting that the descending lithosphere maintains the anisotropic structures which were formed before subducting.

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A model which combines the plate thickening model and the preferred orientation olivine model is proposed to interpret P-wave velocity azimuthal anisotropy in the lithosphere. Two layers of anisotropy are used to interpret the low velocity zone structure introduced by Asada and Shimamura in the Northwestern Pacific as a result of the azimuthal difference of the maximum P-wave velocity axes. We also suggested that the rate of anisotropy changes with depth.

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The group velocities of surface waves in the period range 22-89 s were measured to retrieve the anisotropy pattern of the surface wave velocities in the Eastern Pacific Ocean. WWSSN long period records at KIP, Hawaii, from ridge events in the Gulf of California, the East Pacific rise, and the Easter Island Cordillera were used to cover propagation directions N60°E-N140°E. These station and epicenters are in very favorable positions to minimize the effects of the lateral heterogeneity owing to ocean floor ages. The fast direction of the Rayleigh wave group velocity is normal to ridge, showing 3-5% azimuthal anisotropy. The Love wave group velocity shows little azimuthal anisotropy. A model of a possible type of preferred orientation of constituent olivine beneath the Pacific Ocean is proposed, based on the elastic constants and the petrofabrics of dunite from Hidaka, Hokkaido, Japan, and that of olivine nodules in the ophiolite complex from the Bay of Islands and Oman. In this model, a-axis is concentrated along a particular direction and b- and c-axes form a girdle. Elastic constants of the model are C₁₁=2.44, C₂₂=C₃₃=2.05, C₁₂=C₁₃=0.72, C₂₃=0.67, C₄₄=0.69, and C₅₅= C₈₆=0.75 (10¹² dyn·cm), where X₁-axis, or the direction of the a-axis concentration, is normal to ridge and X₃-axis is vertical. If we assume that the type of the preferred orientation is an age-independent property of suboceanic lithosphere, this model can explain the overall features of the large-scale anisotropy in the Pacific Ocean: the azimuthal anisotropy of surface wave velocities as described above; the SH-SV anisotropy of surface waves of about 0.15 km/s; the P_n, anisotropy of 7.9 km/s to 8.6 km/s; the high S_m, velocity of about 4.8 km/s; and the ScS polarization anisotropy

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According to known laboratory experiments of rock deformation, preferred orientation of a mineral is often closely correlated to a dominant slip system in the crystal. A theoretical consideration reveals how the dislocation gliding that is caused by the resolved shear stress on the slip plane rotates the crystal grain in polycrystalline aggregates. A simple principle to determine preferred orientation associated with such a slip-induced rotation is theoretically derived consistently with the observations. The theory applied to the seismic velocity anisotropy in the oceanic lithosphere predicts that young lithospheres should be subject to an extensional force normal to the ridge axis.

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From fabrics of experimentally and naturally deformed olivine polycrystals, it is shown that the a-axis of olivine tends to the maximum extension direction, the b-axis to the maximum shortening, and the c-axis to the intermediate strain direction. The seismic anisotropy suggests that the a-axis of olivine is nearly parallel to the direction of plate motion in the lithospheric upper mantle under the Western Pacific and normal to the trench axis in the island arc mantle beneath the Honshu arc, Combining the preferred mineral orientation of natural and experimental dunites, we obtain the conclusion that stress acting within the lithospheric upper mantle of the Pacific plate is the tensional one along the direction of plate motion. Consequently, it is suggested that the driving force of plate motion is the pull of subducting slabs at trench, and that the induced flow generating extensional stress appears in the upper mantle under the island arc.

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Syntectonic recrystallization experiments have been done on the synthetic fine-grained aggregate of trevorite (NiFe₂O₄; a spinel type mineral) analogous to the γ-(Mg, Fe)₂SiO₄ in the upper mantle by means of a constant strain rate apparatus under various physical conditions: strain rates from 10^-3 to 10^-6 s^-1, homologous temperatures from 0.56 to 0.80, and confining pressure of 0.6 GPa. An X-ray pole figure goniometry is employed to describe the quantitative orientation distribution of spinel crystallites in uniaxially deformed aggregates. The obtained inverse pole figures indicate that two types of preferred orientation have been produced during the axial deformations both in compression and tension. The lower temperature and faster strain rate yield the preferred orientation of type I: [1 1 0]//compression axis and [1 0 0]//tension axis, while the higher temperature and slower strain rate yield type II: [1 0 0]//compression axis and [1 0 0]//tension axis. The generation of the two types of preferred orientation is caused by grain rotation due to slip in constituent grains in an aggregate, and syntectonic recrystallization, respectively. The present results suggest (1)γ-(Mg, Fe)₂SiO₄ has a considerable degree of preferred orientation in the upper mantle, (2) the preferred orientation is probably of type II rather than of type I under the mantle conditions, and (3) the detectable degree of shear wave anisotropy is expected in the spinel layer of the earth's upper mantle as a result of preferred orientation.

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