J. Phys. Earth, 43, 301-319, 1995
Seismic Anisotropy: A Review of Studies by Japanese Researchers
Satoshi Kaneshima Departmentof Earth and Planetary Physics, Faculty of Science, The Universityof Tokyo, Bunkyo-ku,Tokyo 113,Japan
1. Preface Studies on seismic anisotropy in Japan began in the mid 60's, nearly at the same time its importance for geodynamicswas revealed by Hess (1964). Work in the 60's and 70's consisted mainly of studies of the theoretical and/or observational aspects of seismic surface waves. The latter half of the 70' and the 80's, on the other hand, may be characterized as being rich in body wave observations. Elastic wave propagation in anisotropic media is currently under extensiveresearch in exploration seismologyin the U.S. and Europe. Numerous laboratory experiments in the fields of mineral physics and rock mechanics have been performed throughout these decades. This article gives a review of research concerning seismic anisotropy by Japanese authors. Studies by foreign authors are also referred to whenever they are considered to be essential in the context of this article. The review is divided into five sections which deal with: (1) laboratory measurements of elastic anisotropy of rocks or minerals, (2) papers on rock deformation and the origin of seismic anisotropy relevant to tectonics and geodynarnics,(3) theoretical studies on the Earth's free oscillations, (4) theory and observations for surface waves, and (5) body wave studies. Added at the end of this article is a brief outlook on some current topics on seismic anisotropy as well as future problems to be solved. There have been a few recent reviews of seismic anisotropy, such as Kawasaki (1989), Karato (1989), and Takeshita and Karato (1989). The former emphasizes seismological observations, whereas the latter emphasizes deformation of rocks and petrological aspects of anisotropy.
2. Laboratory Measurementsof Elastic Anisotropy 2.1 Elastic constants of single crystals Determination of elastic constants of minerals and rocks is the basis of any studies on seismic anisotropy. Japan has played a leading role in developing technology for measuring the physical properties of material under high temperature and pressure. Collaborations between Japanese researchers and foreign researchers have also been
ReceivedMay 19, 1992;Accepted May 20, 1993
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fruitful in determining elastic constants. Techniques for measuring the elastic constants of minerals may be separated into two classes. The first measures propagation speeds of acoustic waves through mineral samples in various directions (Fig. 1(a)) and converts them to the elastic constants at the samples. Measuring ultrasonic wave velocities, for instance, Kumazawa determined the elastic constants of single crystal of olivine (Kumazawa and Anderson, 1969), and of orthopyroxene (Kumazawa, 1969). Kasahara and Kumazawa (1969) attempted to find relation between optical and acoustic anisotropies of crystals. The Brillouin spectroscopy method has also been used to measure velocities of elastic waves and to determine elastic constants for geophysically important minerals such as /3 (beta) phase of Mg2SiO4 (e.g., Sawamoto et al., 1984), y (gamma) phase of Mg2SiO4 (e.g., Weidner et al., 1984), and MgSiO3 perovskite (e.g., Yeganeh-Haeri et al., 1989). Compared with the first technique using body waves based on the approximation of geometrical optics, the second method, called the "resonance technique," directly applies the wave equation to the oscillation of samples. The resonance technique measures the eigenfrequencies of free oscillation (resonance) either of parallelepiped (e.g., Ohno, 1976) or of spherical samples (Suzuki et al., 1992) and converts them to the elastic constants using the variational principle. For instance, Oda et al. (1988) determined the elastic constants of spherical samples of anisotropic crystals applying the mathematical formulation of Mochizuki (1988 a). Details of the spherical resonance
Fig. 1. (a) Variation of the quasi-P (qP) and the two quasi-S (qS1 and qS2) body-wave phase velocities for propagation in three ((001), (100), and (010)) orthogonal planes of symmetry of crystalline olivine. In each plane, qS1 and qS2 correspond either qSH or to qSV, where qSH and qSV represent the shear waves polarized nearly parallel and perpendicular to the propagation plane, respectively. (b) Schematic figure of shear wave splitting. A shear wave travelling through an anisotropic medium splits into two orthogonally polarized (quasi-)shear waves with different speeds (from Nicolas and Christensen (1987)).
J. Phys. Earth Seismic Anisotropy: A Review of Studies by Japanese Researchers 303 technique are given in a recent article on measurements of elastic constants of olivine by Suzuki et al. (1992). Mochizuki (1987 a) presented a formula which can be used for resonance of parallelepiped samples. Aside from the papers cited above, a number of measurements have been performed for various single crystals. The contributions by Japanese researchers to the determination of elastic constants of minerals are discussed in a recent review article by Anderson et al. (1992).
2.2 Elastic anisotropy of polycrystalline rocks The elastic constants of polycrystalline rocks are determined by the first of the above class of techniques for measuring body wave speeds. Kasahara et al. (1968 a, b), analyzed the petro-fabric of dunite samples which are composed mostly of olivine, and also measured the compressive and shear velocities of ultrasonic waves in various directions through the samples. They observed anisotropy-related phenomena, indicating azimuthal dependence of body wave propagation speeds as well as splitting of body shear waves. They also confirmed that the elastic anisotropy of dunite is closely related with the preferred alignments of a- and b-axes of olivine crystals (LPO) in the directions parallel and perpendicular to the lineation, respectively. Elastic anisotropy of amphibolite, a typical metamorphic rock in the lower crust, was measured and related to the LPO of hornblende by Siegesmund et al. (1989). For upper crustal rocks such as granite, on the other hand, experimental studies are mainly on the anisotropy due to preferred alignments of cracks. Intensive measurements of the elastic constants of granite have been performed, confirming the presence of three sets of orthogonal cracks, which are commonly known as "rift" and "grain" in quarries (e.g., Sano et al., 1992; Kudo et al., 1987). Since Nur and Simmons (1969) showed azimuthal variation of seismic wave speeds and splitting of shear waves on uni-axially stressed samples, seismic anisotropy of crustal rocks has been related to dilatancy, which is the stress induced opening of cracks before failure of rocks. A number of studies have been performed on anisotropy caused by preferred alignments of microcracks for crustal rocks like granite under deformation due to non-hydrostatic stress (e.g., Soga et al., 1978; Takahashi et al., 1984). The experiment by Soga et al. (1978) was on the velocity anisotropy of uni-axially stressed samples, whereas Takahashi et al. (1984) examined the same phenomenon in triaxially stressed rock samples. Such studies always need a theory for the computation of effective elastic constants of materials that include cracks. The theories are classified into two approaches. The first one applies scattering theory under the assumption of a long wavelength of the incident waves compared with the size of the cracks, and requires the distribution of crack to be sparse so that interaction between the cracks can be neglected (e.g., Hudson, 1981). The second class of methods evaluates the elastic energy due to the presence of an elliptical inclusion under a static stress field (Eshelby, 1957). Using the second approach, Nishizawa (1982) presented a numerical method to compute the effective elastic constants of isotropic materials including non-randomly aligned cracks, which can be applied even to dense distributions of thick cracks. Nishizawa and Yoshino (1991) attempted to extend Nishizawa's method to the case of crack alignment in generally anisotropic materials.
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3. Deformation of Rocks and the Origin of Seismic Anisotropy: Implications for Geodynamics 3.1 Origin of seismic anisotropy As described in the last section, mantle and crustal rocks are usually anisotropic. This means that if elastic anisotropy of in situ rock is uniform within a large enough volume, it can be detected through seismological observations. The anisotropy of in situ rocks has great importance for our understanding of geodynamics. The mechanisms which cause elastic anisotropy of in situ rocks include: (1) lattice preferred orientation (LPO) of the minerals composing the rocks, (2) preferred orientation of pockets of melts in partially molten materials, and (3) preferred orientation of brittle tensile cracks filled with liquid such as water. LPO of olivine is widely believed to be the dominant cause of upper mantle anisotropy and reflect the geometry of flow or stress orientation depending on the ambient condition such as temperature and strain rate. Brittle tensile cracks play a principal role in the upper crust as they are predominantly aligned parallel to the maximum compressive stress, while they are unlikely to pervade in the lower crust and mantle. Partial melting may prevail in the part of the asthenosphere where the geotherm is close to the solidus, and may exist in the mantle beneath island arcs, particularly just under active volcanos. It is not well known, however, what kind of preferred orientation of melt pockets occurs under a certain non-hydrostatic stress field. When Hess discovered the azimuthal variation of P-wave velocity in the eastern Pacific and explained it by the LPO of olivine in the uppermost mantle rocks in his historical paper (Hess, 1964), Kumazawa and Tada (1965) independently presented similar results and proposed a similar idea. Their study was motivated by a ther- modynamical consideration on the preferred orientation of anisotropic crystal under non-hydrostatic stress (Kumazawa, 1963; Kumazawa and Shimazu,1967). Sugimura and Uyeda (1967) tried to relate the LPO of olivine to the occurrence of deep earthquakes, using seismological data around Japan. Aki and Kaminuma (1,963) and Aki (1968) related the hexagonal anisotropy with the vertical symmetry axis (transverse isotropy) which was observed through surface wave studies to a preferred alignment of partial melt pockets. Ishikawa (1984) presented an anisotropic plate thickening model which predicts the presence of two layers within oceanic lithosphere with different anisotropy. This may be relevant to the recent observations of depth dependent anisotropy of surface wave by Nishimura and Forsyth (1988). Kawasaki (1988) provided a com- prehensive survey of seismic observations relevant to upper mantle anisotropy as well as those of a low-velocity layer. He argued that the seismic low velocity zone is caused not by partial melting of mantle rocks but by anelastic relaxation due to dislocation creep.
3.2 Lattice preferred orientation (LPO) and mantle dynamics As described in the last subsection, seismic anisotropy in the mantle is believed to be caused predominantly by LPO of olivine. Orthopyroxene, the second most abundunt mineral of mantle rocks, also has large anisotropy and forms LPO under dislocation creep. However, its effect on seismic anisotropy is minor compared with that of olivine (e.g., Nicolas and Christensen, 1987) and is not discussed here.
J. Phys. Earth Seismic Anisotropy: A Review of Studies by Japanese Researchers 305
Non-random alignment of tensile cracks is unlikely to cause anisotropy of rocks under high ambient stresses in the mantle. LPO within the mantle or crustal rocks is formed by plastic flow. Two different mechanisms exist for the plastic flow; diffusion creep and dislocation creep. The dominant mechanism depends on the physical and chemical conditions of rocks, such as temperature, pressure, stress, grain size, and water saturation. Dislocation creep can form LPO, while diffusion creep does not (e.g., Karato, 1988). It follows that elastic anisotropy is not created under conditions where diffusion creep dominates. The importance of diffusion creep in the deeper part of the mantle has recently been emphasized by Karato and his associates (e.g., Karato and Wu, 1993). The Lehmann discontinuity (at a depth of about 220 km) is related to the transition from dislocation to diffusion-creep and to the transition from anisotropic to isotropic mantle by Karato (1992). The lower mantle is also thought to be isotropic because diffusion creep dominates in perovskite owing to possible reduction of grain size (Karato and Li, 1992). Recent seismological observations seem to support the idea of isotropic lower mantle (Meade et al., 1995; Kaneshima and Silver, 1995). Since Hess's discovery of seismic anisotropy, the formation of LPO of olivine-rich rocks under dislocation creep has been intensively examined to improve our understanding of the genesis of upper mantle anisotropy (for a summary of laboratory deformation experiments see Nicolas and Christensen, 1987). Temperature and strain rate determine the dislocation mechanism by which LPO is formed. Rotation of crystal axes of olivine and/or orthopyroxene due to glide of dislocations is considered to be the dominant mechanism to form LPO under the temperature and strain rate inferred for the upper mantel. This mechanism of the formation of LPO has been explained by the Taylor-Bishop-Hill model (e.g., Takeshita, 1989). On the other hand, Karato (1987, 1989) proposed that, under the higher temperatures and slower strain rates which may exist in the deeper parts of upper mantle, another mechanism may dominate over the rotation of crystal axes due to dislocation glide. The second mechanism, called dynamic recrystallization, is not necessarily understood as well as the first mechanism, but is considered to consist of nucleation of new grains and migration of grain boundaries (Karato, 1987; Toriumi and Karato, 1985; Toriumi, 1984). Karato (1989) studied this problem, compiling seismic observations as well as deformation experiments on olivine polycrystals under high temperature and pressure. He proposed that LPO is formed along the flow direction under low temperatures representing the kinematics of mantle flow, while under relatively high temperatures LPO is formed through dynamic recrystallization representing the stress state. Petrofabric analyses of the deformation of olivine polycrystals and consequent seismic anisotropy is discussed by Takeshita et al. (1990). While a great many experiments have been performed on the anisotropy of olivine, which is the most abundant mineral in the upper mantle, there have been few such experiments for the minerals of the rock in the transition zone, such as spinel and 7 phases), partly because of difficulty in carrying out deformation experiments under high pressures. Fujimura (1984) made a measurement of the anisotropy of an analog material of spinel, and showed that aggregates of these crystals have a large anisotropy after deformation. Although this result suggests that there can be considerably large
Vol. 43, No. 3, 1995 306 S. Kaneshima anisotropy in the transition zone, seismological data appear not to support such anisotropic transition zone models.
4. Free Oscillation Takeuchi and Saito (1972) presented a matrix method to compute the eigenfrequencies and eigenfunctions of free oscillations as well as the phase and group velocities of surface waves in a general transversely isotropic layered half space (for a recent description of their computer program package, see Saito, 1988). Extending the studies by Woodhouse and Dahlen (1978) and Woodhouse (1980) on the perturbation theory of free oscillation taking into account coupling of multiplets due to ellipticity, rotation, and asphericity, Mochizuki (1986) presented a formula for calculating the perturbation matrix for a generally anisotropic Earth using generalized spherical harmonics. He also gave expressions as a function of propagation azimuth and location, in the approximation of high angular orders, for the local frequency perturbation, a quantity which was first introduced by Jordan (1978). This corresponds to the formula showing azimuthal dependence of surface wave phase velocities (Smith and Dahlen, 1973) and of body wave velocities (e.g., Crampin, 1977). Shibata et al. (1990) presented explicit forms of the perturbation matrix which are necessary to calculate the eigenfrequencies of free oscillations for a transversely isotropic Earth, extending the study by Woodhouse (1980) for isotropic cases. Mochizuki (1988 b) presented a method to classify elastic constants dividing the elastic tensor into two parts which are related to compression and shear.
5. Surface Wave 5.1 Polarization anisotropy of surface wave Differences between the 1-dimensional velocity structure determined from Love waves and that from Rayleigh waves (which is called polarization anisotropy of surface waves) have been reported since the early 1960's. Aki and Kaminuma (1963) and Kaminuma (1966 a, b, c) analyzed the dispersion of phase velocities of Rayleigh and Love waves and detected a polarization anisotropy beneath Japan. Such polarization anisotropy was widely observed and was considered to represent the presence of hexagonal anisotropy with a vertical symmetry axis (transversely isotropy) or the presence of laminated structure of thin horizontal layers with large velocity contrasts in the crust and/or uppermost part of mantle (e.g., Aki, 1968; Saito and Takeuchi, 1966; Takeuchi et al., 1968). Mikumo and Kurita (1971) studied the spectra of long period SH and SV waves as well as surface wave dispersion; their results supported the presence of transversely isotropic or laminated structure beneath Japan. Later Thatcher and Brune (1969) criticized these polarization anisotropy studies pointing out that higher mode mixing may have led to erroneous determination of Love wave phase velocities. Yoshida (1983, 1984, 1986) analyzed Rayleigh and Love wave phase and group velocities for the western Pacific region, paying some attention on the effects of higher-mode contamination on phase velocity estimation, and detected a polarization anisotropy of surface waves.
J. Phys. Earth Seismic Anisotropy: A Review of Studies by Japanese Researchers 307
The studies on polarization anisotropy motivated a series of theoretical analyses on partial derivatives of surface wave phase and group velocities due to perturbations of seismic velocities in transversely isotropic layered media by Oguchi and Takeuchi (1968 a, b, 1969, 1970), which can be used to solve inverse problems for the velocity structure of the mantle and crust. Takeuchi and Saito (1972) presented a method to compute phase and group velocities of surface waves in transversely isotropic layered half space. Mochizuki (1990) extended the theory on deviation from great circles of surface wave rays in a weakly heterogeneous media and its effect on surface wave amplitudes by Woodhouse and Wong (1986) to the generally anisotropic cases.
5.2 Azimuthal anisotropy of surface waves Besides polarization anisotropy, another characteristic phenomenon relevant to surface wave propagation in anisotropic media is the azimuthal dependence of phase and group velocities (called surface wave azimuthal anisotropy), which was first discovered by Forsyth (1975) for the upper mantle beneath the Nazca plate. Kawasaki and Kon'no (1984) analyzed Rayleigh and Love waves dispersion of surface waves traveling in the Pacific, and obtained azimuthal dependence of Rayleigh wave phase velocities, but found little variation for Love waves. This observation led Kawasaki to construct a plausible petrological model of mantle rocks and representing transverse isotropy as the azimuthal average of real azimuthally anisotropie mantle (Kawasaki, 1986). His "pseudo azimuthal anisotropic model" of mantle based on seismological as well as petrological data shows a thin lithosphere ( —50 km), as Regan and Anderson (1984) first suggested, and also explains the observed lack of azimuthal anisotropy of Love waves. Tanimoto and Anderson (1984) performed a global mapping of azimuthal anisotropy of Rayleigh wave phase velocity, and related it to the mantle return flows derived by Hager and O'Connell (1979) (Fig. 2(a)). Their study has been extended recently by Montagner and Tanimoto (1990). Suetsugu and Nakanishi (1987), solving an inverse problem with the phase velocities for each station-earthquake pair as data, determined the azimuthal dependence of Rayleigh wave phase velocities as a function of location for the Pacific Ocean. Their results show the fastest direction of phase velocity of 150 s Rayleigh wave coincides with the present-day movement of the plate. Kawasaki and Koketsu (1990) extended the matrix method by Takeuchi and Saito (1972) to allow general anisotropy and to allow coupling of Rayleigh and Love waves. They calculated the phase and group velocities of Rayleigh and Love waves as functions of propagation direction and showed that Rayleigh-Love coupling can be important in some cases.
5.3 Polarization anomaly of surface wave Tajima and Kawasaki (1989) attempted to observe the inclination and tilt of Rayleigh wave particle motion due to the coupling between the fundamental-mode of Rayleigh waves and the first higher-mode of Love waves, which was first indicated by Crampin and his colleagues (e.g., Kirkwood and Crampin, 1981), and was predicted by Kawasaki's anisotropic model of oceanic mantle (Kawasaki, 1986). Kawasaki and Koketsu (1990) calculated the coupling of Rayleigh and Love waves and showed that the polarization of particle motion directions of these waves is twisted along the pair
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Fig. 2. (a) Azimuthal anisotropy of 200 s Rayleighwaves for term 1=1, 2, and 3, reproduced from Tanimoto and Anderson (1984). The lines show the direction of the fastest Rayleigh wave. The length of the lines is proportional to the anisotropy. (b) Faster split shear wave polarization directions (solid bars) shown at the locationsof the WWSSNstations from the ScS study (after Ando (1984)). The dashed bars with a small station code show the linear particle motions possibly representing no splitting of shear waves. The length of the solid bars shows the time differencebetween the two split shear waves. of dispersion curves from Rayleigh- to Love-types and from Love- to Rayleigh-types.
6. Body Wave 6.1 Theoretical studies Most of body wave studies in Japan have been conducted since the late 1970's. This is partly because the quality of waveform data had been largely improved by that time through the earthquake prediction research project of Japan, and partly because ocean bottom seismometers had been developed to record data with high enough quality. There have been several studies on the theoretical aspects of excitation and J. Phys. Earth Seismic Anisotropy: A Review of Studies by Japanese Researchers 309 propagation of body waves in anisotropic media. Kasahara et al. (1968c) experimentally examined the difference between velocity surfaces and wave surfaces, and confirmed the validity of the theories for this problem (for details of these theories see e.g., Musgrave, 1970; Crampin, 1977). Mochizuki (1987 b) formulated ray tracing in an generally anisotropic media showing body wave group velocities as functions of propagation direction as well as of position. Sakai and Kawasaki (1990) presented a Cagniard de Hoop formula which represents the radiation of elastic waves from a line source in a generally anisotropic uniform half space. Kawasaki and Tanimoto (1981) presented approximate representations of the radiation of elastic waves from a point source in generally anisotropic media, which recently have attracted broader attention (e.g., Tsvankin and Chesnokov, 1990).
6.2 Azimuthal anisotropy of body wave propagation Azimuthal variation of P wave propagation speeds, which has been found for continental lithosphere (e.g., Bamford, 1977), is not commonly observed in Japan. Among a few exceptions are the measurement of wave velocities of Sambagawa green schist at a mine in Shikoku (Kitsunezaki, 1964) and a determination of P wave velocities for the same area on a much larger scale based on an analysis of travel time anomalies of explosions (Hashizume, 1977). This lack of observations may be due to strong heterogeneity within the island arc crust beneath Japan. Remarkable observations of P wave azimuthal anisotropy were made for the oceanic lithosphere of the northwestern Pacific, through longshot experiments deploying large arrays of ocean bottom seismometers (e.g., Shimamura et al., 1983) (Fig. 3). Similar observations had been performed on a much smaller scale ( — 100 km) for various sites of ocean (e.g., Raitt et al., 1969), and the anisotropy was explained by the LPO of olivine at the uppermost part of mantle, perhaps right beneath the Moho. The observations by Shimamura et
Fig 3. P wave velocity plot as a function of propagation azimuth. Xis the azimuth measured clockwise from the north. Data of natural earthquakes are expressed by solid circles and those of Longshots by rectangles (modified after Shimamura et al. (1983)).
Vol. 43, No. 3, 1995 310 S. Kaneshima al. (1983) extend more than 1,000 km, and their results suggest that anisotropy exists also in the deeper part of the oceanic lithosphere. The direction of the maximum P wave velocity is perpendicular to the magnetic lineations and coincides with the fossil sea floor spreading direction. Okada et al. (1978) found azimuthal anisotropy of P wave in the sea of Japan. Azimuthal anisotropy of S wave velocity is much more difficult to detect than P wave anisotropy, mainly because deploying controlled sources of S waves with sufficiently large energy for such long distance experiments has been extremely difficult even in on-land experiments, to say nothing of those in ocean. Shimamura et al. (1977) reported that an azimuthal variation of S wave velocity was found in the northwestern Pacific, although the result is less conclusive than the results for P waves. It may be worth noting that even in a refraction experiment with controlled sources any observed azimuthal dependence of propagation speed can also be explained also by isotropic velocity heterogeneity, unless the coverage of ray azimuth of the experiment is extremely good. Recently, Nagumo et al. (1990) analyzed the azimuthal variation of P wave velocity in the northwestern Pacific a few 100 km east of Shimamura's observation site, and observed a relatively high velocity (8.2 km/s) in the "slow" direction proposed by Shimamura et al. (1983). Mapping of the 3-dimensional velocity structure using either P or S wave arrival times (e.g., Aki and Lee, 1976), which is often called tomography, has been remarkably successful in the last decade. In retrieving tomographic images of Earth's structure, the material has been usually assumed to be isotropic. Including anisotropy in a tomographic inversion drastically increases the number of unknown variables to be determined, and the trade-off among those parameters becomes too large to be neglected. There have been some attempts to retrieve P wave velocity anisotropy in the mantle beneath Japan. Hirahara and Ishikawa (1984) applied the tomographic method to determine the velocity anisotropy of P wave using JMA arrival time data. Although their results appear to show the presence of some anisotropy in the subducted slab, it is not necessarily statistically significant and remains inconclusive. Hirahara (1988) later applied an extended version of Hirahara and Ishikawa's method to the upper mantle beneath central Japan where Ando et al. (1983) show the presence of significantly large anisotropy through the analyses of shear wave splitting. He did not, however, obtain substantial anisotropy. Ando (1989) analyzed JMA travel time anomalies of P wave from deep focal-depth events beneath the sea of Japan traversing the Pacific plate subducted beneath Japan. He concluded that the observed P wave travel times anomalies are consistent with the computed travel time anomalies for an anisotropic slab model with a maximum P wave velocity in the direction normal to the expected magnetic lineations of the subducted slab. This is consistent with the results for the oceanic plate under the northwestern Pacific obtained from longshot refraction experiments by Shimamura et al. (1983). If his model is true, anisotropy in the oceanic lithosphere is preserved long after the slab is subducted beneath island arc. These travel time anomalies, however, may be explained by the geometry of the slab, and the above argument seems not to be conclusive. In most body wave travel time analyses the coverage of station-earthquake pairs is poor, so that it is always difficult to separate the effects of anisotropy from those of velocity heterogeneity.
J. Phys. Earth Seismic Anisotropy: A Review of Studies by Japanese Researchers 311
6.3 Sear wave splitting: mantle anisotropy Compared with the analyses of travel time anomalies, splitting of shear waves has been proven to be relatively insensitive to various effects of heterogeneity. This is because the two split shear waves propagate along nearly the same ray path through an anisotropic medium as long as the anisotropy is small and thus should undergo identical waveform distortion (Fig. 1(b)). Kumazawa and his colleagues, encouraged by the successes of their laboratory experiments, attempted to detect shear wave splitting in seismograms of local crustal earthquakes recorded by a micro-earthquake seismic observation network. The quality of the data, however, were not good enough for them to obtain any conclusive results. The first convincing evidence for shear wave splitting was obtained by Ando et al. (1980), who examined shear waveforms of local intermediate-focal depth earthquakes recorded at an local network in central Japan. Shear waves were found to split into two orthogonally polarized waves with different speeds, resulting in arrival time differences up to nearly 0.9 s. The location of anisotropy was not well constrained but was roughly estimated to be in the mantle wedge above the subducted Pacific plate (Ando et al., 1983). The polarization of the faster shear waves obtained at two stations nearly 100 km apart are nearly perpendicular, so that the size of the anisotropic material could be as small as 100 km. Ando (1986) related the anisotropy of the area with a low velocity region within the mantle wedge determined by the P wave tomography by Hirahara et al. (1989), and conjectured that the anisotropy is caused by preferred alignment of cracks filled with melts. Bowman and Ando (1987) analyzed shear waves from intermediate earthquakes at the Fiji islands recorded at a few portable local stations they installed. They found a complicated pattern of the fast shear wave polarization. Splitting of ScS waves from regional deep earthquakes has been intensively analyzed for the detection of mantle anisotropy (e.g., Fukao, 1984; Ando, 1984). Fukao (1984) provided the first clear evidence of this phenomenon on the seismograms recorded at JMA stations from a deep earthquake at Kurile. Ando (1989) also analyzed JMA seismograms of ScS waves from a few deep earthquakes around Japan. He found a pattern of the faster shear wave polarization directions for a deep event at Izu-Bonin which is largely different from that of Fukao (1984). The location of the anisotropy has not been determined yet, but it is likely to exist near the source (Ando, 1989). Ando (1984) systematically analyzed ScS seismograms of the WWSSN stations in the circum-Pacific area. His results show an interesting pattern of the fast shear wave polarization directions, which are mostly parallel to the direction of mantle return flow inferred from the surface plate motions (Hager and O'Connell, 1979) and also to the fast direction of Rayleigh wave phase velocities by Tanimoto and Anderson (1984) (Fig. 2(b)). These results seem to strengthen the hypothesis that mantle anisotropy is caused by the LPO of olivine as originally proposed by Hess (1964) and Kumazawa and Tada (1965). Besides ScS, the splitting of SKS waves has recently been quite successfully applied to the study of mantle anisotropy particularly beneath continents (see e.g., Silver and Chan, 1991). SKS analyses, however, have poor vertical resolution and need closely spaced stations for better resolution of the depth where the anisotropy exists. Silver and Kaneshima (1993) analyzed splitting of SKS waves recorded by a well-designed
Vol. 43, No. 3, 1995 312 S. Kaneshima portable array experiment from the Canadian Shield to Wyoming Craton. The orientations of fast shear wave polarization closely mimic the geological trends which could have been formed by Archean orogenic events, suggesting that the anisotropy was formed at that time and has been preserved ever since. Direct S waves from teleseismic events are starting to be analyzed, although they need more complicated waveform analyses than SKS. An attempt was made by Kaneshima and Silver (1992) to retrieve the anisotropy of the asthenosphere below the slab beneath south America and Kamchatka using direct S waves. The absence of significant anisotropy in the lower mantle which has been proposed by Karato and Li (1992) is supported by analyses of shear wave splitting (Kaneshima et al., 1993; Meade et al., 1995; Kaneshima and Silver, 1995). There have been few body wave studies of transverse isotropy of the Earth, except for Mikumo and Kurita (1971), who analyzed long period SH and SV wave spectra using Phinney's method (Phinney, 1964) and obtained a transversely isotropic mantle structure.
6.4 Shear wave splitting: crustal anisotropy Owing to the dense networks of seismograph stations as well as frequent occurrence of crustal earthquakes all around Japan, analyses of shear wave splitting have been useful for the detection of anisotropy in the crust beneath Japan (e.g., Kaneshima et al., 1987; Saeki and Umeda, 1988; for a summary see Kaneshima , 1990). A systematic comparison between fast shear wave polarization direction and tectonic stress by Kaneshima (1990) suggested that crustal anisotropy is primarily caused by preferred alignment of stress-induced cracks (e.g., Crampin, 1978) (Fig. 4). It is also suggested on the basis of the analyses of shear wave splitting from crustal to uppermost mantle
Fig. 4. Faster split shear wave polarization directions (bars) at the corresponding locations on the map of Japan (after Kaneshima (1990)). Solid and dashed bars indicate reliable and less reliable data, respectively. The trajectories of the maximum horizontal compression are given by thin solid lines.
J. Phys. Earth Seismic Anisotropy: A Review of Studies by Japanese Researchers 313 earthquakes that the presence of such crack-induced anisotropy is restricted to the rather shallow part of the crust (Kaneshima et al., 1988; Kaneshima and Ando, 1989; Kaneshima,1990). Recent studies closely analyzing shear wave forms recorded at closely spaced stations deployed in the Kanto and Tohoku areas also support the presence of stress-induced anisotropy (Tsukada, 1992; Okada et al., 1993).
7. Projects Currently Underway and Future Problems For the further advance of researches on anisotropy both of crust and mantle, analyses of broader band waveforms will be more important. Surface waves should also be closely analyzed to attain better vertical resolution of anisotropic structure beneath Japan. The deployment of the various broad band networks which are expected to cover the eastern part of Asia will be useful in these respects. The possibility of the existence of anisotropy in the slab should be closely checked through P wave travel time and shear wave splitting analyses. Seismic experiments planned in a gold mine in south Africa will reveal more about the anisotropy of in situ rocks by the alignments of cracks, and their response to stress change. Array analyses of waveforms will be essential for further advance in shear wave splitting studies. Anisotropy of the lower crust should be interesting topic to work on. Combinations of various seismological methods, such as shear wave splitting, short period surface waves, and vertical reflection profiling, with electric conductivity surveys may further reveal the structure and dynamics of the lower crust.
I thank E. Mochizuki for his helpful comments on the theoretical aspect of anisotropy studies.Thanks are also givento S. Karato and I. Kawasaki for providing the author with critical and helpful comments on this review and also with their recent manuscripts.
REFERENCES
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