The quadrennium saw an escalation in papers dealing with seismic
anisotropy. It is now recognized that anisotropy is ubiquitous in the upper
mantle and crust. However, the origin of anisotropy and its localization
are enigmatic. Flow in the mantle is thought to orient
olivine crystals resulting in the fast direction for seismic waves
in the direction of flow. In the upper crust cracks which form at right
angles to extension cause the fast direction to be aligned with
the cracks. Also, metamorphic rocks in the crust can have
extreme anisotropies especially those containing phyllosilicates
(McNamara and Owens 1991).
Global sur
Horizontal anisotropy has been recognized in the oceans for some time (e.g., the Moho head wave studies of Raitt et al., 1969, or surface waves, Forsyth, 1975) associated with lateral flow of the mantle way from ridges giving high velocities perpendicular to ridges and low velocities parallel. Recently, Blackman et al. (1993) observed early arriving teleseismic P (compressional) waves on the axis of the Mid-Atlantic ridge which they interpreted to be caused by upward flow-induced anisotropy in the asthenosphere beneath the ridge. Away from the ridge the flow becomes horizontal parallel to the plate motion consistent with the earlier observations. Chastel et al. (1993) model the distribution of anisotropy in a convecting mantle and reproduce the oceanic observations, but in general they find that olivine orientation is inclined to the flow direction rather than precisely parallel to it.
A number of papers attest to the pervasiveness of SKS splitting on the continents. The SKS wave travels from the source to the core as an S wave, then in the core as a P wave, and converts back to an S wave for the path from the core to the surface. Silver and Chan (1991) analyze shear waves at 21 stations world-wide and find splitting at most stations. They conclude that rather than splitting being related to present plate motions, or crustal stress, it is probably due to past and present deformation of the subcontinental mantle by tectonic episodes. For stable continental regions they regard the anisotropy as ``fossil'' anisotropy, frozen in from ancient episodes, whereas for active regions it is due to recent tectonic activity. Savage and Silver (1993) report observations in the western United States and give the first splitting observations attributable to two layers of anisotropy having different fast orientations.
McNamara et al. (1994) analyze shear wave splitting on the Tibetan plateau between Lhasa, Tibet and Golmud, China. They observe fast orientations that are very nearly parallel to the compressional structures caused by the collision of India and Asia, suggesting that recent tectonics is the cause. Similarly, Makeyeva et al. (1992) find that under most of the Tien Shan mountain range the fast orientation is approximately parallel to the strike of the mountains. Also, Gao et al. (1994b) find that in the vicinity of lake Baikal, Siberia, the fast orientation is parallel to the opening of the Lake.
Shear wave splitting measurements in western North America have fast orientations predominantly east-west except for stations close to the San Andreas fault, where the splitting is consistent with two anisotropic layers, the upper aligned with the fault, and the lower in the east-west direction (Savage and Silver, 1993). Further east, including the Rio Grande rift region and stable, eastern North America, (Sandvol et al., 1992; Silver and Chan, 1991) fast orientations are, on average, in the plate motion direction, northeast-southwest. However, when regional variability and structural grain are taken into account, fossil anisotropy from older tectonic events can also provide a satisfactory explanation (Silver and Chan, 1991).
Consistent with the findings of Montagner and Tanimoto, it is generally thought that, although SKS splitting could occur anywhere between the core and the surface, it most likely occurs in the upper several hundred km. Methods to partition the anisotropy between the crust, lid and asthenosphere need to be developed. In one example, McNamara and Owens (1993) present a method to identify crustal anisotropy which involves the first observations of shear wave splitting on Moho Ps (P wave to S wave conversion) phases. They found that the fast orientation of the anisotropy is approximately parallel to Basin and Range extension consistent with stress estimators. Their preferred explanation is flow induced anisotropy in the mid to lower crust, possibly due to alignment of phyllosilicate minerals in metamorphic granodiorite or diorite. Of particular note, the crustal effects (splitting of 0.23 sec) are significantly smaller than mantle effects (splitting of 1 sec, Savage and Silver, 1993). Challenges for the next quadrennium in this area include separating fossil from recent tectonic anisotropy; determining the role of plate motions; partitioning the anisotropy in the crust and layers of the mantle; identifying whether SKS splitting occurs in the asthenosphere; and finer scale measurements of anisotropy of tectonic features such as hot spots, faults, and rifts. A powerful new tool to infer crustal and mantle flow is developing.