Shear wave splitting, also called seismic birefringence, is the phenomenon that occurs when a polarized shear wave enters an anisotropic medium. The incident shear wave splits into two polarized shear waves. Shear wave splitting is typically used as a tool for testing the anisotropy of an area of interest. These measurements reflect the degree of anisotropy and lead to a better understanding of the area's crack density and orientation or crystal alignment. As an analogy, one can think of the anisotropy of a particular area as a black box and the shear wave splitting measurements as a way of looking at what is in the box.
Introduction An incident shear wave may enter an anisotropic medium from an isotropic media by encountering a change in the preferred orientation or character of the medium. When a polarized shear wave enters a new, anisotropic medium, it splits into two shear waves. One of these shear waves will be faster than the other and oriented parallel to the cracks or crystals in the medium. The second wave will be slower than the first and sometimes orthogonal to both the first shear wave and the cracks or crystals in the media. The time delays observed between the slow and fast shear waves give information about the density of cracks in the medium. The orientation of the fast shear wave records the direction of the cracks in the medium. When plotted using polarization diagrams, the arrival of split shear waves can be identified by the abrupt changes in direction of the particle motion. In a homogeneous material that is weakly anisotropic, the incident shear wave will split into two quasi-shear waves with approximately orthogonal polarizations that reach the receiver at approximately the same time. In the deeper crust and upper mantle, the high frequency shear waves split completely into two separate shear waves with different polarizations and a time delay between them that may be up to a few seconds.
History Hess (1964) made the first measurements of P wave azimuthal velocity variations in oceanic basins. This area was chosen for this study because oceanic basins are made of large, relatively uniform homogeneous rocks. Hess observed, from previous seismic velocity experiments with olivine crystals, that if the crystals had even a slight statistical orientation this would be extremely evident in the seismic velocities recorded using seismic refraction. This concept was tested using seismic refraction profiles from the Mendocino fracture zone. Hess found that the slow compressional waves propagated perpendicular to the plane of slip and the higher velocity component was parallel to it. He inferred that the structure of oceanic basins could be recorded quickly and understood better if these techniques were used. Ando (1980) focused on identifying shear-wave anisotropy in the upper mantle. This study focused on shear wave splitting recorded near the Chubu Volcanic Area in Japan. Using newly implemented telemetric seismographic stations, they were able to record both P wave and S wave arrivals from earthquakes up to 260 km beneath the volcanic area. The depths of these earthquakes make this area ideal for studying the structure of the upper mantle. They noted the arrivals of two distinct shear waves with different polarizations (N-S, fast and E-W, slow) approximately 0.7 seconds apart. It was concluded that the splitting was not caused by the earthquake source but by the travel path of the waves on the way to the seismometers. Data from other nearby stations were used to constrain the source of the seismic anisotropy. He found the anisotropy to be consistent with the area directly below the volcanic area and was hypothesized to occur due to oriented crystals in a deep rooted magma chamber. If the magma chamber contained elliptical inclusions oriented approximately N-S, then the maximum velocity direction would also be N-S, accounting for the presence of seismic birefringence. Crampin (1980) proposed the theory of earthquake prediction using shear wave splitting measurements. This theory is based on the fact that microcracks between the grains or crystals in rocks will open wider than normal at high stress levels. After the stress subsides, the microcracks will return to their original positions. This phenomenon of cracks opening and closing in response to changing stress conditions is called dilatancy. Because shear wave splitting signatures are dependent on both the orientation of the microcracks (perpendicular to the dominant stress direction) and the abundance of cracks, the signature will change over time to reflect the stress changes in the area. Once the signatures for an area are recognized, they may then be applied to predict nearby earthquakes with the same signatures. Crampin (1981) first acknowledged the phenomenon of azimuthally-aligned shear wave splitting in the crust. He reviewed the current theory, updated equations to better understand shear-wave splitting, and presented a few new concepts. Crampin established that the solution to most anisotropic problems can be developed. If a corresponding solution for an isotropic case can be formulated, then the anisotropic case can be arrived at with more calculations. The correct identification of body and surface wave polarizations is the key to determining the degree of anisotropy. The modeling of many two-phase materials can be simplified by the use of anisotropic elastic-constants. These constants can be found by looking at recorded data. This has been observed in several areas worldwide.
Physical mechanism
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