seismic anisotropy

seismic anisotropy Seismologists usually make the initial assumption that the material through which a seismic wave travels is mechanically isotropic (see seismic waves, principles). This means the speed of travel is independent of the direction of travel. This assumption greatly simplifies mathematical treatments and is acceptable for most applications. Almost all minerals are anisotropic, but since the individual mineral grains which combine to form a rock are usually randomly oriented, the end result is isotropy on the scale of the wavelengths of typical seismic waves (metres to hundreds of metres). There are, however, many situations in which significant anisotropy occurs, and attempts to measure and understand anisotropy and treat it mathematically are increasingly common. Anisotropy is usually expressed as a percentage representing the variation between the fastest and slowest velocity through a material. Although individual minerals can be highly anisotropic, an anisotropy of 10 per cent would be high for a large body of rock.

It has long been known that foliated rocks, such as gneisses, in which mineral alignments have been produced by the metamorphic effects of pressure and temperature, are anisotropic. Rocks such as shale, which are composed of elongate grains, are also anisotropic because these grains align during the process of sedimentation that forms the rock.

An early significant observation of large-scale anisotropy occurred when seismic refraction measurements (see controlled source seismology) in oceans showed that the P-wave velocity of the upper mantle (Pn) was consistently higher for profiles recorded perpendicular to an oceanic spreading centre (i.e. parallel to the direction of spreading or plate movement) than for profiles recorded parallel to the spreading centre. These measurements have been confirmed by numerous subsequent studies and are attributed to the alignment of olivine crystals in the mantle lithosphere because of flow during the formation of the oceanic plate at the ridge. Similar observations are uncommon for continental areas because there are fewer good determinations of the P-wave velocity in the upper mantle. Anisotropy has, however, been reported in the Basin and Range province of western North America and in the eastern arm of the East African rift.

On a smaller scale, stresses in the Earth can cause bodies of rock to fracture in a consistent manner. If these fractures are aligned, the rock will be anisotropic with the fast direction perpendicular to the fractures and the slow direction parallel to them. In ideal cases, these observations can provide information on the state of stress in the Earth.

G. R. Keller