地震波各向异性：窥测地球深部构造的“探针”

Abstract

Seismic anisotropy has received a lot of attention from seismologists in recent years and is becoming increasingly important in the field of geophysics and geology. It is regarded as the bridge between seismology and structural geology. Seismic anisotropy is discovered at all scales in the Earth's interior and may provide us with valuable information, such as the thickness and structure of lithosphere, mantle convection, and geodynamics, and since the fast wave propagation directions of shear wave correspond to flow directions as implied from plate motions, it is recognized as a good indicator of deformation and mantle flow. Seismic anisotropy plays a central role in revealing the deep structure and geodynamics in the following geological settings, such as subduction zone, continental rift, mantle transition zone and continental collisional orogenic belt (for instance, Tibet). This paper mainly reviews recent studies of the occurrence, geological interpretation and implication of seismic anisotropy for these geological settings. There is no doubt that the existing technologies will be refined and developed further to make estimates of anisotropy and related rock properties more accurate. Problems required to be further considered include the following: (1) resolution of shear wave: SKS wave is poor in vertical resolution, and it is suggested that the combination of surface wave and SKS wave may well constrain the depth of anisotropy; (2) petrofabric analysis: although great advances had been made in investigation of relationship between anisotropy and petrofabric, recent studies reveal that olivine fabric may be different from previously expected under water-rich conditions, which may then induce anomalous seismic anisotropy. Thus, efforts are still required to be taken to further study the petrofabric, and (3) other mechanisms for seismic anisotropy, such as MPO, aligned cracks, etc.. In particular, strain aligns highly anisotropic minerals, such as olivine, orthopyroxene, plagioclase, and so on, in the mantle and crust to form LPO, which is the most likely cause of splitting measured from records of distant earthquakes. As a result, it is emphasized that investigation of seismic anisotropy shall be combined with rheology of rocks and minerals at high temperature and pressure.