Origin of crustal anisotropy: shear wave splitting studies in Japan

Shear wave splitting manifested as leading shear wave polarization has been observed at a number of seismograph stations in Japan and can be attributed to crustal anisotropy. This paper discusses the relation between leading shear wave polarization directions and tectonic features of Japan. To explain the observed shear wave splitting, it is proposed that at least three phenomena should be taken into account: stress-induced microcracks primarily aligned in vertical or subvertical planes; cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes; and intrinsic rock anisotropy resulting from preferred orientation of minerals. Travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes suggest that the crustal anisotropy may be limited to the upper 15-25 km. This implies that the density of nonhorizontally aligned cracks or fractures below 15-25 km and into the upper mantle is much smaller than that in the crust above 15-25 km. -from Author