“…Fortunately, anisotropy with hexagonal symmetry is a proper approximation to the materials in the Earth's crust and mantle, which can reduce the number of physical parameters describing seismic anisotropy (e.g., Christensen, 1984;Park and Yu, 1993;Maupin and Park, 2007). To further simplify the problem, we can assume the hexagonal symmetry to be horizontal when the azimuthal anisotropy is concerned in SWS measurements (e.g., Crampin, 1984;Silver, 1996;Savage, 1999;Huang et al, 2011a,b;Long, 2013) and P-wave velocity studies (e.g., Hess, 1964;Backus, 1965;Raitt et al, 1969;Hearn, 1996;Eberhart-Phillips and Henderson, 2004;Zhao, 2008, 2013); whereas we can assume the hexagonal symmetry to be vertical when the radial anisotropy is concerned in the form of a Vsh/Vsv variation (Vsh and Vsv are the velocities of shear waves polarized horizontally and vertically, respectively) in surface-wave studies (e.g., Nettles and Dziewonski, 2008;Fichtner et al, 2010;Yuan et al, 2011) and in the form of a Vph/Vpv variation (Vph and Vpv are the velocities of P-waves propagating horizontally and vertically, respectively) in P-wave velocity studies (e.g., Ishise et al, 2012;.…”