Study of azimuthal anisotropy in the transition zone of the Earth's upper mantle with the coupling of normal modes

The azimuthal anisotropy in the upper mantle can be determined by studying body wave data or surface wave data, but it is hard to use these studies to find out evidence for azimuthal anisotropy in the transition zone. Some long period normal modes of the earth free oscillations penetrate into the transition zone. According to our estimation of coupling sensitivity kernels using a model of mantle anisotropy, the coupling between fundamental spheroidal and toroidal modes in 250 similar to 400 s, such as S-0(20)-T-0(21) and S-0(25)-T-0(25), shows peak sensitivity to azimuthal anisotropy at 400 similar to 600 km depth. Different from normal mode coupling caused by Earth rotation, the anisotropic coupling modes can be clearly identified only at stations near nodes of spherical harmonics, and most of them have high resolution on 18 similar to 24-hour vertical component spectra. Anisotropic coupling is so strong that sometimes the amplitude of coupled toroidal mode is even larger than that of coupled spheroidal mode on vertical component. These characteristics provide us important clues to distinguish anisotropic coupling from rotational coupling in seismic observations. Anisotropic coupling of long-period normal modes is an important signal that determines azimuthal anisotropy structures in the transition zone of the upper mantle.