利用地球简正模耦合研究上地幔过渡区方位各向异性

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

地震方位各向异性广泛存在于地球上地幔中,目前利用地震体波或面波分析研究上地幔各向异性的地球物理方法有很多种,但是由于各自的局限性均难以分析上地幔过渡区中的各向异性特征.方位各向异性可导致球形简正模和环形简正模之间发生耦合.地球长周期自由振荡的简正模可深入到上地幔过渡区.本文利用各向异性地球模型计算各向异性简正模耦合深度敏感核,表明长周期(250～400s)简正模各向异性耦合(如0S20-0T21和0S25-0T25)的敏感度峰值在400～600km之间.在不受地球自转影响的台站,如位于南极极点的QSPA站,仍然可以观测到强烈的简正模耦合现象.本文的研究表明:只有在地震观测台站靠近长周期球形振荡的节点时,才能在其观测数据中观测到各向异性耦合现象,许多各向异性耦合在震后18～24h期间最强,并可导致垂直方向的环形简正模的振幅大于球形耦合简正模的振幅.这些特征是在地震观测数据中寻找各向异性耦合的重要线索.长周期简正模的方位各向异性耦合为我们提供了一个新的认识上地幔过渡区各向异性的窗口.

The azimut data or surface wave hal anisotropy in the upper mantle can be data, but it is hard to use these studies determined by studying body wave 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 ~ 400 s, such as 0S20-0T21 and 0S2~ 0T25, shows peak sensitivity to azimuthal anisotropy at 400 ~ 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 18-24-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.