GUO GuangRui,
LI Juan*,
CHEN SiDan et al
.0.Seismic Anisotropy in the Big Mantle Wedge and Mantle Transition Zone of the Northwestern Pacific Subduction System Chinese Journal of Geophysics(in Chinese),(): 1-,doi: doi:10.6038/cjg2020O0233
西北太平洋俯冲带大地幔楔及地幔过渡带各向异性证据和特征
郭广瑞1,2，李娟1,2*，陈思丹1,2，Walter D.Mooney3
1 中科院地质与地球物理研究所，北京 100029
2 中国科学院大学，北京 100049
3 美国地质调查局，USA CA 94025
Seismic Anisotropy in the Big Mantle Wedge and Mantle Transition Zone of the Northwestern Pacific Subduction System
GUO GuangRui1,2, LI Juan1,2*, CHEN SiDan1,2, Walter D. Mooney3
1 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2 University of Chinese Academy of Sciences, Beijing 100049, China
3United States Geological Survey, CA 94025, USA
Abstract：The morphologies of subducting slabs have been clearly imaged by seismic tomographic techniques, however, the character of the mantle flow associated with subduction process remains ambiguous. Both trench parallel and trench normal patterns of seismic anisotropy have been reported in various mantle-wedge subduction systems. Here we investigate the anisotropic characteristics in the “Big Mantle Wedge” that is formed by the stagnation of the subducting Pacific slab. A detailed shear wave splitting study was conducted on the SKS phase and the direct S-wave phase of regional deep-source earthquakes of MDJ station, which has a long-term stable observation data. Simultaneous multi-waveform inversion (SIMW) method is used to obtain more accurate SKS phase anisotropy results on the basis of guaranteeing the azimuth coverage. We identified prominent frequency dependence and initial polarization variability in apparent splitting parameters, indicating the presence of multiple possible sources of anisotropy. Two-layer anisotropic model fitting based on the SIMW results in different frequency bands shows a trench-oblique fast direction of ? = 32±12?, ?t = 1.4±0.4 s in the upper layer, attributed to the fossil anisotropy in the lithosphere affected by the Tan-Lu fault zone. And a nearly trench-normal fast direction of ? = -71±6?, ?t = 2.4±0.4 s in the lower layer. The large delay time in the lower layer indicates the presence of anisotropy within the mantle transition zone and possibly deeper. The trench-normal fast axis direction may be due to the combined effects of the lattice preferred orientation (LPO) of olivine caused by asthenosphere flow in the shallow mantle, and the LPO of wadsleyite caused by the lateral mantle flow associated with the stagnation of the Pacific slab.
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