太平洋大型横波低速带东部边界地幔最下部各向异性研究

摘要

针对太平洋大型横波低速带（Large Low Shear Velocity Province，简写为 LLSVP）东部边界的 D″各向异性强度的问题，利用中美洲和南美洲部分台站的地震记录，通过对 SKS 和SKKS 震相进行横波分裂分析，得到22个 SKS-SKKS 震相对的横波分裂结果，其中有6个震相对存在显著差异。对比分析震相对的横波分裂结果差异，可以保守地估计 D″各向异性。横波分裂结果显示，地幔最下部存在各向异性；对 D″各向异性成因的分析结果认为，如果LLSVP 边界上的地幔最下部物质存在变形以及内部存在小尺度的非均匀体，则有助于解释这些观测，但是本文在 LLSVP 边界上并没有看到大量的有差异震相对聚集。结合前人的观测研究推测，该研究区域下方的 LLSVP 及其周围地幔的边界可能不是很陡峭，边界附近没有积累强烈的变形，并在此基础上讨论了地幔最下部各向异性结构的研究意义。%To evaluate the strength of D″ anisotropy beneath the easternmost Pacific Large Low Shear Velocity Province (LLSVP),SKS and SKKS splitting measurements were conducted separately.We examined SKS and SKKS phases for events (Tonga trench)over the life time of 5 seismic stations located in South America and Central America.These phases sample lowermost mantle of easternmost Pacific LLSVP.Finally,22 SKS-SKKS phase-pairs were obtained, of which six pairs show significantly different splitting between the two phases. The results showed the existence of lowermost mantle anisotropy.We attempt to account for these observations with deformation along LLSVP boundary and small-scale lateral heterogeneity in anisotropic structures in LLSVP’s interior,respectively.Based on previous observations,the absence of discrepant pairs clustering along the boundary of LLSVP indicates that the LLSVP boundary is not very sharp here and there is no strong deformation accumulated near the LLSVP boundary beneath the studied region.We also discussed the significance of the research on lowermost mantle anisotropy.