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本文利用三重震相波形模拟获得了藏西北地幔过渡带近东西向剖面的SH波和P波速度结构.震源深度偏移波形叠加的方法可验证数据的一致性,提高三重震相识别的可靠性.研究发现,藏西北过渡带的SH波和P波速度结构并不简单线性相关.过渡带内SH波速度梯度小,其660km间断面速度跳跃大;而P波速度结构则表现为过渡带内速度梯度大,且660km间断面速度跳跃小.与此同时,两种速度结构均显示660km间断面埋深无明显异常.上述结果与藏中和拉萨块体过渡带速度结构基本一致与印度地盾下方相比,研究区过渡带底部SH波速度相当,而P波略显高速,因而Vp/Vs比值稍大.结合矿物物理研究,我们认为地震波速度异常的差异可能主要是由过渡带内化学成分的横向不均匀性,例如铝含量的增多引起的.考虑到青藏高原的构造演化,研究区过渡带内铝含量的增多可能与高原形成过程中特堤斯洋板片的俯冲-拆沉有关.
In this paper, the three-phase seismic facies waveform is used to simulate the SH-wave and P-wave velocities near the east-west section of the mantle transitional zone in northwestern Tibet. The method of superposition of the source depth migration waveform can verify the consistency of the data and improve the reliability of the recognition of the triple seismic facies The study found that the SH and P wave velocity structures in the transitional zone in northwestern Tibet are not linearly correlated simply, and the SH wave velocity gradient in the transition zone is small, with a large jump in the 660km section, while the P wave velocity structure shows as a transitional zone The velocity gradient is large and the velocity jump of the 660km section is small.At the same time, the two velocity structures show that there is no obvious anomaly in the depth of the 660km section.These results are consistent with the velocities of the transitional zone between Tibet and Lhasa, Compared with the lower part of the study area, the SH wave velocity at the bottom of the transitional zone in the study area is fairly high, while the P wave is slightly higher at high velocity, so the ratio of Vp / Vs is slightly larger.Under the study of mineral physics, we think the difference of seismic velocity anomalies may be mainly caused by the chemical composition Such as the increase of aluminum content.Considering the tectonic evolution of the Qinghai-Tibet Plateau, the increase of Al content in the transition zone of the study area may be related to the formation of Tertiz The subducting slab of - delamination related.