青藏高原东北缘上地幔多尺度层析成像

利用布设在青藏高原东北缘地区的甘肃宽频带地震台阵记录到的远震P波走时数据,采用小波域参数化和基于L1范数的稀疏约束反演算法的多尺度层析成像方法,得到了该地区400km深度范围内上地幔的P波速度结构.本文采用的多尺度层析成像方法可以自适应数据非均匀采样的情况,有效降低谱泄漏效应和反问题的多解性,明显提高解的分辨率和可靠性.层析成像结果表明青藏高原东北缘上地幔整体上显示为低速特征,扬子地块上地幔则显示为高速特征,两者之间上地幔存在清晰的块体边界带,该边界带位于东经104°—105°之间并且随深度的增加逐渐东移.该特征暗示了青藏高原上地幔物质向东扩张的机制,但在西秦岭上地幔顶部不存在物质运移的通道.青藏高原东北缘内部也具有明显的分区特征,松潘—甘孜地块上地幔P波速度整体呈低速特征,而柴达木地块的上地幔顶部具有相对高速特征,而在上地幔200km以下这两个地块间的差别逐渐减小.1654年天水地震和1879年武都地震都发生在扬子地块与青藏高原的碰撞交汇区,其震中下方上地幔显示为高低速转换结构.

P-wave structure of upper mantle beneath the Northeastern Tibetan Plateau from multi-scale seismic tomography

The P-wave velocity structure of the upper mantle down to 400 km is obtained by applying multi-scale seismic tomography technique to P-wave arrival time data recorded by the Gansu seismic array. The seismic tomography algorithm used in this study employs sparsity constraints on the wavelet representation of the velocity model via L1-norm regularization. This algorithm can deal with uneven-sampled data, reduce the effect of spectral leakage, and can also improve the resolution and stability of the seismic tomography results. Our seismic tomography results show that there exist the low-velocity anomalies in the upper mantle of the northeastern Tibetan plateau, and the high-velocity anomalies in the upper mantle of the Yangtze block. Between 104°E and 105°E, there exists an obvious boundary between the Tibet and Yangtze block. This boundary shifts eastward with the deepening of the depth, which implies the eastward expansion of the upper mantle material beneath the Tibetan plateau. There is no channel at the top of upper mantle beneath the western Qinling for material transportation. The northeastern Tibetan plateau also shows inhomogeneous feature. The Songpan-Garzê block shows a low-velocity feature, and the Qaidam block shows a high-velocity feature at the upper mantle, and the difference of velocity between the two blocks are gradually disappeared below 200 km depth. The Tianshui earthquake occurred in 1645 and the Wudu earthquake occurred in 1879 located in the collision boundary between the Yangtze block and Tibet plateau, where is the transition zone between high-velocity and low-velocity anomalies.