利用Rayleigh波相速度和群速度联合反演青藏高原东北缘S波速度结构

S-wave velocity structure of the northeastern Tibetan Plateau from joint inversion of Rayleigh wave phase and group velocities

利用青藏高原东北缘地区固定和流动地震台网2007年8月到2012年1月期间记录的远震波形,运用小波变换频时分析方法分别测定了1216和653条周期从15到140s的台站间基阶Rayleigh相速度和群速度频散曲线.通过对上述频散进行反演,重构了青藏高原东北缘分辨率高达0.5°×0.5°的2-D相速度和群速度分布图.然后通过对所提取到的每个格网点Rayleigh波相速度和群速度频散进行联合反演,得到了研究区下方一维S波速度结构.最后通过线性插值,得到了青藏高原东北缘下方地壳上地幔三维S波结构.结果表明,印度板块向北俯冲已经达到班公—怒江缝合带附近;在柴达木盆地北部祁连山下面我们发现了亚洲板块,且其没有表现出明显的向南俯冲的迹象;在两大板块中间,我们观测到延伸到250km深度的低速异常,该低速异常可能是地幔物质底辟上涌现象造成的.

The teleseismic waveforms recorded by permanent and temporary networks in NE Tibet from Aug. 2007 to Jan. 2012 have been used to obtain the fundamental mode Rayleigh-wave dispersion data. The Rayleigh wave phase and group velocities along 1216 and 653 paths respectively, were estimated by the wavelet transformation method. These dispersion data were utilized to construct 2-D phase and group velocity maps for the period range 15~140 s. The tests show that the horizontal resolution across the region is about 0.5° for the periods used in this study. The Rayleigh wave phase and group velocity dispersions at each 0.5° cell were jointly inverted to determine 1-D shear wave velocity structure and then assembled into 3-D model by simple linear method. The results show that the Indian plate is preserved till it gets subducted beneath Bangong-Nujiang suture zone. The Asian plate is imaged beneath north of Qaidam basin and Qilian mountain, and does not show obvious sign of southward underthrusting. Between the two plates, a low velocity anomaly extends from the top of upper mantle down to 250 km, which may be caused by the mantle delamination.