基于密集流动地震台阵的青藏高原东北缘及邻区Rayleigh波相速度层析成像

Phase velocity maps of Rayleigh wave based on a dense coverage and portable seismic array in NE Tibetan plateau and its adjacent regions

本文利用喜马拉雅二期台阵674个流动地震台一年的远震垂直向观测资料,获取了18491条独立路径上的Rayleigh波相速度频散曲线,并反演得到了周期10~80 s的Rayleigh波的相速度分布图.通过对比,本文与已有成像结果具有较一致的高低速分布特征,表明了本文结果的可靠性.结果显示,研究区的相速度分布存在明显的横向非均匀性.短周期(如10~15 s)的相速度分布与地表地形密切相关,中等周期(如20~40 s)的相速度分布受地壳厚度的影响较大.在长周期(如60~80 s),鄂尔多斯块体的高速比阿拉善块体更显著、完整,表明同属于华北板块的阿拉善地块,其上地幔结构并没有鄂尔多斯稳定.从短周期至长周期,与周缘地块相比,青藏高原始终表现出较明显的低速异常,可能暗示其具有较活跃的地壳上地幔结构.松潘一甘孜和北祁连块体的中上地壳均存在低速层.全球参考模型Crust1.0和Lithos1.0均不能很好地解释我们的观测频散,基于本文获得的相速度结果可在很大程度上对Crust1.0和Lithos1.0模型进行补充和完善.

The northeastern margin of the Tibetan Plateau and its adjacent regions are important transition zones, where the orientation of the India-Eurasian collision effect converts to the NE-SW or E-W from the N-S trending. Many strong earthquakes occurred in this region. It is a key place to study the geodynamics of the Tibetan Plateau. In this paper, we collected the vertical component of teleseismic data recorded by 674 broadband stations of the portable seismic array deployed under the project ＂China Seismic Array-northern part of North-South seismic belt＂ from Sep. 2013 to Aug. 2014. A total number of 18491 independent inter-station phase velocity dispersion curves of Rayleigh wave were measured by a frequency-time analysis method based on a continuous wavelet transform. In addition, all of these dispersion data were arranged into a tomographic inversion scheme, called Ditmar ＆Yanovskaya, to obtain the phase velocity maps at periods ranging from 10 s to 80 s in the study region. Our tomographic results are in agreement with previous studies on the distribution of the phase velocities. It shows obviously lateral inhomogeneity of the phase velocity in the study area. At short periods （e.g., 10-15 s）, the distribution of phase velocities is well related to the surface geology and topography. At intermediate periods （e.g., 20-40 s）, the distribution of phase velocities is significantly affected by the thickness of the crust. At long periods （e.g., 60-80 s）, it shows more obvious and integrated high-speed anomalies beneath the Ordos block than that of the Alxa block, indicating that the upper mantle of the Alxa block is not as stable as the Ordos block, although both of these two blocks belong to the North China craton. The Tibetan Plateau is imaged as obvious low-speed anomalies in comparison with the surrounding blocks at all interested periods, indicating that the Tibetan Plateau has active structures of the crust and upper mantle. Low velocity layers （LVL） are imaged in the upper-middle crust of the Songpan-Garzê and the Northwestern Qilian blocks. The dispersion curves calculated from the global reference model Crust1.0 and Lithos1.0 are not consistent with our observed dispersion curves, indicating that the inversion based on the phase velocity dispersion curves obtained from this study may greatly improve the models of Crust1.0 and Lithos1.0.