Abstract:

The northeastern part of the Tibetan plateau, comprising secondary blocks such as Qiangtang, Bayan Har, Qaidam and Qilian, forming a complex tectonic pattern. This region, located at the interface between the South China Block and North China Block, has been at the forefront of the Indo-Eurasian plate collision, experiencing significant tectonic deformation. Consequently, it serves as an ideal natural laboratory for the study of plate tectonics, crustal dynamics, and seismic activity. Shear wave splitting is a method used to study the anisotropy of media, based on the phenomenon where shear waves split into two sets of wave trains, fast and slow, due to the anisotropy of the medium during propagation. In the mid-to-upper crust, this splitting characteristic is often identified through the analysis of local earthquake waveforms. The fast wave direction typically aligns with the oriented arrangement of vertical cracks, governed by the regional principal horizontal compressive stress direction. In contrast, the slow wave is nearly perpendicular to the fast wave, and its time delay is closely related to the crack geometry and internal fluid state, indirectly reflecting the degree of medium anisotropy. In this study, we have collected waveforms of local small earthquakes from January 2010 to September 2021 on the northeastern Tibetan plateau and calculated two anisotropy parameters: fast-wave polarization direction and slow-wave time delay, using shear wave splitting analysis. We subsequently construct a detailed spatial distribution map of the anisotropic parameters of the upper crust. The fast-wave polarization direction is dominated by an ENE direction, roughly parallel to the regional principal compressive stress direction, indicating that the anisotropy of the upper crustal medium is mainly controlled by regional tectonic stress. Several relatively weaker secondary fast-wave polarization directions, including NNW, WNW, and near EW, vary widely across the northern and southeastern parts of the Qilian block and the northern part of the Qiangtang block. These directions are approximately parallel to the widely distributed NW-trending faults, indicating the influence of the fault system. The fast-wave polarization directions on the northeastern edge of the Qaidam block are more discrete, with the northern margin stations showing WNW direction dominance and the north-central part showing NE or weaker NW dominance, affected by the combined effects of stress, faults, rock properties, and other factors. The slow-wave delay time serves as a quantitative indicator of the anisotropy, reflecting variations in stress level within the medium. With the thrust fault system in the northern part of the Qilian block, the slow-wave time delay varies from 1.7ms/km to 6.3ms/km, averaging(3.2±2.1)ms/km. Notably, these time delays are larger in the east than in the west, reflecting differences in the stress environment. The southeastern Qilian block and the northeastern margin of the Qaidam block exhibit a relatively uniform average time delay of(5.1±2.4)ms/km, with an overall range of 2.5ms/km to 5.7ms/km. The similar distribution of time delays may be related to similar rock properties and tectonic environments. At the northern edge of the Qaidam basin, the WNW-oriented fast-wave polarization direction, coupled with a relatively consistent slow-wave time delay ranging from 3.1ms/km to 4.5ms/km, may be a response to the high-pressure metamorphism of fractures in the deep crust. The northern part of the Qiangtang block shows a stable degree of deformation, as evidenced by the slow-wave time delay averaging(4.5±0.8)ms/km with a small standard deviation. Both the northern Qiangtang block and the periphery of the Lajishan faults(encompassing the southeastern Qilian and northeastern Qaidam blocks)host volcanic arcs and reservoir formations. However, the former exhibits shorter time delays compared to the latter, potentially attributed to differences in rock physical properties and the tectonic environment. Due to the heterogeneous distribution of data, further studies are needed to gain a more comprehensive understanding of upper crustal deformation.

Key words: Northeastern Tibetan plateau, shear-wave splitting, seismic anisotropy, upper crust, fast-wave polarization direction, slow-wave time delay, deformation

摘要：

由羌塘、 巴颜喀拉、 柴达木和祁连等几个二级块体组成的青藏高原东北部与华南块体及华北块体接触, 产生强烈的构造变形。文中利用青藏高原东北部2010年1月—2021年9月的小地震波形事件, 使用剪切波分裂分析方法计算得到快波偏振方向与慢波时间延迟2个各向异性参数, 并对参数的空间分布进行分析。快波偏振方向以NEE为主导, 与区域主压应力方向大致平行, 祁连块体北部和东南部、 羌塘块体北部存在NNW、 NWW或近EW等多个相对较弱的第2快波偏振方向, 与区域内广泛分布的NW走向断裂近平行。柴达木块体东北缘的快波偏振方向较离散, 受到应力、 断裂、 岩石性质等因素的共同作用。祁连块体北部逆冲断裂系东部的时间延迟大于西部, 体现了两者应力环境的差异。祁连块体东南部及柴达木块体东北缘大致相当的时间延迟分布可能与其具有类似的岩石类别物性及构造环境相关。柴达木盆地北缘存在NWW向快波优势偏振方向和相对一致的时间延迟, 可能是断裂在深部高压变质作用的响应。羌塘块体北部的时间延迟小于拉脊山断裂周缘(祁连块体东南部与柴达木块体东北缘), 可能与岩石类别物性与构造环境的差异有关。

关键词: 青藏高原东北部, 剪切波分裂, 各向异性, 上地壳, 快波偏振, 慢波时间延迟, 变形