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

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