关键词: 背景噪声, 面波直接成像, 方位各向异性, 滇西北中上地壳

Abstract:

The northwestern Yunnan region is located at the collision boundary between the Indian and Eurasian plates. Owing to its unique tectonic setting, complex geological structures, and intense crustal deformation, it represents a key area for geoscientific research. Detecting the fine crustal structure of this region not only helps to elucidate crustal motion and lithospheric deformation mechanisms, but also provides an important basis for understanding plate tectonic evolution, mineral resource formation, and geological environment protection.

Seismic anisotropy refers to the phenomenon in which the propagation velocity and polarization direction of seismic waves vary with propagation direction in anisotropic Earth media. It mainly includes body-wave anisotropy and surface-wave anisotropy, the latter of which can be further divided into azimuthal anisotropy and radial anisotropy. Azimuthal anisotropy arises from differences in surface-wave phase velocities with azimuth and is an important parameter for characterizing medium deformation. Investigating crustal azimuthal anisotropy in northwestern Yunnan can reveal variations in the stress field and crustal motion during deformation processes, thereby providing critical insights into the background of crustal evolution. As a transitional layer of the lithosphere, the middle and upper crust is also an earthquake-prone zone, and its internal structure and deformation play a key role in understanding plate tectonic evolution and crustal dynamics. In recent years, with the successive deployment of short-period dense seismic arrays through active-source detection and sub-instability experiment projects, the number of seismic stations in northwestern Yunnan has increased significantly, enabling the acquisition of short-period surface-wave signals. Although short-period, high-frequency surface waves are strongly attenuated, they are more sensitive to shallow velocity structures, making them particularly valuable for resolving three-dimensional azimuthal anisotropy of the shallow crust and for understanding shallow structural and deformation patterns.

Based on two years of continuous vertical-component waveform data recorded by 74 seismic stations in northwestern Yunnan, phase-velocity dispersion curves of fundamental-mode Rayleigh waves with periods of 1-20 s were extracted. Using a direct inversion method for three-dimensional surface-wave azimuthal anisotropy, a three-dimensional azimuthal anisotropy model of the middle and upper crust above 20km depth was constructed, and the deformation characteristics and stress field state of northwestern Yunnan were analyzed. The main conclusions are as follows.

(1)Azimuthal anisotropy in northwestern Yunnan exhibits pronounced spatial zonation, generally bounded by the Weixi-Qiaohou Fault, with significant differences between its eastern and western sides. West of the fault, azimuthal anisotropy is relatively uniform and shows little variation with depth; fast-wave directions are predominantly NNW and NW, consistent with the regional principal compressive stress direction. East of the Weixi-Qiaohou Fault, azimuthal anisotropy differs markedly above and below a depth of 10km. At depths greater than 10km, fast-wave directions generally rotate clockwise, transitioning from NNW and NW in the north to nearly NS south of 26°N, indicating that crustal deformation is mainly controlled by regional strike-slip motion. Within the 10~20km depth range, dominant fast-wave directions are NNE and NE, consistent with the strike of the Chenghai Fault. This anisotropy is likely related to strong compression or strike-slip deformation near the Chenghai fault zone, resulting in the preferred alignment of minerals such as mica and hornblende along the fault strike.

(2)At the southern termination of the Weixi-Qiaohou Fault(south of Yangbi), the dominant fast-wave directions are SWW and near EW. These deformation characteristics are preliminarily interpreted to be associated with a normal-fault dislocation mechanism driven by horizontal compression in the NNW direction and horizontal extensional tectonic stress in the SWW direction.

(3)The three-dimensional layered azimuthal anisotropy model obtained in this study provides information on anisotropic variations at different depth levels in the vertical direction. Horizontally, compared with shear-wave splitting and Pms splitting methods, which only provide anisotropy information beneath seismic stations, the surface-wave-based approach enables azimuthal anisotropy imaging over the entire study area, including regions without stations, thereby offering superior horizontal and vertical resolution. Given that the lateral resolution of surface-wave methods mainly depends on station density and wavelength, three-dimensional azimuthal anisotropy imaging based on dense seismic arrays is an effective means of resolving crustal deformation characteristics and stress field states. With the implementation of the China Earthquake Science Experimental Site project, denser seismic networks are expected to be deployed in northwestern Yunnan in the near future, which will facilitate the construction of more refined three-dimensional azimuthal anisotropy models. The results of this study provide valuable reference data for regional geophysical research and scientific assessment of future earthquake hazards, and contribute to a deeper understanding of the tectonic background and dynamic mechanisms of northwestern Yunnan.

Key words: ambient noise, direct surface wave tomography, azimuthal anisotropy, middle-upper crust of northwest Yunnan