利用面波直接反演方法研究滇西北地壳三维S波速度结构

Three-dimensional S-wave velocity structure of the crust in Northwest Yunnan using the surface wave direct inversion method

摘要:

我国滇西北地区(25°~27°N,99°~101°E)是川滇菱形地块西边界所在区域,由于构造运动活跃,是我国地震灾害比较严重的地区,非常有必要深入研究其孕震结构。文章利用该地区69个短周期流动台站2020年记录的连续波形,通过背景噪声互相关计算和时频分析方法,提取所有台站对Rayleigh面波相速度频散曲线,采用基于射线追踪的面波频散直接反演方法,得到研究区地下三维S波速度结构。研究结果表明，研究区域上地壳速度结构存在明显差异,特别是大型断裂带的分段特征明显。如维西—乔后断裂带浅层5 km内整体表现为西北部高速,东南部低速;程海断裂带北段高南段低,在宾川盆地存在厚度约5 km的低速异常,说明宾川盆地存在较厚的沉积层。地壳浅层波速和地形起伏有较好的对应关系,在盆地区域整体表现为低速,高山区域为高速。2021年漾濞M_S6.4地震发生在S波高低速过渡区域,研究区中下地壳内存在不同程度的低速区,可能为地震的发生提供了孕震环境。

Abstract:

The northwestern region of Yunnan Province (25°—27°N, 99°—101°E) lies along the western boundary of the Sichuan—Yunnan rhomboid block, a zone marked by active tectonic movements. This region is among the most seismically hazardous in China, requiring a comprehensive understanding of its seismogenic structures. In 2020, we analyzed continuous waveforms from 69 short-period stations in the area. We extracted Rayleigh surface-wave phase-velocity dispersion curves using cross-correlation of ambient noise and time-frequency analysis. We derived the Dl S-wave velocity structure beneath the study area using the direct inversion method of surface-wave dispersion based on ray tracing. The results reveal significant variations in the crustal velocity structure, particularly in segmented features along major fault zones. For example, at a depth of 5 km beneath the Weixi—Qiaohou fault zone, high velocities are observed in the northwestern section, whereas low velocities dominate the southeastern section. Similarly, the Chenghai fault zone exhibits high velocities in its northern segment and low velocities in the southern segment. In the Binchuan Basin, a low-velocity anomaly of approximately 5 km suggests the presence of a substantial sedimentary layer. Shallow crustal wave velocity correlates closely with terrain relief, with low velocities observed in basin areas and high velocities in mountainous regions. The 2021 Yangbi M6.4 earthquake occurred in a transition zone between high and low S-wave velocities. In addition, low-velocity zones of varying extents are identified in the middle and lower crust, potentially providing a seismogenic environment for the earthquake.