Abstract:

Northwest Yunnan is one the key regions for earthquake monitoring in China due to its intense tectonic activity, the presence of major deep-seated faults, and frequent strong earthquakes. Investigating the crustal structure in this region is critical for understanding earthquake mechanisms, variations in physical properties, and tectonic evolution—insights that are essential for seismic hazard assessment and disaster mitigation. However, current research is limited by the sparse and uneven distribution of seismic stations and inconsistencies in imaging techniques. Consequently, studies have primarily focused on large-scale structures, with insufficient resolution of the three-dimensional fine structure, particularly in shallow sedimentary layers and basement formations. As a result, existing velocity models of the crust in northwest Yunnan remain relatively coarse. Given the complexity and heterogeneity of the middle and upper crust in both horizontal and vertical directions, high-resolution imaging is necessary to improve our understanding of seismogenic processes and meet the practical needs of earthquake risk mitigation. When integrated with deeper structural information from previous studies, such imaging can provide a more complete, multi-scale, three-dimensional view of the crustal architecture.

In this study, we processed two year of continuous vertical-component waveform data from 74 seismic stations in northwest Yunnan. Fundamental-mode Rayleigh wave phase velocity dispersion curves were extracted for periods ranging from 1 to 20 seconds. Using the direct surface wave imaging method, we inverted a high-resolution three-dimensional S-wave velocity model of the crust to a depth of 20km. This model provides new insights into the velocity structure and seismogenic environment of the region. The key findings are as follows:

(1)The S-wave velocity structure of the middle and upper crust exhibits significant lateral and vertical heterogeneity. Between depths of 0~8km, a low-velocity layer with variable thickness shows alternating uplift and depression features. Around 10km depth, high-velocity anomalies with thicknesses of approximately 5~10km are observed. These anomalies are mainly distributed across the Yongsheng-Binchuan region(central Chenghai fault), the Eryuan-Yangbi region(central Weixi-Qiaohou fault), and the intersection of the Longpan-Qiaohou and Weixi-Qiaohou faults. These high-velocity bodies are likely related to magmatic activity from the Emeishan Large Igneous Province(ELIP), composed of mafic and ultramafic materials emplaced by mantle plume activity during the Permian. In particular, the high-velocity body in the Eryuan-Yangbi region may be associated with the distribution of Proterozoic Cangshan Group rocks.

(2)From a seismogenic perspective, there is a strong spatial correlation between earthquake distribution and the velocity structure. Horizontally, seismicity is concentrated in weak zones adjacent to high-velocity bodies or at the boundaries between high- and low-velocity zones, often skewed toward the high-velocity side. Vertically, most earthquakes occur in the brittle upper crust, just above high-velocity anomalies and within low-velocity transition zones in the middle to lower crust. These transitional zones are characterized by significant contrasts in lithology and physico-mechanical properties, which facilitate stress accumulation and earthquake nucleation.

Key words: Northwest Yunnan, Ambient noise tomography, Direct inversion of surface wave, S-wave velocity structure in the middle and upper crust, Seismogenic environment

摘要：

滇西北构造运动剧烈、 强震多发, 对其进一步开展精细结构探测, 了解孕震机理, 可满足防范地震风险的实际需求。文中通过对滇西北74个台站2年记录的垂直分量连续波形数据进行处理, 在提取1~20s周期的基阶Rayleigh波相速度频散曲线的基础上, 采用面波直接成像方法反演了0~20km深度范围内的高分辨率三维S波速度模型, 并据此对滇西北地区的速度结构及孕震环境进行了研究。结果表明: 1)研究区中上地壳S波速度存在明显的横向和垂向非均匀性。在0~8km深度范围内存在厚度不均匀的低速层, 隆起和凹陷特征明显。10km深度附近存在厚5~10km的高速异常体, 主要分布在程海断裂中部的永胜—宾川之间、 维西-乔后断裂中段的洱源—漾濞之间、 维西-乔后断裂与龙蟠-乔后断裂的交会区, 推测这些高速体可能与峨眉山大火成岩省岩浆活动有关, 其应是二叠纪时期地幔柱活动残留在地壳内部的基性和超基性幔源物质。洱源—漾濞之间的高速体可能还与元古界苍山群岩的分布有关。2)从孕震环境来看, 研究区地震的空间分布与速度结构之间表现出了很好的对应关系。横向上, 地震主要集中在高速体的薄弱区域或高、 低速过渡带偏向高速体的一侧; 垂向上, 地震大多发生在中下地壳存在低速层且上覆高速体的脆性地壳中。在速度结构高、 低速过渡区, 地壳介质岩石组分及其物理-力学性质存在较大差异, 壳内应力易于积累, 有利于诱发地震。

关键词: 滇西北, 背景噪声成像, 面波直接反演, 中上地壳S波速度结构, 孕震环境