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

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