华南及南海北部地区瑞利面波层析成像

基于华南及周边地区106个宽频带地震台站多年记录的MS≥5.0中浅源地震事件, 开展瑞利面波层析成像和速度结构特征研究, 获得了华南大陆及南海北部地区10~100 s瑞利波群速度分布图像和典型剖面下方地壳上地幔速度结构, 为理解该地区构造演化和深部过程提供约束.考虑到实际地震射线的覆盖情况以及华南地区主要构造的主体展布特征, 本文同时采用传统的规则网格剖分和平行主要构造走向的非规则网格剖分方法, 分别进行分格频散反演, 开展了不同参数化方案对反演结果影响的对比分析研究.基于瑞利面波层析成像结果, 进行了典型剖面横波速度结构反演, 重建了华南地区由内陆至南海海域主要构造单元的壳幔横波速度结构.研究结果表明, 扬子和华夏块体地壳上地幔结构特征差异显著, 扬子块体地壳和岩石圈厚度均大于华夏地块, 且扬子块体上地幔顶部速度较华夏块体低, 岩石圈厚度在雪峰山造山带下方发生过渡和转换;南海北部陆缘和南海海盆上地幔速度较高且形态相对完整, 表现为非火山型大陆边缘和已停止扩张海盆的壳幔结构特征.

Surface wave tomography for South China and the northern South China Sea area

South China is located at the southeastern part of the Eurasia plate, which consists of South China Continent and South China Sea. In tectonics, it mainly includes the Yangtze and Cathaysia blocks, and the South China Sea Basin. Since the Mesoproterozoic, South China has evolved in the dynamic settings of assembly and break-up of the supercontinent, the Southern and Northern continents for a long time. During the Meso-Cenozoic, it suffered influences from the westward subduction of the Pacific plate and the uplift of the Qinghai-Tibet plateau. Due to the complicated history of its geological evolution, some fundamental questions, such as the location of the convergence boundary between Yangtze and Cathaysia blocks, the origin of the Xuefengshan Orogenic belt, and the rifting type of the northern margin of the South China Sea, etc, are still controversial. Seismic investigations can provide reliable constraints on the tectonic evolution and the deep process.We carried out a study of surface wave tomography based on seismic waveform data, which were triggered by events of M_S≥5.0 with a shallow or moderate focal depth, and recorded by 106 broadband digital stations in South China and surrounding areas. Rayleigh wave group velocity maps at periods from 10 s to 100 s were derived, and a typical profile of S-wave velocity structures, down to a depth of ~200 km, was determined.Considering the actual ray-path coverage and the major tectonic features in South China, we developed an irregular parameterization scheme with slant grid-lines paralleling to the strike of the major tectonic features based on a method of coordinate transformation, besides the traditional gridding scheme with regular gird-lines paralleling to the latitude and longitude. Using the Frequency-Time Analysis method, the group velocity dispersion was firstly measured for ray paths across the study area. And then, the pure dispersion in a grid of 1.5°×1.5° was obtained using a damped least-squares approach. The comparison of the group velocity maps, derived from the regular-gridding and slant-gridding schemes, demonstrates that gridding scheme has certain effects on the detailed shapes of the anomalies in the case of the ray-path coverage of this study.We extracted the group velocity dispersion and inverted for the S-wave velocity along a typical NW-SE trending profile, which crosses the Yangtze and Cathaysia blocks, and the South China Sea Basin progressively. Both the Rayleigh wave group velocity and the S-wave velocity sections reveal distinct differences of the crust and upper mantle among the major tectonic units in South China. From the mainland to the deep sea in South China, both the crustal and lithospheric thicknesses decrease gradually along the profile, and the lithosphere-asthenosphere boundary depth is sharply changed beneath the Xuefengshan orogenic belt. The velocity at the top of the upper mantle beneath the Cathaysia Block and South China Sea Basin is higher and more intact than that of Yangtze Block, which indicates the tectonic nature of a passive non-volcanic continental margin for the northern margin of South China Sea, and an inactive marginal basin of the South China Sea Basin.