中国南北带地壳和上地幔的三维速度图象

本文采用作者提出的地震层析成象法得到了中国南北带地壳和上地幔的三维速度图象。通过误差和分辨分析,以及同爆炸地震测深剖面的比较,证明了成象结果的可靠性。 成象结果表明:1.南北带的地壳和上地幔存在显著的横向不均匀性,深达450km这种不均匀性还依然存在;2.地壳上部的速度图象与地表的已知地质特征明显相关:四川盆地显著低速,康滇地轴显著高速;3.中地壳在很大范围内存在低速层,其最低速度值达5.60km/s;4.在25°N-38°N和100.0°E-103.2°E的长条带内,上地幔顶部出现低速异常,异常速度值约为7.49km/s。 成象结果还清晰地勾划了各块体间的焊接边界。120km深度的速度图象表明,扬子准地台自秦岭以南以龙门-大巴和盐源-丽江台缘褶带为其西部边界;西南以哀劳山褶皱带为界;东南则以右江褶皱带内的南盘江为界。 统计表明,地震活动与南北带的速度结构相关:从20km以上的速度图象发现,大地震大都发生在高速和低速间的过渡条带上。

THREE DIMENSIONAL VELOCITY IMAGES OF THE CRUST AND UPPER MANTLE BENEATH NORTH-SOUTH ZONE IN CHINA

In this paper three-dimensional velocity images of the crust and upper mantle beneath the North-South zone are obtained by means of the seismic tomographic method proposed by the authors. The error and resolution analyses and comparison with that of the DSS profile proved the reliability of the image results.The image results indicate that 1. there is a significant lateral heterogeneity in the crust and upper mantle beneath the North-South zone, persisting down to 450 km depth; 2. the velocity images of the upper crust are closely related to the well known geological features of the surface; 3. the low velocity layer exists in a very wide range of the mid-crust. The lowest velocity value is near 5.60 km/s; 4. in a long and narrow area between 25°N-38°N and 100.0°E-103.2°E, the low velocity of about 7.49 km/s appears at the top of the upper mantle. The image result at a depth of 120 km also outlines clearly the welded boundaries between the different geological blocks.The statistical data show that seismic activity is related to the velocity structure of North-South zone: almost all of the major earthquakes took place in the transition strips between high-and low- velocity zones in the crust above 20 km. From the velocity images at 20+0 and 50+0km depths we find that most of epicenters of strong earthquakes with magnitude larger than 7.0 are distributed in the low velocity zones or on their boundaries.