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LIUF JAN-HUA LIU Fu-TIAN Wu HUA LI QIANG HU GE 1) Institute of Geophysics, Academia Sinica, Beijing.Institute of Geophysics, State Seismological Bureau, Beijing.  
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.
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