| Abstract: | Abstract: In this paper, 3‐D velocity images of the crust and upper mantle beneath the Nanbei tectonic zone of China are constructed using P‐wave travel time residuals of earthquakes, with the data supplied by China's seismic networks and the International Seismic Centre. During the model parameterization in the tomographic inversion the interpolation function of grid node velocities is used as the space function of velocity, and the velocity function is allowed to be discontinuous. The difficulties caused by large memory demand and high computing cost in solving the system of equations are avoided by utilizing the least squares QR decomposition algorithm. Thus, the stability of the algorithm is ensured. Though resolution images are not derived in the inversion process based on the resolution theory of Backus and Gilbert, a covariance resolution method is put forward by the authors. The resolution analyses have proved the reliability of the image results. 3‐D ray tracing is conducted to obtain the ray paths in a spherical coordinate system. The imaging results indicate that there is a significant lateral heterogeneity in the crust and upper mantle beneath the North‐South tectonic zone of China persisting down to 200 km in depth. The velocity images of the upper crust show features closely related to the tectonic features on the surface. A low‐velocity layer exists in a very wide range of the mid‐crust. The lowest velocity value is about 5.60 km/s. In a long narrow area between 25.0°N‐38.0°N and 100.0°E‐103.2°E, a low‐velocity anomaly of about 7.49 km/s appears at the top of the upper mantle. Almost all of the major earthquakes (ML≥6.0) took place in the transitional strips between high and low velocity zones at depths above 20 km in the crust. From the velocity images at 20+0 km and 50+0 km depth respectively, we find that the epicentres of strong earthquakes with magnitudes larger than 6.0 are almost entirely distributed in the low‐velocity zones or on their boundaries. |
