Determination of 3-D velocity anomalies of the Nanbei tectonic zone of China based on local earthquakes

In this paper, 3-D velocity images of the crust and upper mantle beneath th e Nanbei tectonic zone of China are constructed using P-wave travel time re siduals 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 interpol ation function of grid node velocities is used as the space function of vel ocity, and the velocity function is allowed to be discontinuous. The diffic ulties caused by large memory demand and high computing cost in solving the system of equations are avoided by utilizing the least squares QR decompos ition algorithm. Thus, the stability of the algorithm is ensured. Though re solution images are not derived in the inversion process based on the resol ution theory of Backus and Gilbert, a covariance resolution method is put f orward 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 heterogene ity 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 lo west velocity value is about 5.60 km/s. In a long narrow area between 25.0 degrees N-38.0 degrees N and 100.0 degrees E-103.2 degrees E, a low-velocit y anomaly of about 7.49 km/s appears at the top of the upper mantle. Almost all of the major earthquakes (M(L)greater than or equal to 6.0) took place in the transitional strips between high and low velocity zones at depths a bove 20 km in the crust. From the velocity images at 20(+0) km and 50(+0) k m depth respectively, we find that the epicentres of strong earthquakes wit h magnitudes larger than 6.0 are almost entirely distributed in the low-vel ocity zones or on their boundaries.