oa Three Dimensional Velocity Images of the North-South Zone in China with LSQR Algorithm

In this paper three dimensional velocity images of the crust and upper mantle beneath the North South Zone are reconstructed using 10277 P wave arrivals of regional earthquakes recorded at 157 stations in the study area with LSQR algorithm. 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 in depth. 2. The velocity images of the upper crust show the features closely related to the well known tectonic features on 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 narrow piece of area between 25°N-38°N and 100.0° E-I03.2°E, the low velocity anomaly about 7.49 km/s appears at the top of the upper mantle. The image result at a depth of 120 km also outlines dearly the welded boundaries between the different geological blocks. The statistical data show that seismic activity is related to the velocity structure of the 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.