Seismic tomography of 3d velocity structure on the central south crust of the North China block

摘要

The North China Fault-block is located in the north of Henan (113°E-116°E, 35°N-37N°), which is an important earthquakes active object. There are three different geological elements in the region. Since the faults almost entirely control the geological structure motion of the block, they are surely closely related to the earthquake activities as well. Now, there are more earthquakes occurring in the area. Therefore, it will be meaningful to use seismic tomography to study the distribution of velocity structure of the crust in the area and to explore the earthquake activity in the future. In the study, no less than 7000 travel time records of P-wave and S-wave of 293 local earthquakes were selected from 22 earthquake-stations in this area. The crust 3D velocity structure and earthquake activity features were studied on the basis of the data. Three discontinuities (15km, 27km, 34km) were considered in the initial model base on the former study outcomes. During selecting the mesh scale, the principle which was assured to discern some depth clearly meanwhile as to promote resolution of discern as possible was complied with. Whereas above, we divided the study area into blocks with an equality grid scale 5.56kmx5.56kmx3.00km. According to the feature of the geological structure and earthquake's distribution of the area, we sliced the studied region along such as the 35.3°N, 35.7°N, 36.0°N, 36.4°N and 113.9°E, 115.4°E lines and make the slice pass through the earthquake belts and the merge of faults as more as possible. The conclusions we got from the study are the following, (a) We find that earthquake tends to occur in higher velocity zone in this area, (b) The mid crust (the depth from 20km to 26km) is more uniform and velocity-changing area is smaller, (c) There is a larger obvious high velocity zone in the area (114.5°E-115.0°E, 35.6°N-35.8°N, in the south-center of the study area) and it extends to the depth of 12km. But at the depth of about 30km the area is changing gradually into lower velocity. And there is few earthquakes occurred in here before. So, whereas above, we advice that should focus on earthquakes activity of the area in future.