Inversion for regional tectonic stress field in the North-South Seismic Belt of China

Using the regional stress tensor damping inversion method and focal mechanism data （466 M_L≥3.5 earthquakes focal mechanism solutions from January 2009 to August 2017 and 259 M≥4.5 earthquakes focal mechanism solutions from January 1976 to August 2017 from the GCMT）, we have obtained the distribution of the stress field by 1.0°×1.0° grid region in the North-South Seismic Belt and its adjacent regions. The spatial distributions of stress field show that, because of the complex dynamic environment, there are differences between the maximum principal stress direction inside the North-South Seismic Belt which is the east boundary of the Tibetan Plateau. The maximum principal stress direction in the northern segment of the North-South Seismic Belt is NE. In and surround the middle segment of the North-South Seismic Belt, the maximum principal stress direction is nearly EW except the NE segment of the Longmenshan fault zone whose direction is NW-NNW. The direction of the maximum principal stress in the middle and southern segments of the North-South Seismic Belt changes gradually, from nearly EW to NW/NE then to nearly NS. Overall, the maximum principal stress in the North-South Seismic Belt and its adjacent regions has a clockwise rotational pattern from north to south. Inside the Sichuan-Yunnan rhombic block the maximum principal stress is in NNW direction, while to the west of the west boundary the direction is NNE, to the east of the east boundary the direction is NW. The stress direction transition bands have well corresponded with the Sichuan-Yunnan rhombic block boundary. According to the analysis of regional tectonic stress field, it is difficult to apply the North-South Seismic Belt as a unified seismic belt to the medium and long-term earthquake prediction.