Three-dimensional velocity structure imaging of the crust in the middle and northern section of the north-south tectonic belt of China

The middle and northern section of the North-South Tectonic Belt of China (NTBC) is located at the junction of three large blocks: the Qinghai-Tibet plateau, the north China block and the Yangtze block. On the whole, the tectonics in this section shows the characteristics of "overpass" with the deep SN-trending structure and the shallow EW-trending structure. In addition, strong seismic activity is very frequent in the region. To establish a three-dimensional (3D) fine velocity model is of great significance to understanding the scientific problems such as the deep dynamics process and the seismogenic background of strong earthquake. The study area is located between 102°E-108°E and 32°N-37°N. We use earthquake body-wave arrival times (P and S) and differential times (S-P) to image the 3D P- and S-wave velocity structure and Vp/Vs ratio of the crust recorded by permanent stations of the Chinese Seismic Network and a high density temporary seismic array from 2010-2015. An improved double-difference (DD) tomography algorithm (Guo et al., 2018) which is based on the local-scale version of tomoDD (Zhang and Thurber, 2003) is applied. The initial 3D forward velocity model is formed by interpolation of several deep seismic sounding profiles in the study area. The new inversion results show high-resolution velocity structure of the middle and northern section of the NTBC in 3D. Based on the geological structure distribution characteristics of different blocks and the distribution of historical strong earthquakes in the region, we discuss the relationship between the velocity distribution of Vp、Vs 、Vp/Vs ratio and the structure and strong earthquakes. The study is supported by the National Natural Science Foundation of China (41704058).