Three-dimensional high-resolution velocity structure imaging and seismicity study of the Tangshan fault

Based on the 4415 earthquakes body wave arrival times recorded by the 47 fixed and 33 temporary stations in Tangshan and its surrounding areas, the Tangshan fault’s area focal parameters and three-dimensional fine velocity structure model were inversed by using the consistency-constrained double-difference tomography method. The results revealed that the relocated earthquakes were roughly vertical to the fault’s strike, with the maximum depth exceeding 25 km. The Tangshan fault’s area Vp, Vs and Vp/Vs values demonstrate obvious inhomogeneity, and high speed bodies prone to the brittle fault are dominant on both sides of the fault in the upper crust. Small low velocity bodies appeared initially at the Ms7.8 Tangshan earthquake source in the 14–16 km range of the middle crust, followed by a gradual expansion of the low velocity area with an increase in depth. Variation in the velocity structure below the Tangshan earthquake source shows that the high temperature mantle source invading the crust has weakened the upper crust’s high-speed area, leading to stress accumulation in the local area, which is an important factor for the occurrence of earthquakes in the Tangshan region. The relationship between the regional seismic structure and Tangshan seismic sequence, causing migration of surrounding blocks and seismic activity in recent years was analyzed based on the Tangshan fault’s 3D highresolution velocity structure imaging. It is considered that the Tangshan area unique velocity structure not only plays a pivotal role in promoting horizontal tectonic stress during seismic activity, but is also the root cause of the aftershock evolution process and the fault’s morphological characteristics.