Investigating the fine structure of fault zones is essential for understanding the mechanism of large earthquake nucleation. In this paper we study the complex fault geometry of the Tangshan fault zone in North China using a new seismic data set from 2010 to 2020 that spans ∼10 years. Based on the well-located hypocenters and focal mechanism solutions, we obtained more detailed information on the fault zone structural features: (1) the fault zone is divided into five segments according to different dip angles of the fault plane; (2) the spatial distribution of seismicity reveals conjugate fault branches and a flower-shaped structure at the northern part of the fault zone; and (3) the fault zone is dominated by a strike-slip type, but their intersecting segments are characterized by a normal-fault type. We further obtained high-resolution seismic images of Vp, Vs and Vp/Vs around the Tangshan source region. Combining the fault geometry with the low-velocity anomalies below 10 km depth, we infer that the ascending of hot mantle material into the lower crust may have weakened the Tangshan fault zone and generate the 1976 Ms. 7.8 Tangshan earthquake. •The Tangshan fault zone is composed of five fault segments according to different dip angles of the fault plane.•Low velocity anomalies are imaged in the middle-lower crust of the Tangshan source region.•The Tangshan Ms. 7.8 earthquake is associated with the thermomechanical weakening of the fault zone.