Fault structures of the Haichenghe fault zone in Liaoning, China from high-precision location based on dense array observation

The Haichenghe fault zone (HFZ), the site of the 1975 M 7.3 Haicheng earthquake, is one of the most seismically active zones in eastern China. To better understand the fault structures in HFZ, we deployed a dense array of 23 broadband seismic stations in 2021, with an average distance interval of ~ 6 km. Utilizing neural network-based phase picking, earthquake association, and relocation methods, we analyzed data from the dense array and the Liaoning Seismic Network from Aug. 9, 2021, to Sep. 8, 2023. The relocations clearly reveal a conjugate fault system within the HFZ, consisting of WNW-striking and NE-striking subvertical faults with different scales. The Haichenghe Fault (HF) appears as a WNW-trending en echelon fault, with a 30-km-long main segment (MHF) to the northwest and a 5-km-long Xiuyan segment (XYF) to the southeast. The MHF is further divided into NW and SE segments by two NE-trending faults. Additionally, our data resolve the asymmetric conjugate rupture area of the Haicheng M 7.3 earthquake and a triangular seismic gap near the intersection of the MHF and the main NE-trending conjugate fault (MCF), indicating a strong heterogeneity of the subsurface medium in this region. Furthermore, we identified new conjugate fault structures with a V-shaped seismicity pattern in the Xiuyan area, extending along WNW and NE directions. Our findings stress the importance of dense array observations in the HFZ, providing essential seismological insights into its complex fault structures and seismogenic environment.