The Tangshan-Hejian-Cixian fault zone is one of the most important active tectonic belts in the North China Plain and has generated a series of destructive earthquakes, including the 1966 Xingtai M_S6.8 earthquake sequence and the 1976 Tangshan M_S7.8 earthquake. However, main segment of this fault zone is buried beneath thick Cenozoic sediments of the Bohai Bay basin, resulting in poorly developed surface ruptures and limited geological evidence at the surface. Consequently, the geometry, kinematics, and seismogenic significance of the concealed faults remain controversial. In particular, the seismogenic structures that generated the 1966 Xingtai earthquake sequence remain long debated. Based on newly acquired high-resolution three-dimensional seismic data and drilling information from the North China Oilfield, this study conducts a detailed structural analysis of the Shulu segment of the Tangshan-Hejian-Cixian fault zone to clarify its tectonic evolution and identify the principal seismogenic faults.

The Shulu sag is located in the southern Jizhong depression of the Bohai Bay basin and represents one of the most seismically active regions in North China. Stratigraphic data indicate that Cenozoic strata directly overlie Paleozoic basement rocks, while Mesozoic deposits are absent because of long-term uplift and erosion prior to basin formation. The study integrates three-dimensional seismic interpretation, drilling and stratigraphic correlation, and regional earthquake data to investigate the structural framework of the Shulu segment and its relationship with seismic activity. Earthquake catalogs obtained from the China Earthquake Networks Center （CENC） and the United States Geological Survey （USGS） are further used to examine the spatial distribution of seismicity and its correlation with major fault systems. The results reveal that the tectonic evolution of the Shulu sag can be divided into two major stages. During the Paleogene, the region was dominated by NW-SE-oriented extensional deformation. The Xinhe fault acted as the principal basin-controlling listric normal fault and controlled the development of a series of asymmetric half-graben basins. Thick Paleogene succession accumulated within these fault-controlled depressions. Seismic profiles display typical wedge-shaped growth strata and syn-rift geometries, indicating intense fault activity and rapid subsidence during basin evolution. The Paleogene extensional fault system therefore established the fundamental structural framework of the Shulu Sag.

A major tectonic transition occurred during the Neogene. The activity of the Paleogene listric normal faults gradually weakened and eventually ceased, and the Bohai Bay basin entered a stage of regional subsidence. The Guantao Formation unconformably overlies the Paleogene syn-rift succession, marking the end of large-scale extensional faulting. At the same time, a new right-lateral strike-slip fault system developed within the Shulu segment. The Xinhedong fault and Xinhexi fault cut through and modified the earlier extensional structures, and became the dominant tectonic elements in the region. Seismic interpretation shows that these faults are characterized by steep fault planes and well-developed negative flower structures, which are diagnostic features of strike-slip deformation. Their geometry and crosscutting relationships demonstrate that strike-slip activity postdated Paleogene extension and fundamentally reshaped the pre-existing basin architecture. The observed tectonic transformation reflects a significant change in the regional stress regime of eastern China. Paleogene extension was associated with lithospheric thinning and extensional tectonism in the Bohai Bay basin, whereas Neogene deformation was dominated by right-lateral shear related to regional tectonic reorganization. Under this new stress regime, previously formed extensional structures became inactive and were progressively modified by strike-slip faulting. As a result, the present-day structural framework of the Shulu segment records both Paleogene extensional basin development and Neogene strike-slip reactivation.

To further constrain the seismogenic structures, the interpreted faults were compared with historical earthquake epicenters and focal mechanism solutions. The Xinhedong and Xinhexi faults exhibit strong spatial and kinematic consistency with the seismogenic structures inferred for the 1966 Xingtai earthquake sequence. Their near-vertical geometry, right-lateral strike-slip characteristics, and close correspondence with earthquake distribution indicate that they are the principal active faults controlling modern seismicity in the study area. In contrast, the Paleogene listric normal faults show no evidence of significant late-stage activity and are unlikely to represent the main seismogenic structures of the Xingtai earthquake sequence. These observations provide direct structural evidence supporting previous interpretations that emphasized strike-slip faulting as the dominant mechanism responsible for large earthquakes in the region. The identification of the Xinhedong and Xinhexi faults also has important implications for understanding the coupling relationship between deep and shallow structures in the North China Plain. Previous geophysical studies suggested that strong earthquakes in this region are associated with deep crustal fault systems extending into the lower crust. The present study demonstrates that such deep tectonic processes are expressed in the shallow subsurface through a well-developed strike-slip fault network that can be clearly imaged using high-resolution three-dimensional seismic data. The Shulu segment therefore represents a typical example of structural inheritance and multi-stage tectonic evolution within an intraplate tectonic setting.

In summary, the Shulu sag initially developed as a Paleogene extensional basin controlled by the Xinhe listric normal fault system. Since the Neogene, extensional activity has ceased and a right-lateral strike-slip fault system represented by the Xinhedong and Xinhexi faults has become dominant. These strike-slip faults cut and modify the earlier extensional structures and constitute the principal seismogenic faults of the 1966 Xingtai earthquake sequence. This study provides direct three-dimensional seismic evidence for the concealed seismogenic structures of the Shulu segment of the Tangshan-Hejian-Cixian fault zone and offers new insights into tectonic evolution, structural inheritance, and seismic hazard generation in the North China Plain.