The 1976 Tangshan M_S7.8 earthquake, a catastrophic event in modern seismology, occurred along the Tangshan-Hejian-Cixian fault zone. This fault zone is recognized as a major contemporary right-lateral shear rupture system within the North China Basin. To the south of its extensive aftershock zone, a segment characterized by a notable absence of historical large earthquakes has been identified and designated as the Tianjin Seismic Gap. Based on a comprehensive integration of historical earthquake catalogs, geological field investigations, and crustal deformation data, it has been proposed that this seismic gap possesses the potential to generate a large earthquake of approximately M_W7.5. The occurrence of such an event would pose a severe threat of destruction to the megacities surrounding the gap, making the identification of seismogenic structures within this area a critical priority for seismic hazard assessment.

A fundamental challenge for understanding the seismic potential of the Tianjin Seismic Gap lies in determining the spatial distribution and current activity state of deep crustal faults within it. Previous studies have demonstrated that the seismogenic structures for several destructive earthquakes in the broader region are high-angle strike-slip faults located in the deep crust. These structures are consistent with the regional strike-slip system governed by the contemporary neotectonic stress field. However, the Tianjin Seismic Gap area is characterized by a thick sedimentary cover. Moreover, during its early rifting stage, the region was dominated by NW-SE extensional tectonics, which resulted in the development of numerous shallow, low-angle normal faults in the upper crust. This complex geological setting, particularly the thick sediment layer and the prevalence of shallow extensional features, poses significant difficulties for the direct geophysical detection of deeper fault structures. Under these constraints, the detailed analysis of local moderate-to-small magnitude earthquake sequences using digital seismic networks emerges as a uniquely effective tool for investigating the geometry and kinematics of deep fault structures.

On 11 December 2003, anM_L4.0 earthquake occurred in Tanggu, Tianjin. This event represents the largest earthquake to have occurred in the northern segment of the Tianjin Seismic Gap since the establishment of the Capital Circle Digital Seismic Network in 2001. The high-quality digital waveform data recorded by this network offer a valuable opportunity to probe the deep seismogenic fault system of this gap. In this study, we present a detailed seismological analysis of the Tanggu M_L4.0 earthquake sequence. Our analytical workflow integrates multiple advanced methods, including micro-earthquake detection to build a more complete event catalog, precise seismic relocation to refine hypocentral parameters, calculation of focal mechanisms to define fault geometry and slip sense, and inversion of rupture direction and process to characterize source kinematics.

Our results show that the seismogenic structure of the Tanggu M_L4.0 earthquake is a fault with a strike of approximately 219°, a dip angle of about 76°, and a sense of motion that is predominantly right-lateral strike-slip with a minor extensional component. The focal depth of the mainshock is determined to be in the range of 18–20 km, placing it in the middletolower crust. The rupture propagated from the northeast toward the southwest. This seismogenic fault is not a direct downward or upward extension of previously mapped, near-surface NE-trending faults in the epicentral area, such as the Chadian fault or the Cangdong fault. Instead, it represents a distinct, high-angle right-lateral shear fault located in the deep crust. Through comparative analysis with the 1976 Tangshan M_S7.8 mainshock and three strong （M_S≥6.0） aftershocks located in the southern part of the Tangshan aftershock zone, we identify important similarities. The focal mechanism of the Tanggu earthquake is closely comparable to those of the Tangshan mainshock and these three large aftershocks. Furthermore, like those three large aftershocks, the Tanggu earthquake is located in the middle-to-lower crust. While the Tanggu epicenter lies outside the defined spatial extent of the Tangshan aftershock zone, its location falls along the southwestward linear extension of the fault system as delineated by relocation studies of the Tangshan sequence.

Based on these observations, we propose a tectonic interpretation. The NE-trending deep shear fault revealed by the Tanggu earthquake likely represents a southwestward continuation of the deep right-lateral shear structure that is active within the Tangshan aftershock zone. This deep shear system appears to extend southwestward, passing beneath the shallow crustal Beitang sag, a region where the upper crust is dominated by low-angle normal faults formed during the early extensional rifting phase of the basin. The identification of this deep structure has direct implications for seismic hazard assessment. The Beitang sag area, situated in the northern segment of the Tianjin Seismic Gap, exhibits tectonic conditions that are favorable for the generation of a strong earthquake. This area lies within the loading zone of Coulomb stress changes induced by the 1976 Tangshan M_S7.8 mainshock and its strong aftershocks, a factor that increases the likelihood of future failure. By estimating the distance from the southern end of the Tangshan aftershock zone to the hypocenter of the Tanggu M_L4.0 earthquake, and by applying empirical relationships between fault rupture length and moment magnitude, we estimate that the Beitang sag and its surrounding areas have the potential to host an earthquake of approximately M_W6.5.

The detailed characterization of the seismogenic structure of the Tanggu M_L4.0 earthquake provides crucial seismological evidence for the identification of strong-earthquake hazard sources and for the improvement of seismic risk analysis in the densely populated Capital Circle region of North China. This study demonstrates the value of analyzing moderate earthquakes in complex geological settings to reveal deep fault structures that are otherwise difficult to detect.