The 1976 Tangshan M_S7.8 earthquake is a highly representative intraplate strong earthquake in North China. Although nearly half a century has passed, moderate and small earthquakes in the source region have remained active, and the source region has remained continuously active for nearly 50 years. Such long-term seismic activity is extremely rare for continental intraplate earthquakes. The driving mechanism of long-term seismic activity and the rupture differences between the mainshock and subsequent earthquakes remain unresolved key scientific questions. Affected by sparse seismic networks, narrow instrument frequency bands and clipped near-field records, previous studies produced discrete focal mechanism parameters for the Tangshan mainshock. Existing research pays little attention to non-double-couple components （CLVD） and fails to reflect complex mechanical features of the source zone. Therefore, redetermining the high-precision focal mechanism and comparing moment tensor characteristics are of great significance for exploring seismic activity rules and assessing regional seismic hazard.

This study takes the Tangshan mainshock and earthquakes on the rupture zone as research objects, and adopts multi-source data and multiple methods for cross-constraint. We collected 148 P-wave first-motion polarity data of the mainshock and 567 high-precision focal mechanism solutions from digital seismic networks during 2002−2025. Three approaches were applied to determine the focal mechanism: P-wave first-motion inversion with the CHNYTX program, central solution calculation based on published results, and fuzzy clustering of nodal planes. We conducted moment tensor superposition and decomposition, and compared DC and CLVD components to analyze the evolution of rupture modes.

The results show that the nodal plane parameters from P-wave first-motion inversion are strike 222°, dip 81°, rake 179°, and strike 313°, dip 89°, rake 9°, with a misfit ratio of 0.13, indicating high reliability. Fuzzy clustering of 567 nodal planes identifies two dominant planes consistent with the mainshock fault. By integrating three sets of results, the optimal nodal plane parameters are determined as follows: nodal plane I: strike 219.01°, dip 80.57°, rake 174.12°; nodal plane II: strike 309.98°, dip 84.2°, rake 9.48°, reconfirming that the mainshock is a typical right-lateral strike-slip earthquake.

Both the mainshock and modern earthquakes are dominated by the DC component, with basically identical P-axis and T-axis orientations, matching the regional stress field controlled by Pacific Plate subduction. However, their CLVD components differ greatly. The mainshock shows near east-west compression plus extension in other directions, while recent earthquakes present near north-south extension plus compression in other directions, with a much higher CLVD proportion. The rupture mode of the rupture zone has changed obviously instead of simply inheriting the mainshock’s moment tensor release pattern.

In summary, this study acquires the high-precision focal mechanism of the Tangshan mainshock and clarifies the evolutionary differences of moment tensor and CLVD components between the mainshock and subsequent earthquakes. Continuous stress adjustment occurred in the source zone after the mainshock. The variation of CLVD components acts as a key indicator for the long-lasting seismic activity, providing a new quantitative basis for studying seismic activity and seismic hazard assessment in North China.