Abstract: Fault-plane solutions of the mainshock and 17 large aftershocks are computed on the DJS-6 computer from the data of the first motions of P waves. Based on the model of finite moving source, the fracture processes and the source parameters of the mainshock and three largest aftershocks are estimated. The Mainshock occurred on a nearly vertical right-lateral strike-slip fault, striking N30°E, with an asymmetric bilateral fracture which propagated 70 km northeastward and 45 km southwestward with an average velocity of 2.7 km/sec. Source parameters of the mainshock such as the average dislocation, seismic moment and stress-drop are estimated to be 136 cm, 1.24×10²⁷ dyne, cm and 12 bars respectively.Most of the aftershocks with M_L>5.0 occurred near the main fault plane and along the fracture branches at both of its ends. These fracture branches are located in the dilatational quarters and make about 80 angles with the mam fault plane. On the two fracture branches, most of the major aftershocks are concentrated. A mechanical analysis of this phenomenon of fracture propagation is attempted in this paper.It is shown that for complex deformations, the fracture propagation no longer takes place along the original plane for brittle materials but deviates from it by a certain angle. The mechanical model used to calculate this deviation angle of fault yields results which agree essentially with the observation.Based on the above results, it can be inferred that the characteristics and mechanical conditions of the Tangshan earthquake as well as those of the Haicheng earthquake are quite different from those of the earthquakes occurring on some great faults which may be explained by stick-slip mechanisms. These two earthquakes are not only caused by the horizontal stress fields but also by local vertical forces induced from the movement of the underground materials. It is also worthy of note that Tangshan earthquake is different from Haicheng earthquake in that it occurs in a relatively homogeneous and brittle medium and therefore no foreshocks took place before the mainshock.