The 1976 Tangshan earthquake sequence that occurred on the northern margin of the North China semdimentary basin is one of the strongest and most destructive intracontinental earthquake sequences ever recorded by worldwide long-period seismograph networks. We have studied the source process of the six largest events of this earthquake sequence by formally inverting long-period P and SH wave seismograms from the World-Wide Standardized Seismographic Network. Our analysis shows that the main shock was caused by motion on at least three fault segments, each with a different orientation. The initial, and dominant, faulting occurred on two segments of a north-northeast trending right-lateral strike-slip fault system. The last stage of faulting had a significant component of thrusting and occurred on an east-northeast trending subsidiary fault at the southern end of the main fault. The largest aftershock occurred at the northeastern end of the main fault and had a nearly pure normal faulting mechanism with east-west striking nodal planes. This event occurred in a pull-apart region where the north-northeast trending main strike-slip fault system steps to the right. This region subsided by more than 1 m during the earthquake sequence. To the north of this step the main fault is delineated by numerous aftershocks whose mechanisms are by-and-large consistent with right-lateral slip on this segment. The large event in 1945 (M=6.3) probably occurred on this part of the fault. A region of substantial subsidence (up to 1.5 m) associated with the earthquake sequence was also observed in the south, where the main fault system takes another step to the right. Two of the large aftershocks occurred nearby and had strike-slip mechanisms. The overall faulting process of the Tangshan sequence is well characterized by right-lateral slip on a set of right-stepping faults.

The mechanisms and the points of nucleation of the strongest shocks, and the extent of the ruptures, appear to be closely tied to preexisting fault geometries. A compilation of the mechanisms of large earthquake sequences in the North China Basin strike-slip systems trending north-northeast. The basin seems to have formed and evolved as a large transtensional structure at an en echelon step of a large-scale strike-slip system. The deformation within the basin is both strike slip and extensional and appears to occur on smaller scales in an approximately self-similar fashion, as evidenced by the deformation during the Tangshan sequence. This interpretation provides and explanation for the variable rate of subsidence in both space and time and the scatter in measured heat flow values, as well as the overall shape of the North China Basin. It also provides an explanation for occasional thrust-faulting earthquakes that occur, apparently, as a result of geometrical complexities of the strike-slip faults (transpression). These observations are otherwise difficult to fit into a simple model of regional crustal extension and thermal subsidence. ¿ American Geophysical Union 1987