Several earthquakes with M S≥5.0 occurred in the Datong seismic region in 1989, 1991 and 1999. The precise focus location of the earthquake sequence was made by the records of the remote sensing seismic station network in Datong. Using that data together with macro-intensity distribution and focal mechanism solutions, we analyze the difference among three subsequences. The results show that the focal fault of the 1999 M S5.6 earthquake was a NWW-trending left-lateral strike-slip fault. It is 16 km long and 12 km wide. It developed at the depth of 5 km and is nearly vertical in dip. The two previous earthquake subsequences, however, were generated by activity along NNE-trending right-lateral strike-slip fault. It can be found that the rupture direction of the 1999 earthquake has changed. It is generally found that a rupture zone has more than two directions and has different strength along these two directions. The complicate degree of focal circumstance is related to the type of earthquake sequences. There is the NE-trending Dawangcun fault and the NW-trending Tuanbu fault in the seismic region, but no proof indicates a connection between focal faults and these two tectonic faults. The feature that focal faults of three subsequences are strike-slip is different from that of the two tectonic faults. It is suggested that the 1999 earthquake subsequence was possibly generated by a new rupture.
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