A M5.6 earthquake occurred in Datong of Shanxi Province on Nov.1

1999. There was a group of small earthquakes that was located finely near the hypocenter 10 months before the event. The three-dimensional distribution of these earthquakes showed that there is a special seismic gap with NNW direction in the vertical profile. The occurrence of these gap-surrouding earthquakes showed clockwise order. The focal mechanism solutions of these earthquakes were obtained with point source dislocation model in layered medium. In inversion

the maximum amplitude ratio of and wave in vertical records observed was fitted to that of theoretical seismogram. Data recorded by Datong network and some stations from other networks were used. These focal mechanism solutions showed different property in the 4 quadrants. And the property in opposite quadrant was close. The M5.6 earthquake occurred on the edge of the seismic gap. The seismic gap was filled up by aftershocks. The direction of long axis for the aftershock area is same as the direction of the seismic gap.