A PRELIMINARY STUDY OF FOCAL DEPTH DISTRIBUTION IN THE BEIJING-TIANJIN-TANGSHAN AREA

When locating the earthquakes along the line Tangshan to Zhangjiakou in the Beijing-Tianjin-Tangshan area,we select the differences of arrival times at station-pairs to determine the position of the epicenter and use the differences in arrival times of seismic phases to determine depths of foci,seperately. This can raise the location accuracy. The earthquake sources of this area distribute between the depths of 5—22 km. This is in agreement with the estimated distribution,with depth,of crustal rock strength. So,we deduce that the crust here has the following characterisic: the upper part is fragile,and the lower part is plastic. The transition depth is about 20 km.