RECENT TECTONIC MOVEMENT OF THE BABAOSHAN FAULT NEAR BEIJING AND ITS RELATION TO EARTHQUAKE OCCURRENCES

Basing on the geological framework, ground level deformation, underground water level changes and seismicity data, the character of the recent tectonic movement of the Ba Bao Shan Fault has been investigated. It is shown that during the time period from 1972 to the Tang Shan earthquake of July 1976, the type of tectonic movement of this fault was different for different intervals of time. Before September 1973, it was ten-sional and right lateral; after that time, the movement became compressional and left lateral and till the outbreak of the Tang Shan earthquake, there was accelerated movement along that fault for a short duration accompanied by variations of underground water level, starting from the southern portion towards the north in three deep wells drilled into bed rock on the western side of the fault.In June 1973, water level began to rise first in the Xie Xi Gang well to the south, and then in July and in October, the water level in the Da Hui Chang well and the Ban Qiao well to the north rose respectively. On the contrary, from April to July of 1975, obvious lowering of water level was observed in the wells. First, water level in the Xie Xi Gang well began to descend in April and then that of the Da Hui Chang well and Ban Qiao well also lowered in May and in July respectively.It was noted that while the water level of the Da Hui Chang well was lowering, there was also ground level deformation around that well.All such phenomena may be explained as the effect of the creep of the Ba Bao Shan Fault which was transmitting from the southwest to the northeast gradually. For the last more than ten years, small earthquake in the vicinity of the fault concentrated in a zone to the northwest of this fault and to the southwest of the northwest-trending Nan Kou-Sun He fault. It might be explained as the result of the barrier effect of the latter to the northeasterly creep movement of the Ba Bao Shan Fault.