关键词: 隐伏断裂, 断裂活动性, 第四纪, 上海

Abstract: In this paper, the Quarternary activity of two main faults in Shanghai has been studied by means of shallow seismic prospecting. The nine seismic reflection profiles crossing the Fengjing-Chuansha Fault have shown that the fault can be divided into three segments. The southwestern segment is a middle Pleistocene active fault. The middle segment is composed of two normal faults which make up a horst. The northeastern segment displays only the northern branch fault. The latter two segments were active during the late stage of early Pleistocene. The Taicang-Fengxian Fault is under the control of five seismic reflection profiles. The northern segment of the fault is a normal fault that was active during middle Pleistocene. The southern segment consists of two normal faults bounding a horst. The age of the latest activity of the fault is about 100ka B.P. The offsets of the upper reflector by the two faults are 2～5m in general. The average rate of vertical displacement is about 0.02mm/a for the Taicang-Fengxian Fault, and n ×10^-3mm/a for the Fengjing-Chuansha Fault. The characteristics of the activity of these two buried faults in Shanghai can be summarized as follows: (1) The NNW-, NE-, and E-W-trending faults are predominant over the fault structures of Shanghai. (2) The latest activity of the faults occurred generally in early-middle Pleistocene. (3) The age of the latest activity of the NNW-trending fault is younger than that of the other faults. The latest activity occurred generally in middle Pleistocene, but in some segments it might continue to late Pleistocene. (4) Normal fault is the only fault style in this area. The dislocation produced by the faults is 5～ 15m, and the maximum dislocation may reach up to 40～50m. It seems that the younger the age of activity, the smaller the dislocation is. (5) The Taicang-Fengxian Fault has the greatest average displacement rate, it is about 0.01～ 0.028mm/a.

Key words: Buried fault, Fault activity, Quaternary, Shanghai