青海大柴旦断裂古地震特征

通过卫星影像解译发现，青海海西地区大柴旦东侧山前洪积扇上发育了一条影像特征清晰、连续性较好的活动断裂带，被称为大柴旦断裂。野外追踪调查确认该断裂为一条总体走向NNW(340°左右)，长度约125 km的活动断裂带。该断裂属柴达木盆地北缘断裂系的重要组成部分，晚第四纪构造活动具挤压逆冲特征兼右旋走滑分量。山前冲洪积扇保存有多级较小的断层陡坎、断层沟槽和冲沟右旋等断错地貌现象。利用差分GPS技术对断层陡坎进行测量，单条小陡坎的高度一般为0.5~1 m，多条陡坎组合累积高度为3~5 m。通过对2个天然古地震剖面和3个探槽剖面进行综合分析，最前缘的主控断层为逆冲断层，后缘表现为张性正断层的组合形式，断裂新构造活动具有逐渐向山前扩展的特征。根据样品14C测年结果大致确定了5次古地震事件，其中全新世以来有4次；古地震时间具有准周期性，其复发间隔约为2 000年。最近一次古地震事件的离逝时间是(1 935±60) a B.P.之后，说明大柴旦断裂离逝时间与复发间隔较接近，地震危险性比较高。

Paleoseismic Characteristics of Dachaidan Fault in Qinghai

By means of satellite image interpretation, a distinct continuous active fault zone, which is called Dachaidan fault, has been found on the diluvial fan in the east piedmont of Dachaidan of Haixi area, Qinghai. Field investigation confirms that the fault is a NNW-direction active fault (about 340°) with a length of about 125 km. This fault is an important part of fault system in the northern margin of Qaidam Basin; the fault in Late Quaternary shows extrusion and thrust with dextral strike-slip component tectonically. There are faulted landforms including multi-level small fault scarps, fault ditch and dextral gully preserved in the piedmont alluvial-diluvial fan. Differential GPS surveying results show that the height of a single small scarp is 0.5-1 m, and generally up to 3-5 m for a plurality of scarps. According to the comprehensive analysis of 2 natural paleoseismic profiles and 3 trench profiles, the main fault in the front is thrust fault with the performance of a group of tensional normal faults in the back. The latest tectonic activity suggests that the fault has gradually extended to the piedmont. 14C dating results indicate that 5 paleoseismic events are confirmed on Dachaidan fault, particularly 4 events since Holocene; occurrence time of paleoseismic events has the characteristics of quasi periodicity at the recurrence interval of about 2 000 years. The elapsed time for the recent paleoseismic event is (1 935+60) a B.P., which indicates that the elapsed time and recurrence interval of Dachaidan fault are close, resulting in high earthquake risk.