关键词: 阿尔金断裂, 断层分段, 地表破裂, 断错地貌

Abstract: Identifying correctly the segmentation of an active fault is very important for the evaluation of earthquake hazard on this fault. In a fault system, earthquake rupture occur on different segments of the fault, resulting in the segmentation of the fault system. The segmentation of a fault, therefore, may reflect the propagation process of earthquake rupture, and may provide an insight into the development process and seismic behavior of the fault. The segmentation of a fault system can be identified through different methods, among which the most practical one is to identify the segments by using surface ruptures, which is the direct product of earthquake faulting. This paper deals mainly with the segmentation of the surface rupture on the eastern segment of the Altyn Tagh Fault zone. The Altyn Tagh Fault is one of the major east west trending strike slip faults in China. It is the northern boundary of the Qinghai Tibet Plateau, and its linear character is very conspicuous on satellite images. Basing on the offset landforms observed during fieldwork and spot images analysis of the eastern Altyn Tagh Fault, we divide the eastern Altyn Tagh Fault into three segments from west to east according to its surface ruptures: (1) Qingyazi-Lucaowan segment. The extension of the fault on this segment is simple and its sinistral strike slip motion can be recognized from the offsets of streams, alluvial fans and terrace ridges. Here the youngest terraces of 5.24?0.40ka B.P. in age are offset by the fault, indicating that the latest earthquake on the segment is younger than 5.24?0.40ka B.P. (2) Lucaowan-northern Qilianshan segment. The length of this segment is about 250km. By analyzing the offset landforms and combining with the previous results of paleoearthquake research, we find that the youngest terraces here are not offset by the fault, and we conclude that the latest earthquake on the segment occurred at about 7.080?0.57ka B.P. (3) East of the north Qilianshan segment. The displacement of the fault on this segment is much smaller than those on the other segments, and the fault trace disappears gradually eastward. Here the youngest terraces are offset by the fault, and we estimate that the age of the last earthquake on this segment is younger than 5ka B.P.

Key words: Altyn Tagh Fault, Segmentation, Surface rupture, Offset landform