Abstract: The Zhongwei-Tongxin fault zone is one of the arcuate active fault zones in northeastern margin of Tibetan plateau. An earthquake of M =7 1/2 occurred on the middle segment of the fault zone in 1709 A.D. The structures of the fault zone are complicated and composed of a series of secondary faults. The fault zone can be divided into three segments according to the differences in the style of movement, strength and time of activity of the secondary faults. The study of the behavior of paleo-seismicity on the fault zone, therefore, is of great significance to better understanding of the segmented rupturing and the assessment of future seismic risk. The western segment of the fault zone is nearly E-W-trending with a total length of about 60km, most of which is covered by wind carried sand. The recent activity of the faults on this segment is displayed most distinctly at Xiaohongshan area. The middle segment trends NWW with a length of 55km. This segment is the most active one among the three segments of the fault zone. A series of streams, terraces, ridges and alluvial fans were sinistrally offset along the fault. The average left lateral-strike slip rate since Holocene is 3.58mm/a. The eastern segment changes from NW-trending to NNW-and S-N-trending, having a length of about 40km. This segment is located in the compressional area of Zhongwei-Tongxin left-lateral strike slip fault zone, where folding deformation is predominant and the fault is activated weakly. Seven trenches were excavated recently along the fault zone. Five of them are located on the middle segment and the rest on the western segment. Combining new data from the seven trenches and results obtained before, we discuss in this paper the recurrence behavior of paleoearthquakes on Zhongwei-Tongxin fault zone. The seven trenches have revealed six paleoearthquake events of the past 14 000 years along Zhongwei-Tongxin fault zone. One of them occurred in late Pleistocene and had ruptured the whole fault zone, while the others all occurred in Holocene and had ruptured only the middle or western segment. The 1709 Zhongwei earthquake of M =7 1/2 had ruptured only the middle segment of the fault zone. We postulate, therefore, that the magnitude of paleoearthquake that ruptured the middle or western segment of the fault zone should be about 7 1/2,while that of the event ruptured the whole fault zone should be about 8. In addition, we find that the temporal distribution of paleoearthquakes on the Zhongwei-Tongxin fault zone was neither uniform nor evidently clustered.

Key words: Zhongwei-Tongxin fault zone, Paleoearthquake, Recurrence interval

摘要： 通过对中卫-同心断裂带中段和西段7个新探槽的古地震研究,并结合前人对中段古地震的研究结果,分析确定出14000a以来中卫-同心断裂带共发生6次古地震事件。其中仅有1次是破裂全带的,发生在晚更新世末,其它都为全新世以来的次级破裂事件,3次破裂中段,两次破裂西段。公元1709年7 1/2级历史地震只破裂中段,因此推断只破裂中段或西段的古地震震级约为7 1/2级左右;破裂全带的古地震震级应为8级左右。从时间上看,这6次事件的分布是不均匀的,但没有明显的丛集现象。

关键词: 中卫-同心断裂带, 古地震, 重复间隔