关键词: 大探槽, 古地震, 全新世, 口泉断裂, 山西

Abstract: The NNE-trending Kouquan Fault Zone is located on the western side of the Datong Basin, Shanxi Province. It is one of the three major boundary faults controlling Cenozoic sedimentation of the Datong Basin. The Huairen depression, where the greatest thickness of Cenozoic sediments may reach up to 1,800m, is located on the eastern side of the fault (see Fig. 1 in text). It is indicated by field investigation that a segment as long as 120km of the Kouquan Fault zone has been active in Holocene, and that three levels of morphologic surfaces formed in Holocene have been offset by this active segment. The highest one among the three surfaces is the back edge of a pluvial fan located on the western side of the Datong Basin. The upper layer of this pluvial fan consists of mid Holocene deposits, dated as 5ka BP. The other two lower surfaces are the 1st terrace and the 2nd terrace embedded into the pluvial fan. Among them, the 2nd terrace is a denudation terrace with a height of 8m; while the 1st terrace is accumulated with a height of 1.5～4m. The dating data show that the near surface layer of the 1st terrace is of 2.52ka BP in age (See Fig.2 & 3 in the text). Moreover, field investigation also shows that the Kouquan Fault has had obvious right lateral strike-slip component in Holocene as more than ten east west directed gullies were offset right laterally for 8～20m when crossing the fault trace. Two exploratory trenches were dug during our investigation on the northern and southern segments of the Kouquan Fault, respectively. Their locations were chosen right beneath the fault plane where the fault offsets the 1st terrace of the gully (see Fig. 4 & 6 in the text). The two trenches are of 40km apart. Their dimensions are 11m in length with a depth of 4～5.5m and 22m in length with a depth of 3.2～8.8m, respectively. The two trenches reveal a line of evidence of the Holocene activities of the Kouquan Fault. The Wudao trench has revealed 3 fault planes, and in the middle part of the profile a drag structure in dark brown clay layer has been observed, appearing as the ascending of the western side and the descending of the eastern side (Fig. 5). The Shanghuangzhuang trench reveals a fault bundle of 13m in width that consists of 5 groups of faults. A vertical displacement of 2.9m for the two sides of F₁～F₄ Faults can be determined from the vertical dragging of the dark gray clay layers on the trench profile. The F₅ Fault at the easternmost end of the profile consists of two vertical fault planes about 0.8m apart. No vertical displacement of clay layers on both sides of F₅ is observed, but the layers between the two fault planes have nearly horizontal layering and show stratum discontinuity on both sides of the fault, indicating a strike-slip activity of the fault. Moreover, the shanghuangzhuang trench log shows multiple yellow sand soil veins in the vicinity of the fault planes (Fig. 7). The time sequence of the paleo-seismic activities revealed by the Wudao and the Shanghuangzhuang trench logs might correspond to each other. That is to say that there have been 4 paleo-earthquake events since 13.7ka BP. Three of them occurred about 2.25, 5.68, and 13.73ka BP, respectively. The rest event occurred between 6.76ka and 10.82ka. The average recurrence time of these 4 events would be 3,737yr, and the interval between the latest and its previous one is 3,160yr. The average minimum co seismic vertical displacement of the 4 events reaches 1.8m. These new data are of great significance to earthquake safety assessment of the Datong area.

Key words: paleo-earthquake, trenching, Holocene, Kouquan Fault, right-lateral strike-slip with normal faulting