山西大同盆地口泉断裂全新世古地震活动

HOLOCENE PALEO-SEISMIC ACTIVITIES ON THE KOUQUAN FAULT ZONE, DATONG BASIN, SHANXI PROVINCE

野外调查表明 ,口泉断裂断错了断面附近的 3级地貌面 ,包括大同盆地西侧全新世形成的洪积扇后缘及位于洪积扇冲沟内的Ⅰ ,Ⅱ级阶地。其中冲沟内Ⅱ级阶地为剥蚀阶地 ,Ⅰ级阶地为堆积阶地 ,Ⅰ级阶地面的地层时代距今 2 52ka。在该断裂的悟道及上黄庄 2个地点开挖的大探槽表明 ,在距今 1 2 3万年以来该断裂曾发生 4次古地震事件 ,其中 3次分别发生在接近距今 2 52 ,5 6 8,13 73ka。另一次古地震事件发生在距今 6 76～ 10 82ka。这些数据有可能反映了口泉断裂具备准周期的强震活动。这 4次古地震事件的平均间隔约为 3 74ka ,最新一次古地震与上一次事件的时间间隔约为 3 16ka。 2个大探槽各次事件的平均最小同震垂直位移为 1 8m。这些资料对重新评价口泉断裂未来的地震潜势具有重要意义。

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 F 1～F 4 Faults can be determined from the vertical dragging of the dark gray clay layers on the trench profile. The F 5 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 F 5 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.