内蒙古色尔腾山山前断裂(乌句蒙口-东风村段)的断层活动与古地震事件

Active Fault and Paleoearthquakes of the Piedmont Fault (Wujumengkou-Dongfeng Village) for Seertang Mountains, Inner Mongolia

通过对色尔腾山山前断裂乌句蒙口 -东风村段的遥感资料解释、野外地质地貌考察 ,并通过对重点地段的古地震探槽开挖 ,获得了该断裂段晚更新世晚期以来的垂直位移速率是 0 88～ 1 83mm a ,全新世中期以来的垂直位移速率是 0 89mm a。通过 2个大型探槽的开挖、古地震事件分析和相关堆积物的断代研究 ,以及用逐次限定方法分析整个断层段上的古地震事件 ,认定该断裂段上全新世以来发生了 5次古地震事件 :事件 1发生在距今 90 0 0± 130 0年 ,事件2发生在距今 6 5 0 0± 5 0 0年 ,事件 3发生在距今 5 5 70年左右 ,事件 4发生在距今 4 2 0 0± 30 0年 ,事件 5发生在距今 32 5 0± 2 5 0年。晚更新世晚期到距今 1万年之间 ,古地震事件很不完整。全新世以来的 5次古地震事件表现出一定的丛集特征。最早的一丛事件发生在距今 890 0年左右 ,第2丛发生在距今 6 5 0 0～ 5 70 0年之间 ,第 3丛事件发生在距今 32 5 0～ 4 2 0 0年之间。第 1丛与第 2丛古地震事件之间间隔为 2 4 0 0年左右 ,而第 2丛与第 3丛古地震事件之间仅间隔 15 70年左右。距今 32 5 0年以来 ,该断裂段上还没有发生过错断地表的地震事件 ,已经超出了古地震丛之间的重复间隔。因此 ,它是色尔腾山前活动断裂带上具备潜在危险的一个活动断裂段。

The remote sensing data, field investigation and trench measurement along the fault segment (Wujumengkou-Dongfeng village) on piedmont fault of Seertang mountains, which is one of the most active fault surrounding the Ordos block have been explained. The results show that perpendicular displacement is about 42m, and the rate is 0.88～1.83mm/a since late period of late Pleistocene. The height of the fault scarps on the first terrace is about 6.25m, and the rate is 0.89mm/a since middle period of Holocene. We have excavated two large trenches across fault scarps on deferent points, mapping the geological profiles maps in the two trenches, analyzing paleoearthquake events using the relation between faults and stratums, and dating some accumulation samples for related to fault activity. We use progressive constraining method in paleoseismic study to analyse paleoearthquake events along the fault segment on the piedmont fault of Seerteng mountains. The results show that five paleoearthqukes have occurred on the fault segment of Wujumengkou-Dongfeng village since Holocene. The first paleoearthquake happened about 8900±1900 a B.P., the second event was about 6500±500a B.P., the third paleoearthquake occurred on about 5770a B.P., the fourth events took place on about 4200±300 a B.P., the last one was about 3250±250a,B.P. The late period of late Pleistocene to early period of Holocene, about 38ka to 11ka B.P., many paleoearthquakes may be missed as some reason. For all paleoearthquke events, they have displayed some cluster grouping characteristic on a certain extent. The first cluster occurred at the 8900a B.P., the second cluster was 5700～6500a B.P., the last cluster too place at 3250～4200a B.P. The interval between the first cluster to second cluster is about 2400 a, but the interval of second cluster to third cluster is only 1570a. The eathquake event which cut the ground surface do not take place since 3250a B.P., on the active fault segment, lapse time is more than the repeated interval between two paleoearthquake clusters. Therefore, this active fault segment has potential seismic risk.