大青山山前断裂全新世古地震的湖相沉积记录研究

The main purpose of paleoearthquake study is to reveal deformation evidence of large earthquakes recorded by geologic and geomorphic features beyond the historical record, and to apply it to large earthquakes prediction as the integrity of paleoearthquake record is crucial importance. Trenching exploration is not competent for paleoearthquake’s integrity because of its limitation on the selection of trenches’ location, the trenches’ length and the distinguishment of identification marks. This proposal focuses on lacustrine strata in Holocene of Hasuhai lake which is located along the Daqingshan piedmont fault. We will excavate exploratory pits in the lacustrine strata and measure grain size and magnetic susceptibility variation, recognize vertical distribution of soft sediment deformation about the lacustrine strata along with high resolution chronology measurement. We will identify periodic variation of grain size and magnetic susceptibility which is a sudden increase and slow decrease, eliminate the influence of climate change using the high resolution climate evolution history from previous studies, and get the paleoearthquake record in the grain size, magnetic susceptibility and soft sediment deformation which is complete paleoearthquake sequence of Daqingshan piedmont fault. This study is significant to predict seismic risk about Daqingshan piedmont area. It also provides complement to paleoearthquake study means, which can be used on study of paleoearthquakes with similar tectonic structure.

古地震研究是为了恢复历史地震记录以前地质地貌记录中的大地震事件，用来评估未来大地震发生的危险性，其完整性评价至关重要。常规采用的探槽方法中因探槽位置的选取，探槽长度的局限和识别标志的辨别等因素而难以保证古地震事件完整性。本项目选取位于大青山山前哈素海湖相地层为研究对象，在靠近大青山山前断裂下降盘湖相地层中开挖探坑，结合高分辨率的地层年代学测试，系统测定全新世以来粒度、磁化率随时间变化的曲线和软沉积物变形的垂直分布特征。对粒度和磁化率呈现出的突然增高然后缓慢降低的周期性变化进行识别提取，对照前人研究得出的呼包盆地高分辨率气候演变历史，剔除气候成因的影响，得到湖相沉积粒度、磁化率曲线以及软沉积物变形层对于古地震事件的记录序列，从而获得断裂全新世以来古地震序列。本项研究对于预测未来大青山山前断裂地震危险性具有重要意义，是传统古地震研究方法的补充，且对于构造类似的断裂古地震研究具有推广价值。

探槽作为古地震研究的传统方法，结合定年技术可以识别出古地震事件与地震重复间隔，但依然存在较大的不确定性和局限性。近些年学者尝试用其他方法揭露断层古地震信息取得很多进展。紧邻断层的湖相沉积物是否可以记录断裂活动的历史？哈素海紧邻内蒙古大青山山前断裂中段，全新世以来区域气候属于温带大陆半干旱性气候，受干扰小。全新世以来哈素海及周缘水动力条件弱，是开展湖泊沉积古地震研究的理想场所。利用哈素海周缘开挖的3个探槽采集的沉积物样品和年龄样品，通过沉积物粒度和磁化率变化特点，发现哈素海沉积物的沉积条件为风力与流水共同作用。依据沉积物粒度不同粒径组分和磁化率受地震事件扰动出现的变化特征，结合碳十四测年数据，并与前人通过探槽、河流裂点、古土壤等方法对大青山山前断裂中段识别出的古地震事件序列对比，我们识别出晚更新世晚期以来哈素海湖共记录7次古地震事件，其中1.6万年以来发生6次古地震事件，平均重复间隔2704年。利用湖泊沉积物变化特征，结合测年技术是研究古地震事件的一种有效的方法。