郯庐断裂带中段最新活动断裂F5在淮河以南的活动特征研究

F5, the most recently active fault in eastern graben of the middle segment of Tanlu fault, has been accumulating seismic risks at Xiyi-Sihong segment. Prelimilary studies have shown that F5 had stretched southward to Huaihe River in Anhui Province. However, present studies could not confirm the region and extent to which F5 is confined. Researches aimed at revealing its activity in Quaternary are not so thorough. This project focuses on the most recently active F5 fault of middle segment of Tanlu fault located to the south of Huaihe River, With the following work to be conducted: (1) Map the geometric distribution and structural characteristics of the section of F5 fault south of Huaihe River, including remote sensing interpretation, field investigation and trench excavation for the Tanlu secondary fault on the eastern side of the Ziyang Mountains and the Tanlu Mingguang City to the Chihezhen section of Dingyuan County, south of the Nushan Lake. (2) Obtain the vertical slip rate and paleoseismic sequence in the south section of the Huaihe River, including fine height survey and time determination of the fault scarp in the section of F5 fault south of the Huaihe River, determine the vertical slip rate of the fault, select the key sections for special paleoseismic study, and establish the paleoseismic sequence. (3) Discuss the tectonic activity period and seismic risk of the section of F5 fault south of the Huaihe River, mainly based on the outcrops of the fault, the trench profiles and many other data, synthetically analyze the tectonic activity period of the fault, and discuss the paleoseismic recurrence behavior of the section based on the paleoseismic sequence of the F5 fault. The results of this study can provide a basis for understanding the seismic risk in the boundary area between Jiangsu and Anhui Province and for the overall activity characteristics of F5 fault.

F5为郯庐断裂带中段东地堑内最新活动断裂，多种资料表明其新沂-泗洪段（江苏段）发生强震的危险性逐渐增大。最新研究发现，F5新沂-泗洪段已延伸至淮河以南的安徽境内，但现有工作还无法确定F5在安徽境内的具体展布范围，已揭示段落的活动性研究也不深入。本课题以郯庐断裂带中段最新活动断裂F5淮河以南段为研究对象，主要开展以下几个方面的研究工作：（1）揭示F5淮河以南段的几何展布与结构特征，包括对紫阳山东侧郯庐次级断层、女山湖以南郯庐明光市至定远县池河镇段断层进行遥感解译、野外调查及探槽开挖。（2）获取F5淮河以南段的垂直滑动速率及古地震序列，包括对F5淮河以南段断层陡坎高度进行精细测量及时代的测定，确定断裂的垂直滑动速率；选取关键地段进行专题古地震研究，建立古地震序列。（3）讨论F5断裂淮河以南段的构造活动期次与地震危险性，主要基于断层露头剖面、探槽剖面及多方面资料，综合分析断裂的构造活动期次；基于F5淮河以南段揭示的古地震序列讨论其古地震复发行为特征。本课题研究成果可为认识苏皖交界地区地震危险性及F5断裂的整体活动特征提供依据。

郯庐断裂带中段最新活动断裂F5为中国大陆东部重要的地震活动断层，发生过1668年郯城8.5级地震。现有研究表明，F5断裂已延伸至淮河以南的安徽境内，但目前对其在淮河以南的具体展布范围及活动特征认识并不深入。本课题以郯庐断裂带淮河-定远池河镇段为研究对象，通过遥感解译、野外地质调查、探槽开挖、断错地貌测量及地层测年，明确了F5断裂在安徽境内的延伸范围和定量活动参数。结果表明，F5断裂发育于淮河至女山湖之间，段落长度约20km，断层迹线单一、连续，最新活动时代达全新世早期。郯庐断裂带明光南-定远池河镇段与紫阳山东侧断裂活动时代均为中更新世，不属于F5的延伸或组成部分。F5淮河-女山湖段结构特征复杂，或表现为单一断面，或表现为宽10余米的断层破碎带并具花状构造特征；断裂性质多变，剖面可见低角度逆冲断层也有高角度正断层，普遍断错晚更新世地层。该段晚更新世晚期以来存在至少3次古地震事件，形成年代依次为：（28.8±2.1）ka BP~20.36 ka BP，（15.7±2.0）ka BP~13.46 ka BP，（10.6±0.8）ka BP~7830a BP，断裂垂直滑动速率介于0.05-0.18mm/a之间，属于弱活动断层。综合古地震研究结果和其它学科资料分析认为，F5泗洪-明光段最近一次古地震离逝率已接近或超过1.0，现今小震不活跃，闭锁程度较高，易于应力积累，存在发生7级或7级以上强震的危险性。本研究成果丰富了对F5断裂整体活动特征的认识，为苏皖交界地区地震中长期预测及危险性评价提供了重要资料。