郯庐断裂带安丘-莒县断裂江苏段古地震研究新进展

doi:10.3969/j.issn.0253-4967.2023.04.005

摘要/Abstract

摘要：

郯庐断裂带是中国东部活动性最强的断裂带, 郯庐断裂带江苏段主要由5条分支断层组成, 并于更新世强烈活动, 其中安丘-莒县断裂持续活动至全新世, 是1668年郯城8½级特大地震的发震断裂。文中采用古地震探槽方法研究安丘-莒县断裂江苏段全新世以来的古地震事件, 并采用¹⁴C测年方法确定古地震的时间。结合前人通过探槽揭露的古地震时间进行综合分析, 认为安丘-莒县断裂江苏段全新世以来共有3次古地震事件, 时间分别为距今3 00a以来、距今约6 000a和1 1000a, 垂直同震位移均约1m。1668年郯城8.5级地震在安丘-莒县断裂新沂段山前出露区存在地表破裂的迹象, 在隐伏区表现为大量喷砂冒水现象, 在探槽揭露的晚全新世地层中有密集的裂缝和砂脉。

关键词: 安丘-莒县断裂江苏段, 古地震, 地表破裂

Abstract:

Paleoseismology is a discipline that studies prehistoric earthquakes or earthquakes that occurred before instrumental records using geological and geomorphological methods, mainly by trench excavation and Quaternary chronology. It focuses on the time and intensity distribution of large earthquakes, to reveal the recurrence characteristics of large earthquakes and provide basic data for evaluating the probability of future earthquakes. The Tanlu fault zone is the most active fault zone in eastern China. The Jiangsu section of the Tanlu fault zone is mainly composed of five branch faults, which are strongly active in the Pleistocene. Among them, the Anqiu-Juxian Fault continued to be active until the Holocene, which is the seismogenic fault of the Tancheng 8½ earthquake in 1668. The Xinyi-Sihong section is likely to generate strong earthquakes in the future in the south-central section of the Tanlu fault zone.

The total length of the Jiangsu section of the Anqiu-Juxian Fault is about 170km, with an overall strike of 5°~15°, extending southwards from the north Maling Mountain to the Chonggang Mountain. The geomorphic features are distributed from north to south by the alternation of the bedrock mountain and the sedimentary basin. The Anqiu-Juxian Fault shows a single exposed fault on one side of the bedrock mountain, extending to the basin into two branches in the east and west, of which the east branch is the active late Pleistocene Fault and the west branch is the Holocene active fault. The Jiangsu section of the Anqiu-Juxian Fault is dominated by dextral strike-slip and has both dip and thrust components.

Lots of research have been done on the ancient earthquakes of the Anqiu-Juxian Fault. The trenches are mostly located in Maling Mountain, Zhangshan Mountain and Chonggang Mountain, which are in the state of uplift and denudation. The Holocene is very thin, and the dating method is mostly optical luminescence. The identification of ancient earthquake events is less since the Holocene, with the accuracy of ancient earthquake time not high and the ancient earthquake sequence not complete. According to the topographic and geomorphological characteristics of the Jiangsu section of the Anqiu-Juxian Fault, three trenches were excavated along the Anqiu-Juxian Fault, of which two were in exposed areas and one in a buried area. Three trenches completely revealed the Holocene sedimentary strata in the Jiangsu section of the Anqiu-Juxian Fault, in which MLTC2 revealed the early Holocene strata, MLTC1 revealed the middle Holocene strata, and HSTC revealed the late Holocene strata. The determination of the age of earthquake events is one of the most uncertain factors in the study of paleoearthquakes and is the main indicator of the recurrence period of paleoearthquakes. At present, most of the paleoearthquake events studied have occurred since the late Pleistocene, and the accuracy of ¹⁴C dating is the highest. A total of 13 ¹⁴C samples were collected from the trenches. Combined with the paleoearthquake events and time revealed by previous trenches, it is concluded that there have been three paleoearthquake events in the Jiangsu section of the Anqiu-Juxian Fault since the Holocene, with theelapsed time of ~3000aBP, ~6000aBP and ~11000aBP, and the coseismic vertical offset are all nearly 1m.

The 1668 Tancheng M8½ earthquake showed signs of surface ruptures in the exposed area of the Xinyi section of the Anqiu-Juxian Fault, accompanied by a large amount of sandblasting and water gushing in the buried area. Dense fissures and sand veins are observed in the late Holocene strata overlying the fault, indicating the impact of the 1668 Tancheng earthquake. More representative chronological data are needed as to whether the 1668 Tancheng earthquake ruptured Suqian City.

Key words: Jiangsu segment of Anqiu-Juxian Fault, Paleoearthquake, Surface rupture

张浩, 李丽梅, 蒋新, 章东, 许汉刚. 郯庐断裂带安丘-莒县断裂江苏段古地震研究新进展[J]. 地震地质, 2023, 45(4): 880-895.

ZHANG Hao, LI Li-mei, JIANG Xin, ZHANG Dong, XU Han-gang. NEW PROGRESS IN PALEOEARTHQUAKE STUDIES OF THE JIANGSU SEGMENT OF THE ANQIU-JUXIAN FAULT IN THE TANLU FAULT ZONE[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(4): 880-895.

图/表 11

图 1 安丘-莒县断裂江苏段的构造分布图 F1昌邑-大店断裂; F2白粉子-浮来山断裂; F3沂水-汤头断裂; F4鄌郚-葛沟断裂; F5安丘-莒县断裂; F6蒙山-山前断裂; F7苍山-尼山断裂。图b中的虚线为第四系等厚线(厚度单位为m)

Fig. 1 Tectonic distribution of the Jiangsu segment at Anqiu-Juxian Fault.

表 1 14C测年结果

Table 1 Ages of the 14C samples

采样剖面	样品编号	实验室编号	样品类别	测试方法	校准年龄
MLTC1	¹⁴C-1	504607	炭屑	AMS	(6770±30)a BP
MLTC1	¹⁴C-2	504608	炭屑	AMS	(7040±30)a BP
MLTC1	¹⁴C-3	504609	炭屑	AMS	(6960±40)a BP
MLTC1	¹⁴C-4	504610	炭屑	AMS	(5950±30)a BP
MLTC1	¹⁴C-5	504611	炭屑	AMS	(6880±30)a BP
MLTC1	¹⁴C-6	504616	炭屑	AMS	(6840±30)a BP
MLTC1	¹⁴C-7	504618	炭屑	AMS	(6770±30)a BP
MLTC1	¹⁴C-8	504617	炭屑	AMS	(6900±60)a BP
MLTC2	¹⁴C-1	504619	炭屑	AMS	(9640±40)a BP
MLTC2	¹⁴C-2	504620	炭屑	AMS	(10140±40)a BP
MLTC2	¹⁴C-3	504621	炭屑	AMS	(670±30)a BP
MLTC2	¹⁴C-4	504622	炭屑	AMS	(70±30)a BP
HSTC	HSTC	515330	黑灰色黏土	AMS	(2790±30)a BP

图 2 MLTC1探槽的地理位置

Fig. 2 Geographical location map of MLTC1.

图 3 MLTC1剖面及解释图 U1 红褐色-黑褐色基岩弱风化层, 断层面附近分布断层泥, 宽约40cm, 断层泥墙面比较光滑, 夹破碎石块, 棱角状, 夹透镜体, 片理化, 粉末化, 定向分布, 下部岩石的变质作用更加强烈。U2 棕红色基岩强风化层, 夹砾石, 砾径最大约10cm×10cm。U3 灰黄色、红褐色黏土, 夹砾石。U4 灰黄色粉砂质黏土, 硬塑, 较光滑。U5 灰色含砾中粗砂夹粉砂质黏土, 含砾中粗砂以透镜体状分布。U6 灰黄色砂质黏土, 呈楔状分布。U7 灰黄色粉砂质黏土夹透镜体状中粗砂。U8 黄灰色含砾砂质黏土, 含植物根茎

Fig. 3 The MLTC1 trench and its interpretation.

图 4 MLTC1探槽南壁的重点部位图 ①黏土层; ②粗砂砾石层

Fig. 4 The main point of MLTC1 trench.

图 5 MLTC2探槽的地理位置

Fig. 5 Geographical location map of MLTC2.

图 6 MLTC2剖面及解释图 U1 紫红色王氏组砂岩, 完整性较好, 透水性较好。U2 粗砂砾石, 砾石含量约40%, 棱角状, 分选差, 砾径最大约10cm×5cm, 洪积层。U3 黄灰色粉砂质黏土夹灰褐色含砾中粗砂, 含砾中粗砂以透镜体状分布, 砾石直径为1~2cm, 个别达4~6cm, 呈水平薄层状分布, 在f3东侧呈倾斜状分布, 倾向E, 角度为50°, 宽达1.0~2.0m。U4 粉砂质黏土, 灰黄色, 墙面较光滑, 见水平层理, 夹少量薄层状透镜体, 成分为花岗岩风化产物, 风干后见裂纹。U5 粉砂质黏土, 黏土含量较高, 较光滑, 中细砂以透镜体状分布。U6 耕植层, 黑灰色, 较光滑, 干裂缝发育, 见植物根系

Fig. 6 The MLTC2 trench and its interpretation.

图 7 黄沭路钻孔联合地质剖面钻孔名下方数值为孔深

Fig. 7 Combined borehole segment of Huangshu Road.

图 8 HSTC北壁剖面及解释图 U1 灰黄色黏质粉砂, 夹黄色黏土块。U2 黑灰色细砂, 夹黄色黏土块, 偶含砾石。U3 灰黑色黏土, 切面光滑, 黏性较大。U4 棕灰色黏土。U5 浅棕黄色砂质黏土, 上部黏土含量较高, 底部灰黄色粉细砂含量较高。U6 棕灰色黏土, 切面光滑。U7 黄灰色粉砂质黏土, 粉砂含量约20%, 可塑, 黑色铁锰结核呈斑状密集分布, 斑径约5mm。U8 灰黄色粉砂质黏土, 粉砂含量约40%, 黑色铁锰结核呈斑状密集分布, 斑径约5mm, 锈黄色粉砂质黏土呈斑状分布。U9 浅棕色粉砂质黏土, 粉砂含量约10%, 黑色铁锰结核呈斑点状分布, 斑径<3mm。U10 深灰色黏土, 切面光滑, 偶夹砾石, 砾径<5mm。U11 回填土

Fig. 8 The northern wall of the HSTC trench and its interpretation.

图 9 典型砂脉分布特征

Fig. 9 Distribution characteristics of the typical sand vein.

图 10 MLTC2地表破裂

Fig. 10 The surface ruptures of MLTC2.

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