Abstract:

The Anqiu-Juxian Fault is the latest active fault in Tanlu fault zone, which is also the seismogenic fault of Tancheng M8.5 earthquake in 1668. In order to probe the shallow structure and the characteristics of faults in the eastern graben of Tanlu fault zone, we applied the high-resolution shallow seismic reflection method with multifold overlaying and stacking. In addition, we laid out two shallow seismic reflection lines across the Anqiu-Juxian Fault and the eastern graben of Tanlu fault zone. The shallow seismic profiles clearly reveal the stratigraphic interface morphology and shallow fault characteristics. The results show that the eastern graben of Tanlu fault zone is a graben basin consisting of multiple faults, and the thickness of Quaternary strata and graben structure characteristics are obviously affected and controlled by Changyi-Dadian Fault F₁ and Baifenzi-Fulaishan Fault F₂. Also, the eastern and western sides of the graben are the basement uplift areas, and the sediment thickness of the Quaternary strata in uplift areas is less than 30m. There are thick Cenozoic strata deposited in the barben, the stratigraphic morphology changes greatly laterally, showing an inclined form which is shallow in the west and deep in the east, and the Cenozoic strata are in angular unconformity contact with the overlying strata. The deepest part of Quaternary strata in the graben is located near the horizontal distance of 7400m, and its depth is about 190m. The Anqiu-Juxian Fault revealed by the shallow seismic reflection profile is composed of two branch faults dipping in opposite direction, which merge into one fault in the deep section. According to the discernible buried depth of the upper breakpoints of these faults and the characteristics of the Quaternary activity, the activity of Baifenzi-Fulaishan Fault on the western boundary of the eastern graben of Tanlu fault zone is relatively weak and the discernible depth of the upper breakpoint is 53m, we infer that the Baifenzi-Fulaishan Fault is a pre-Quaternary fault. The Changyi-Dadian Fault on the eastern boundary of the eastern graben of Tanlu fault zone not only cut the bedrock’s top interface, but also revealed signs of dislocation since Quaternary. The discernible depth of the upper breakpoint of Changyi-Dadian Fault is about 26~33m. The Anqiu-Juxian Fault is the latest active fault in the study area, which possess the characteristics of large scale and large penetration depth. The fault controls the deposition of the Cenozoic strata in the graben and plays an important role in the formation of the the eastern graben of Tanlu fault zone. The discernible depth of the upper breakpoint of Anqiu-Juxian Fault is about 17~22m. Therefore, we infer that the active ages of Changyi-Dadian Fault and Anqiu-Juxian Fault are the late Pleistocene and Holocene, respectively. The research results can provide seismological evidence for further understanding of activity mode and activity age of the seismogenic fault of the 1668 Tancheng M$8\frac{1}{2}$ earthquake, as well as the near-surface characteristics and activity of the Banquan segment of the Tanlu fault zone.

Key words: TanLu fault zone, Anqiu-Juxian Fault, Changyi-Dadian Fault, fault activity

摘要：

安丘-莒县断裂是郯庐断裂带内活动性最强的一条断裂, 也是1668年郯城$8\frac{1}{2}$级地震的发震断裂。跨郯庐断裂带东地堑和安丘-莒县断裂完成的浅层反射地震剖面清晰地揭示了地层界面形态和断裂的浅部特征。研究结果表明: 郯庐断裂带东地堑是一个由多条断裂组成的断陷盆地, 且第四纪地层厚度和堑状构造特征明显受到昌邑-大店断裂(F₁)和白芬子-浮来山断裂(F₂)的影响和控制; 地堑东、 西两侧为基底隆起区, 第四纪地层厚度<30m; 地堑内部沉积了较厚的新生代地层, 呈现西浅东深的倾斜形态, 与上覆地层呈角度不整合接触。安丘-莒县断裂由2条相向而倾的分支断层组成, 在剖面深部合并为1条断裂。从断裂可分辨的上断点埋深和第四纪活动特征来看, 郯庐断裂带东地堑的西边界——白芬子-浮来山断裂的活动性相对较弱, 为前第四纪断裂; 东边界昌邑-大店断裂和地堑内安丘-莒县断裂错断了基岩顶界面, 且第四纪以来存在错断迹象, 其活动时代分别为晚更新世和全新世。文中的研究结果可为进一步理解1668年郯城$8\frac{1}{2}$级地震发震断层的活动方式、 活动时代以及郯庐断裂带板泉段的近地表特征及其活动性提供地震学依据。

关键词: 郯庐断裂带, 安丘-莒县断裂, 昌邑-大店断裂, 断层活动性