COMPREHENSIVE STUDY OF THE CURRENT CONNECTION MODE OF A NORMAL FAULT STEPOVER: AN EXAMPLE OF THE CHANFANG STEPOVER ON THE KOUQUAN FAULT IN THE SHANXI RIFT SYSTEM, CHINA

doi:10.3969/j.issn.0253-4967.2024.04.005

Abstract

Abstract:

The overlapping area between the ends of adjacent fault segments is known as a fault stepover. The normal fault stepover has two endmember connection modes, i.e., soft-link mode and hard-link mode. The soft-link stepover's border faults are connected through a relay ramp, and the border faults' displacements are transmitted through the bending deformation of the relay ramp. The hard-link stepover's border faults are connected through a breaching fault, and the border faults' displacements are transmitted through the faulting deformation of the breaching fault. Distinguishing the current connection mode of a normal fault stepover can shed light on the evolution stage of the normal fault. It can also indicate the potential earthquake rupture pattern in the stepover, which is important for evaluating the seismic hazard of engineering sites within the stepover. The straightforward technique to distinguish the current connection mode of a normal fault stepover is to determine whether an active breaching fault exists within the stepover. However, in many cases, due to the small amount of accumulated offset and human modification of the breaching fault, it is always hard to observe fault scarps in the field even though the fault stepover is deforming under the hard-link mode.
The Shanxi Rift System is a prominent intracontinental rift zone in East Asia. It comprises a series of left-stepping en échelon grabens bounded by high-angle normal faults. It is distributed in an S-shaped geometry with a narrow, NNE-trending zone in the middle and two broad, NEE-trending extensional zones in the north and south. The Shanxi Rift System is one of the strong earthquake-prone regions in China. Since 780 BC, the Shanxi Rift System has hosted three M8 earthquakes, five M7-7娻 earthquakes, and a series of M6-7 earthquakes. The Kouquan fault is the western border fault of the Datong Basin in the northern part of the Shanxi Rift System. A stepover is developed near the Chanfang village on the Kouquan fault, which we named the Chanfang stepover.
In this study, we use a combination of the tectonic geomorphological investigation in the field, high-resolution topographic data analysis, and Ground Penetrating Radar(GPR)surveying to study the current connection mode of the Chanfang stepover. Three fault outcrops on the border faults of the Chanfang stepover are investigated. The outcrop D1 is in an alluvial fan covered by loess on the southwestern boundary fault of the Chanfang stepover. Two branch faults are present at this outcrop. One offsets a bedrock surface and the alluvial fan's gravel layer. The other is the boundary between a gravel layer and the loess, where imbricated gravel can be observed. The fault outcrop D2 is also on the southwestern boundary fault of the Chanfang stepover. The fault at the outcrop D2 offsets a gravel layer and the vertical offset of the top of the gravel layer is approximately 2m. The fault outcrop D3 is located on the northeastern boundary fault of the Chanfang stepover. At the outcrop D3, the fault separates the gneiss of the Archean Jining Group from the loess. Based on the Chinese GF-7 satellite stereo imagery, we obtain the high-resolution digital elevation model(DEM)covering the Chanfang stepover and identify two levels of geomorphic surfaces, i.e., T1 and T2. The surface T1 is an alluvial fan, mainly developed in the piedmont areas. The surface T2 is an erosion surface distributed in the bedrock mountain. To quantify the deformation pattern within the Chanfang stepover, we construct a series of topographic cross-sections on the surface T1 and find a gentle geomorphic scarp within the stepover. We conduct two GPR surveying lines across the Chanfang stepover. On the GPR images, we identify two known faults, F₁ and F₃, that previous researchers have mapped and a buried fault, F₂, that has not been constrained previously.
The Fault F₂ observed by the GPR is consistent with the geomorphic scarp constrained on the DEM, suggesting that a breaching fault exists in the Chanfang stepover. The existence of the Chanfang breaching fault indicates that the connection mode of the Chanfang stepover is the hard-link mode. We thus infer that the future earthquakes on the Chanfang stepover may cause concentrated surface ruptures on the breaching fault. This study shows that the combination of the tectonic geomorphologic investigation in the field, high-resolution topographic data analysis, and the GPR survey can effectively locate near-surface, slow, active normal faults. This comprehensive technique can be used for the connection mode of a normal fault stepover.

Key words: normal fault, stepover, link mode, Kouquan Fault, Shanxi Rift System

摘要：

正断层阶区有软连接(soft-link)和硬连接(hard-link)2种端元连接模式。判定正断层阶区现今的连接模式不但有助于理解正断层的生长演化过程, 也能指示未来正断层上发生的地震是否会在阶区内形成地表破裂, 这对于评价阶区内工程场地的地震安全性具有重要意义。判定正断层阶区内是否存在活动的连接断层是区别正断层阶区现今连接模式的直接方法。然而, 在多数情况下, 由于正断层阶区内的变形量相对小, 加上人类活动对阶区内地貌的改造, 导致在本以硬连接模式生长的正断层阶区内也很难直接看到断层陡坎, 这为判定正断层阶区内是否存在活动的连接断层带来了困难。山西裂谷系是东亚大陆显著的活动裂谷带, 也是中国重要的强震带。口泉断裂位于山西裂谷系北部, 是大同盆地的西缘边界断裂。口泉断裂在禅房村附近发育一个正断层阶区, 文中称之为禅房阶区。文中以口泉断裂禅房阶区为例, 通过构造地貌野外调查、地貌数据分析和地质雷达探测研究禅房阶区的连接模式。结果显示, 口泉断裂禅房阶区内部存在活动的连接断层, 说明禅房阶区的连接方式为硬连接模式。文中研究基于构造地貌野外调查、地貌数据分析和地质雷达探测的综合方法, 可有效定位弱活动断层的空间展布, 降低单一方法的不确定性。

关键词: 正断层, 阶区, 连接方式, 口泉断裂, 山西裂谷系

HUA Chun-yu, SU Peng, SHI Feng, XI Xi, GUO Zhao-wu. COMPREHENSIVE STUDY OF THE CURRENT CONNECTION MODE OF A NORMAL FAULT STEPOVER: AN EXAMPLE OF THE CHANFANG STEPOVER ON THE KOUQUAN FAULT IN THE SHANXI RIFT SYSTEM, CHINA[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(4): 837-855.

花春雨, 苏鹏, 石峰, 席茜, 郭钊吾. 正断层阶区现今连接模式综合研究——以山西裂谷系口泉断裂禅房阶区为例[J]. 地震地质, 2024, 46(4): 837-855.

Figures/Tables 10

Fig. 1 End-member connection models of normal fault stepovers (modified from Peacock et al., 1991, 1994; Larsen, 1998; Gürboğa, 2014).

Fig. 2 Regional tectonic geomorphology map of the Datong Basin.

Fig. 3 Maps of tectonic geomorphology, geology, and GPR survey lines of the Chanfang stepover.

Fig. 4 Fault outcrop in the Chanfang stepover.

Fig. 5 Using high resolution topographic data to analyze the tectonic geomorphology of the Chanfang stepover.

Fig. 6 Topographic cross sections in the relay ramp of the Chanfang stepover.

Fig. 7 Result of the Ground Penetrating Radar(GPR) survey line 1 and its tectonic interpretation.

Table1 Ground penetration radar(GPR) constrained the connection fault locations in the Chanfang stepover

测线编号	断层编号	空间地理坐标
		北纬	东经
GPR1	F₁	39°43'53.35″	112°50'53.32″
	F₂	39°43'46.84″	112°50'58.62″
	F₃	39°43'42.33″	112°51'3.62″
GPR2	F₁	39°43'51.48″	112°50'41.47″
	F₂	39°43'45.34″	112°50'45.72″
	F₃	39°43'41.59″	112°50'56.76″

Fig. 8 Result of the Ground Penetrating Radar(GPR) survey line 2 and its tectonic interpretation.

Fig. 9 Summarizing the study results of the Chanfang stepover and the stepover connection mode.

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