3D STRUCTURAL MODELLING OF THE ANNINGHE-ZEMUHE-XIAOJIANG FAULT ZONE IN THE EASTERN BOUNDARY OF SICHUAN-YUNNAN BLOCK USING MULTI-DATA AND IMPLICIT MODELING METHODS

doi:10.3969/j.issn.0253-4967.2024.01.002

Abstract

Abstract:

The Anninghe-Zemuhe-Xiaojiang fault zone is located at the intersection between the Qinghai-Xizang Plateau and the Yangtze block, representing the eastern boundary of the Sichuan-Yunnan block with frequent seismic activities. Its overall kinematic characteristics involve left-lateral strike-slip motion, and the fault structures along its strike are complex, posing significant challenges in accurately characterizing the 3D structural features of deep faults. The main issues include the structural complexity of the fault surfaces, uncertainties in the intersection relationships of fault systems, spatial constraints of blind faults, and the definition of fault surfaces in regions with weak seismic activity. Traditionally, 3D structural modeling for fault geometry heavily relies on high-resolution seismic reflection profiles, 3D seismic data volumes, and borehole data. It defines the geometric shapes of objects with limited nodes in a triangular mesh, and then simulates the topological structure of objects by connecting these nodes. However, obtaining high-resolution seismic reflection data in active tectonic areas like the eastern boundary of the Sichuan-Yunnan block is challenging, and even when available, it is often sparse in space. Alternatively, a large amount of relocated earthquakes and surface fault traces are generally used to create initial three-dimensional models of active faults. However, this approach overlooks the contributions of focal mechanism solutions in constraining the modeling, with more subjectivity in the selection of relocated seismicity, and does not adopt a differentiated weight strategy for various data sources. In this study, a 3D implicit modeling approach, combining deep and shallow geological and geophysical data that are generally available in active tectonic environments, was used to construct a detailed 3D structural model of the Anninghe-Zemuhe-Xiaojiang fault zone at the eastern boundary of the Sichuan-Yunnan block. The modeling process effectively integrated the fault plane constraints provided by focal mechanism solutions with surface fault traces and relocated seismic data, using a multi-iteration process with differentiated weight to increase the accuracy of the fault models. This approach ultimately represented the 3D complex structural features of the eastern boundary of the Sichuan-Yunnan block using multiple data sources. The modeling results show that the Anninghe-Zemuhe fault zone is characterized by a steep strike-slip fault structure with along-strike geometry variations. The Anninghe Fault shows its steepest dip angle in the central segment and gradually becomes gentler to both ends. Meanwhile, the Zemuhe Fault exhibits several asperities that are perpendicular to the direction of fault slip at a depth of 5~15km. By contrast, the north-to-central segment of the Xiaojiang fault zone is more complex. The western branch of the Xiaojiang Fault, which is an east-dipping, left-lateral strike-slip fault, is characterized by a relatively gentle fault plane with an average dip angle of 76° to 78°. The west-dipping segment of the eastern Xiaojiang Fault has a steeper dip with an average angle of 85°. The detailed 3D structural model of active faults constructed through implicit modeling can be used for analyzing fault roughness and fault system studies, which are crucial for understanding the distribution of asperities on fault planes and conducting seismic rupture simulations. Implementing the implicit modeling approach allows for the development of improved fault surface representations that can contribute to Community Fault Models in active tectonic environments, and support fault system modeling, rupture simulations, and regional hazard assessments.

Key words: active faults, implicit approach, 3D structural modeling, Anninghe-Zemuhe-Xiaojiang fault zone, Sichuan-Yunnan block

摘要：

安宁河-则木河-小江断裂带位于青藏高原与扬子地块相互拼接的交会部位, 代表了川滇菱形块体的东部边界, 地震活动频繁。其总体表现为左旋走滑的运动学特征, 沿走向断裂结构复杂, 为精细表征深部断裂的三维结构特征带来了很大挑战。目前存在的问题主要包括: 主断层面本身的结构复杂特性; 断裂交切与主次关系的不确定性; 隐伏断层的空间约束; 微震活动区域的断层面限定等。传统上, 三维构造建模主要依赖于高分辨率的地震反射剖面、三维地震数据体和钻孔等数据约束, 可以三角面网的形式在三维空间内使用数量有限的节点定义任意物体的几何形态, 进而连接节点模拟物体的拓扑结构。然而, 川滇块体东边界等大部分活动构造区域缺少这些高分辨率的数据, 即使有相关数据, 通常在空间上也非常稀疏。以往有研究利用大量精定位地震和断层地表迹线等数据建立了活动断裂的初始三维模型, 但这种方法忽视了震源机制解提供的节面信息对建模的贡献, 且未对多元数据采取差异化的权重分配策略。文中基于三维构造隐式建模方法, 结合不同约束条件下的深、浅地质与地球物理数据, 构建了川滇块体东边界安宁河-则木河-小江断裂带的三维精细构造模型。建模流程充分利用震源机制解提供的节面约束信息, 将其与地表断层迹线、重定位地震震中等数据融合, 纳入一个考虑差异化权重分配的多次迭代过程, 最终实现利用多元数据建立川滇块体东边界断裂三维复杂结构特性的目标。基于隐式建模方法构建的活动断裂的精细三维结构模型可用于断面粗糙度和断裂系统分析, 对约束断面上的凹凸体分布与开展地震破裂模拟具有重要意义。

关键词: 活动断裂, 隐式建模方法, 三维构造建模, 安宁河-则木河-小江断裂带, 川滇块体

WANG Mao-mao, HU Shun-yang, MA Hao-ran, LIANG Bo-yu, ZHANG Jin-yu, LU Ren-qi. 3D STRUCTURAL MODELLING OF THE ANNINGHE-ZEMUHE-XIAOJIANG FAULT ZONE IN THE EASTERN BOUNDARY OF SICHUAN-YUNNAN BLOCK USING MULTI-DATA AND IMPLICIT MODELING METHODS[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(1): 19-34.

王毛毛, 胡顺阳, 马皓然, 梁铂雨, 张金玉, 鲁人齐. 川滇块体东边界安宁河-则木河-小江断裂带的三维构造模型: 基于多元数据与隐式建模技术[J]. 地震地质, 2024, 46(1): 19-34.

Figures/Tables 10

Fig. 1 Structural outline map of the Anninghe-Zemuhe-Xiaojiang(AZX) fault zone in the eastern boundary of the Sichuan-Yunnan block.

Fig. 2 Implicit modeling approach for 3D structural modeling of active fault(modified from Plesch et al., 2020).

Fig. 3 Spatial distribution of data collected for constructing 3D models of active faults in this study illustrated by the SKUA-GoCAD modeling platform.

Table1 The type, source, and weight factor of multiple kinds of data used in modeling the Anninghe-Zemuhe-Xiaojiang fault zone in the eastern boundary of the Sichuan-Yunnan block

断层名称	数据类型	数据来源	权重分配
安宁河-则木河断层	断层迹线	中国地震断层信息系统^①	100
	震源机制解	中国地震科学实验场^②	10
	震源机制解	中国震源机制解数据库^③	10
	精定位地震	中国地震科学实验场^②	0.01
	远离断层点	远离断层(off-fault)点自设	1
小江断裂带中北段	断层迹线	中国地震断层信息系统^①	100
	震源机制解	中国地震科学实验场^②	10
	震源机制解	中国震源机制解数据库^③	10
	精定位地震	中国地震科学实验场^②	0.01
	精定位地震	Feng等(2022)	0.01
	远离断层点	远离断层(off-fault)点自设	1

Fig. 4 The technique route map used in this study(adapted from Riesner et al., 2017).

Fig. 5 Scalar field of tetrahedral mesh properties built for the Anninghe-Zemuhe fault zone.

Fig. 6 Initial fault surface of Anninghe-Zemuhe fault zone extracted from the tetrahedral grids.

Fig. 7 The two-stage model suppression for the Anninghe-Zemuhe fault belt by calculating the hypocenter-fault distances to select closer seismic events.

Fig. 8 Final three-dimensional structural model of the Anninghe-Zemuhe fault zone.

Fig. 9 Final three-dimensional structural model of north-central segments of the Xiaojiang fault zone.

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