Abstract:

The complex stress field and distinct internal tectonic framework of North China contribute to the frequent occurrence of strong earthquakes, particularly around the Ordos Basin, the Bohai Bay region, and the North China Plain. The generation and spatial distribution of these earthquakes are closely associated with the geometric structure and dynamic behavior of active tectonic blocks. Therefore, understanding the motion and deformation of these blocks is crucial for assessing the timing, location, and intensity of seismic events in the region. Current research on the deformation characteristics and mechanisms behind strong earthquakes in North China mainly focuses on kinematic methods such as fault slip rate and GNSS velocity field inversion. However, the dynamic mechanism underlying these kinematic characteristics remain debated. Moreover, while many active blocks exist in North China, the interaction and dynamic effect among them have received limited attention.

This study integrates active block division data, GNSS velocity fields, and the spatiotemporal distribution of strong earthquakes to construct a three-dimensional finite element model covering North China and adjacent regions. Using this model, we simulate the regional stress and strain fields under varying block division schemes(primary, secondary, and tertiary levels)and assess the influence of tectonic activity along the Haiyuan, Liupanshan, and Longmenshan fault zones on block motion and deformation. The aim is to explore how the geometric configuration and hierarchical structure of active blocks affect the tectonic evolution of North China, thereby providing insight into the mechanisms of strong earthquakes in the region.

The main findings are as follows:

(1)As the number and resolution of active blocks increase, the motion and rotation rates of the Ordos Block, Taihang Mountain Sub-block, Jilu-Yuwan Sub-block, and Ludong-Huanghai block all show upward trends. The simulation results under the three-level block division scheme align best with current observational data. The contribution of secondary blocks to deformation is approximately three times that of tertiary blocks. This suggests that primary and secondary blocks, along with their boundaries, play dominant roles in the current tectonic pattern of North China, while tertiary structures contribute less significantly.

(2)The collision between the Indian and Eurasian plates, along with the subduction of the Pacific and Philippine plates, exerts shear forces on North China from the south and north, resulting in a regional counterclockwise rotational pattern. The rotation rates of the Ordos, Taihang, Jilu-Yuwan, and Ludong-Huanghai blocks(referenced to the South China Block)are estimated at 2.3, 2.2, 2.0, and 3.4 nanoradians/year, respectively. These differences arise from two main factors: the heterogeneous distribution of the regional stress field and the compressive and extensional effects at block boundaries due to block interactions. The resulting uncoordinated block deformation leads to stress concentration and strain along fault zones, both within North China and at its margins, potentially triggering strong seismic events.

(3)Since the late Miocene, tectonic extrusion from the eastern margin of the Tibetan plateau has influenced the southwestern Ordos region and the South China block through the left-lateral strike-slip motion of the Haiyuan fault, the thrusting of the Liupanshan fault zone, and deformation along the Longmenshan fault zone. The Haiyuan and Liupanshan faults directly enhance stress accumulation along the southwestern margin of the Ordos block, promoting deformation and movement in North China. Meanwhile, the Longmenshan thrusting enhances stress within the western South China block, facilitating its eastward motion. This movement establishes a left-lateral shear zone between the South China and Amur blocks, intensifying tectonic activity in the relatively weak North China region and indirectly driving block deformation across the area.

Key words: North China, active blocks, stress field, earthquake, numerical simulation

摘要：

华北地区强震活动频繁, 主要集中分布在鄂尔多斯盆地周缘、 渤海湾盆地及华北平原内部等构造活跃区域, 其强震的空间分布格局与该地区活动地块的几何结构及其动力学行为密切相关。文中综合了华北地区活动地块划分方案、 GNSS速度场及强震时空分布等数据, 构建了华北及其周边地区的三维数值模型, 采用有限元方法模拟了华北地区在一级、 二级、 三级活动地块划分方案下的构造应力场和应变场, 并探讨了华北地区西部海原断裂带、 六盘山断裂带、 龙门山断裂带的活动对华北地区构造变形的远程效应。模拟结果显示, 华北地区现今的地块构造格局主要受一级、 二级活动地块及其边界带的控制, 相较之下, 三级活动地块及其边界带对华北地区构造变形的贡献相对有限。印度板块与欧亚板块的碰撞作用及太平洋板块和菲律宾板块的俯冲作用, 在华北地区南部和北部形成了一对剪切力偶, 使得华北地区整体呈现逆时针旋转的运动特征, 但华北地区内部的鄂尔多斯地块、 太行山次级地块、 冀鲁-豫皖次级地块及鲁东-黄海地块的逆时针旋转程度不同, 以稳定的华南地块为参考系, 这些地块的旋转速率分别为 2.3×10^-9rad/a、 2.2×10^-9rad/a、 2.0×10^-9rad/a、 3.4×10^-9rad/a。华北地区内部活动地块的差异性运动主要受控于2个关键因素: 一方面是华北地区内部应力场在空间上的非均匀性分布, 另一方面是活动地块在运动变形过程中引起的地块边界处的挤压和拉张效应。华北地区内部活动地块间的不协调变形运动, 导致华北地区内部活动地块间及其与周缘地块间的边界带产生了不同程度的应力集中和变形特征, 形成了强震孕育的重要构造背景。此外, 海原断裂带的左旋走滑运动、 六盘山断裂带和龙门山断裂带的逆冲推覆作用通过作用于鄂尔多斯地块西南缘或华南地块, 直接或间接地影响了华北地区活动地块的运动变形, 其中海原断裂带的走滑运动和六盘山断裂带的逆冲推覆作用对华北地区构造变形的影响尤为显著。

关键词: 华北地区, 活动地块, 应力场, 地震, 数值模拟