Abstract:

Beijing locates in the North China active tectonic block, where the NW- and NE-trending active faults are widely distributed, such as the Nankou-Sunhe Fault, the Shunyi-Liangxiang Fault, and the Xiadian Fault. Historically, large earthquakes frequently occurred along these faults, especially in the intersection of these two sets of faults, e.g. the 1679 Sanhe-Pinggu earthquake(M~8). Thus, it is of great significance to quantitatively study the faults’ basic parameters, including the fault trace, slip distribution, and rupture behavior, for accurate assessment of seismic hazard of Beijing area.
The Xiadian active fault locates at the eastern boundary of Beijing, near the Beijing Municipal Administrative Center. The 1679 Sanhe-Pinggu earthquake(M~8)occurred on this fault. Previous studies on this area have revealed clearly the bedrock geology, fault geometry, seismicity distribution as well as co-seismic deformation of this earthquake, which greatly improves the understanding of the activity behavior of the Jiadian active fault.
However, the previous studies have focused on the surface rupture of the 1679 earthquake, the complete rupture geometry and slip distribution have not yet been constructed, due to the restriction of high-resolution topographic data. Furthermore, the triangular slip distribution has widely occurred along active faults, especially along the typical normal faults. Whether the fine slip distribution of Xiadian Fault conforms to the case or not is still unclear.
In order to explore all those issues above, using low-costing high-resolution(0.5m)satellite images, we derived 1.0m grid size DEM to quantitatively explore the surface rupture along the Xiadian Fault. Detailed mapping and offset measurements revealed 5 left-stepping branches(~3km), with a total length of 12.3km for the coseismic rupture of the 1679 Sanhe-Pinggu earthquake. Slip distributions along the fault exhibit the arc-shaped geometry, and the maximum and average vertical offsets are ~3.2m and ~1.8m, respectively.
Such triangular shaped slip distribution has also been found along other typical normal faults, for instance, the Wairarapa Fault in New Zealand, the Afar Fault in East Africa, and Owens Valley Fault in California. Modeling of these measurements revealed 2 earthquakes with co-seismic vertical offset of ~1.8m and 1.7m, respectively. Reasonably, the maximum ~3.2m vertical offset possibly represents the cumulative vertical offset of 2 earthquakes, including the 1679 Sanhe-Pinggu earthquake.
Based on the relationships among the surface rupture length, average offsets, as well as moment magnitudes, the calculated size is comparatively small. Based on the cutting shape of the 2 sets of faults and the upper crust imaging by shallow seismic reflection profile, we propose that the current right-lateral shear deformation of the fault is decoupled with the existing extensional structures, and this hypothesis has been verified by the current focal mechanism solution.

Key words: Xiadian Fault, high-resolution topographic data, vertical offsets, surface rupture

摘要：

文中利用高分辨率“吉林一号”卫星影像数据, 获得了夏垫断裂沿线高精度DEM数据(1.0m)。基于高精度地形数据对夏垫断裂开展了定量研究, 揭示出1679年三河-平谷地震地表破裂由5条大致呈左阶平行展布的、 长约3km的分支断层组成, 地表破裂的总长度约为12.3km。位错测量揭示出断裂垂直位移沿断层呈不对称三角形分布, 存在2个明显波峰。断裂沿线的平均位移值为1.8m, 约3.2m的最大位移值分布在潘各庄附近。位错模拟分析表明, 其可能代表了2次地震事件的累积位错。根据夏垫断裂上代表华北地区最新构造变形的地表破裂带相关定量参数估算的震级(约 M_W6.3 或 M_S6.25 ), 与1679年三河-平谷地震的历史震级(约M8.0或 M_S8.0 )存在较大差异。这可能与华北地区现今剪切走滑构造变形与早期分布于上地壳的断陷拉张构造在孕震层存在的解耦作用, 或华北地区巨厚的第四纪覆盖层对构造变形的吸收作用相关, 这种假设也被浅层地震反射剖面揭示的断层切割关系和现今走滑型的震源机制所印证。

关键词: 夏垫断裂, 高精度地形数据, 垂直位错, 地表破裂