The latest deformation style along the North China block revealed by high-resolution topographic data: A case study on Xiadian Fault

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. © 2022, Editorial Office of Seismology and Geology. All right reserved.