关键词: 夏垫断裂 高精度地形数据 垂直位错 地表破裂

Abstract: Beijing located at the North plain active tectonic block of China. The NW and NE orientation active faults widely distributed along this region. For instance, the Nankou-sunhe, Shunyi-liangxiang, and Xiadian faults. Historically, large earthquakes frequently occurred along this region, especially the converging section of these two pair of faults, for instance, the 1679 Sanhe-pinggu earthquake (M~8). Thus, it is necessary to quantitively study the faults' basic parameters, including the fault trace, slip distribution, and rupture behavior, to accurately assess the seismic hazard along this region. Xiadian active fault located at the eastern boundary of Beijing, near the Beijing Municipal Administrative Center. The 1679 Sanhe-pinggu earthquake (M~8) occurred along this fault. Previously, lots of studies have performed along this area, clearly revealed the bedrock geology, fault geometry, seismicity distribution as well as co-seismic deformation of this earthquake, which highly improve the understanding of this seismically active fault. However, although previous studies have focus on the surface rupture of 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 shaped distribution of slip has widely occurred along active faults, especially along those typical normal faults. Whether Xiadian fault's slip distribution is 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 1.0 m grid size DEM, to quantitively explore the surface rupture along the Xiadian fault. Detailed mapping and offset measurements revealed 5 left-stepping branches (~3 km), with total length of 12.3 km for the 1679 Sanhe-pinggu Earthquake. Slip distributions along the fault exhibit the arc-shaped geometry, and the maximum and average vertical separations are ~3.2 m and ~1.8 m respectively. This triangular shaped slip distribution has been also found along the 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 ~1.8 m and 1.7 m, respectively. Reasonably, the maximum ~3.2 m vertical offset possibly represents cumulative vertical offset of 2 earthquakes, including the 1679 Sanhe-pinggu earthquake. Based on the relationships among the surface rupture length, average displacements, as well as moment magnitudes, the calculated size is comparatively small. On account of the cutting shape of the 2 pair of faults, on 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 have been verified by the current focal mechanism solution.

Key words: Xiadian fault, high-resolution topographic data, vertical separation distribution, surface rupture