The geometrically complex segments of strike-slip faults affect the initiation, propagation, and the end of major earthquake rupture, ultimately determining the rupture length and final magnitude of major earthquake. The area from Ningnan to Qiaojia is the intersection area of the Zemuhe fault on the eastern boundary fault system of the Qinghai-Xizang Plateau. Although previous 1 ∶ 50, 000 active fault mapping work has been carried out on the three faults, but the understanding of the intersection relationship of the three faults was still insufficient, and the geometric distribution and motion characteristics of the faults in this region were not understood from the perspective of the fault system. This article conducts aerial photo interpretation, drone aerial survey, and detail field investigation on the intersection area of the three faults. Based on the clarification of the geometric distribution of the three faults, we conducted targeted field investigations of the target faults and obtained the movement properties of the three faults through field geological profiles, thereby analyzing the intersection relationships among them. It is identified that the northern section of the Xiaojiang fault has undergone deflection of about 25° north of Qiaojia, and the Ningnan basin forms the segmentation boundary between the Zemuhe fault and the Xiaojiang fault, with a width of about 4.5 km pull-apart basin. Meanwhile, the Daliangshan fault experiences a minor bend across the Jinsha River and gradually connects with the Xiaojiang fault near Qiaojia, with no apparent barrier between the two.
The property of the fault movement in this area is primarily left-lateral strike-slip with some normal faulting. The Ningnan basin and the bending structure on the Xiaojiang fault constitutes the Ningnan-Qiaojia pull-apart zone. From the perspective of seismic rupture segmentation, whether earthquakes can propagate through fault junction zones determines the final magnitude and destructiveness of earthquakes. This pull-apart zone could serve as an obstacle for the propagation of large earthquakes along the eastern boundary fault. However, the Dalingshan fault and the Xiaojiang fault intersect near Qiaojia, and there is no obvious geometric barrier between them, making it difficult to impede the propagation of large earthquake ruptures. This was evident in the intensity distribution map of the earthquake that occurred near Dongchuan on August 2, 1733. The surface rupture zone was distributed near Dongchuan but extended northward to Qiaojia without deviating toward Ningnan, indicating that the major earthquake near Dongchuan failed to propagate across the approximately 4.5 km wide Ningnan basin and the bend towards the Zemuhe fault. Instead, it propagated through the junction between the northern segment of the Xiaojiang fault and the Daliangshan fault. The Xiaojiang fault and the Zemuhe fault are obstructed by bend and pull-apart basins, which could serve as segment boundaries hindering the propagation of seismic ruptures, thereby impeding the spread of earthquake energy.
Through the study of the geometric distribution and kinematic characteristics of active faults in the intersection area, the segmental features of seismic rupture are analyzed, providing a basis for predicting the types and characteristics of strong earthquake disasters in cities along the fault line. The intersection area of these faults poses a significant risk of experiencing major earthquakes. This area encompasses three cities, Ningnan County, Qiaojia County and Dongchuan City. Ningnan County is primarily located within the Ningnan pull-apart basin, with active faults mainly distributed on both sides of the basin, and no apparent active faults traversing through the interior of the basin. In the event of a strong earthquake in this area in the future, attention should be paid to the amplification effect of thick-layered Quaternary deposits within the basin on strong ground motion, leading to building damage in Ningnan County, without the risk of fault displacement, nor the need to address active fault avoidance issues. The county town of Qiaojia and Dongchuan City are situated on an active fault, with many residential buildings and schools distributed along its course, necessitating active fault detection work. Additionally, the northern segment of the Xiaojiang fault has experienced severe landslide disasters, with multiple ancient landslides distributed in the area, among which the county town of Qiaojia is located on one such ancient landslide body. Therefore, in considering the avoidance of active faults, Qiaojia must also take into account the potential impact of landslide disasters. With the completion and impoundment of the Baihetan Hydropower Station, the county town of Qiaojia has undergone relocation and reconstruction, establishing multiple centralized residential resettlement communities primarily consisting of mid-rise and high-rise buildings. These resettlement communities are located relatively close to the reservoir, and any occurrence of landslide disasters would result in significant losses and increase the difficulty of rescue efforts.
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