关键词: 晚更新世, 古地震, 三危山断裂, 敦煌

Abstract: The Sanweishan fault is located in the front of the northwest growth of the northern margin of the Tibetan Plateau. It belongs to the branch fault of the Altyn Tagh fault which extends to the northwest. The latest seismic activity of the Sanweishan fault reflects the tectonic deformation characteristics of the northern plateau. Meanwhile, it is of great significance for the seismic risk assessment of Dunhuang and its adjacent areas to understand the characteristics of earthquake recurrence. The sanweishan fault runs along the western piedmont of the Sanwei Shan, with a total length of 175 km. Based on the geometry, the fault can be divided into three segments, i.e. Shuangta-Shigongkouzi, Shigongkouzi - Shugouzi and Shugouzi-Xishuigou from east to west. Previous studies about the paleoearthquakes on the Sanweishan fault mainly focus on the middle and East segments of the fault, while the west segment of the fault has not been studied. Meanwhile, the available research does not involve the recurrence characteristics and possible magnitude of the paleoearthquakes. Based on high-resolution satellite images, we found that the main fault has extended to the basin in the western segment of the Sanweishan fault. Based on trench excavation and chronological study on the latest fault scarps, this paper determined the sequence of fault paleoseismic events, and discussed the recurrence characteristics and possible magnitude for the Sanweishan fault. In the western segment of the fault, through satellite image interpretation and field investigation, we found new fault scarps distributed on the alluvial fan in front of the mountain near Gedajing. The activity characteristics of the fault scarps may reflect the latest seismic events in the western part of the Sanweishan fault. Based on the analysis of paleoseismological trenches and optical stimulated luminescence dating, two paleoseismic events are determined. Event E1 occurred at approximately 35.1±3.7 ka ~ 36.7±4.1 ka; event E2 occurred at approximately 76.5±8.8 ka ~ 76.7 ± 8.3 ka. The vertical slip rate of the Sanweishan fault is determined to 0.03±0.01 mm/a, with a corresponding shortening rate of 0.09±0.01 mm/a. Together with the previous results, we consider that the Sanweishan fault is characterized by segmentation characteristics. The middle and east segments may have the ability of independent rupture behavior, as well as the characteristics of cascading rupture behavior with the west segment. According to the existing results, we conclude that the recurrence interval for cascading rupture behavior on the Sanweishan fault is approximately 40 ka, which shows a characteristic of long-term recurrence. The best estimated magnitude is inferred to range between Mw 7.1 and Mw 7.5 based on the empirical relationships between moment magnitude and rupture length.

Key words: Late Pleistocene, Paleoearthquake, the Sanwei Shan fault, Dunhuang