| The Danghenanshan North Piedmont Fault(DHNF), which stretches to the AltynTagh Fault(ATF) at west end, locates in the western of Qilian mountain range. It plays an important role in the strain transformation from horizontal to vertical in the middleeast section of ATF. There are multistep coseismic thrust fault scarps in the Yanchiwan segment of DHNF, and the surface rupture zone of the last earthquake was also well preserved. These deformation features make Yanchiwan to be a well condition for the study on the slip behavior and the recurrence model of large earthquakes of DHNF. In this study, photogrammetry with low-flying drone is used to obtain the high resolution topography data of the Yanchiwan segment of DHNF, and conduct distribution characteristics of the multistep coseismic scarps of this fault. Several trenches were also dug to reveal the paleoseismic sequence and the subsurface geometry of DHNF. Based on these field data, we further analyzed and discussed the slip behavior, the recurrence pattern of strong earthquakes, and the probability of seismic potential of DHNF. The conclusions as follows:(1) With the 3-D reconstruction technology with low-flying sUAV photogrammetry, we can reconstruct the surface morphological characteristics, and obtain terrain data with high resolution and precision. Compared to traditional mapping method and Light Detection and Ranging(LiDAR), it shows high flexibility and lower costs with outputting higher resolution and precision data. That makes the low altitude sUAV photogrammetry a perfect approach for the quantitative research of active tectonics and tectonic geomorphology. But for the limit form its principle, it may work well in the arid or semi-arid regions with very sparse vegetation, such as the northwest region of China.(2) Vertical displacements of multistep scarps of DHNF was measured and calculated with error estimation. Combining the result with the vertical displacements formed in single event and multiple event revealed in the paleo-earthquake trenches, we suggest that the recurrence pattern of large earthquakes in this fault is probably the same as characteristic earthquake model, with an average vertical displacement of 70±20cm.(3) The movement model of DHNF is considered as the piggyback propagation of thrust structure extension mode. A series of thrust fault scarps with the strike of NWW formed, due to the SSW layers thrusting to NNE along the faults planes. The average dip angle of the seismogenic fault is about 25°. Small back thrust often formed near the main fault plane in the hanging wall. However, it also forms a narrow graben, which is associated with some normal fault and controlled by the tensile stress, at the rear of the hanging wall.(4)The elapsed time of the most recent earthquake occurred on DHNF revealed by the paleo-earthquake trenches and radiocarbon samples dated by AMS ~ 720±30a. The earthquake probably occurred after 1290~1295 A.D., with an average 0.7±0.2m coseismic vertical offset. Referencing from the historical earthquake record around Subei, it seems that the occurrence time of the historical large earthquake occurred near Dunhuang in 1297~1303 matches age of the most recent earthquake on DHNF very well. Therefore, DHNF may be the causative fault of this historical earthquake.(5)We also assessed the probability of large earthquake risk of DHNF using the method of time dependent potential probability assessment, combining with the latest study result of Paleoseismology and reliable data provided by previous study. Finally, the calculated seismogenic probability in the future 50 a, 100 a, 200 a, and500 a of this fault are 0.006, 0.012, 0.039, and 0.28 respectively. This result shows that the ruputure probability of strong earthquake is low in the future hundreds of years. |
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