The northern margin of the Qinghai-Tibet Plateau is currently the leading edge of uplift and expansion of the plateau. Over the years, a lot of research has been carried out on the deformation and evolution of the northeastern margin of the Qinghai-Tibet Plateau, and many ideas have been put forward, but there are also many disputes. The Altyn Tagh Fault constitutes the northern boundary of the Qinghai-Tibet Plateau, and there are two active faults on the north side of the Altyn Tagh Fault, named Sanweishan Fault with NEE strike and Nanjieshan Fault with EW strike. Especially, studies on the geometric and kinematic parameters of Sanweishan Fault since the Late Quaternary, which is nearly parallel with the Altyn Tagn Fault, are of great significance for understanding the deformation transfer and distribution in the northwestward extension of the Qinghai-Tibet Plateau. Therefore, interpretation of the fault landforms and statistical analysis of the horizontal displacement on the Sanweishan Fault and its newly discovered western extension are carried out in this paper. We believe that the Sanweishan Fault is an important branch of the eastern section of the Altyn Tagh fault zone. It is located at the front edge of the northwestern Qinghai-Tibet Plateau and is a left-lateral strike-slip and thrust active fault. Based on the interpretation of satellite imagery and microgeomorphology field investigation of Sanweishan main fault and its western segments, it's been found that the Sanweishan main fault constitutes the contact boundary between the Sanweishan Mountain and the alluvial fans. In the bedrock interior and on the north side of the Mogao Grottoes, there are also some branch faults distributed nearly parallel to the main fault. The main fault is about 150km long, striking 65°, mainly dipping SE with dip angles from 50° to 70°. The main fault can be divided into three segments in the spatial geometric distribution:the western segment(Xizhuigou-Dongshuigou, I), which is about 35km long, the middle segment(Dongshuigou-Shigongkouzi, Ⅱ), about 65km long, and the east segment(Shigongkouzi-Shuangta, Ⅲ), about 50km long. The above three segments are arranged in the left or right stepovers.
In the west of Mingshashan, it's been found that the fault scarps are distributed near Danghe Reservoir and Yangguan Town in the west of Minshashan Mountain, and we thought those scarps are the westward extension of the main Sanweishan Fault. Along the main fault and its western extension, the different levels of water system(including gullies and rills)and ridges have been offset synchronously, forming a series of fault micro-geomorphology. The scale of the offset water system is proportional to the horizontal displacement. The frequency statistical analysis of the horizontal displacement shows that the displacement has obvious grouping characteristics, which are divided into 6 groups, and the corresponding peaks are 3.4m, 6.7m, 11.4m, 15m, 22m and 26m, respectively. Among them, 3.4m represents the coseismic displacement of the latest ancient earthquake event, and the larger displacement peak represents the accumulation of coseismic displacements of multi-paleoearthquake events. This kind of displacement characterized by approximately equal interval increase indicates that the Sanweishan Fault has experienced multiple characteristic earthquakes since the Late Quaternary and has the possibility of occurrence of earthquakes greater than magnitude 7. The distribution of displacement and structural transformation of the end of the fault indicate that Sanweishan Fault is an "Altyn Tagh Fault"in its infancy. The activities of Sanweishan Fault and its accompanying mountain uplift are the result of the transpression of the northern margin of the Qinghai-Tibet Plateau, representing one of the growth patterns of the northern margin of the plateau.

Sanwei Shan Fault is located in the north of Tibet, which is a branch of eastern segment of Altyn Tagn fault zone. This fault is distributed along the boundary of fault facet and the Quaternary, with the total length of almost 150km. The fault is a straight-line structure read from the satellite image. Based on the spatial distribution of the fault, three segments are divided, namely, Xishuigou-Dongshuigou segment, Dongshuigou-West Shigongkouzi segment and West Shigongkouzi-Suangta segment, these three segments are distributed by left or right step.Though field microgeomorphology investigation along Sanwei Shan Fault, it has been found that two periods of alluvial-pluvial fans are distributed in front of Sanwei Shan Mountain, most of which are overstepped. Comparing the distribution of alluvial-pluvial fans with their formation age in the surrounding regions, and meanwhile, taking the results of optical stimulated luminescence(OSL) dating, it's considered that the formation age of the older alluvial-pluvial fans, which are distributed in northern Qilian Shan, inside of Hexi Corridor and western Hexi Corridor(including the Sanwei Shan piedmont fans), is between later period of late Quaternary and earlier period of Holocene. The gullies on the older fan and ridges have been cut synchronously. The maximum and minimum sinistral displacement is 5.5m and 1.7m, but majority of the values is between 3.0~4.5m. Taking the results from the OSL dating, we conclude that the minimum sinistral strike-slip rate is(0.33±0.04) mm/a since 14 ka BP and(0.28±0.03) mm/a since 20 ka BP.

The spatial-temporal evolution process of strain field and acoustic emission(AE)events was investigated during the deformation of 5° bend faults,with 96-channels strain acquisition system and 16-channels distributed AE acquisition system in the laboratory.The loading was applied by controlling the Y-displacement and holding the X-load in a biaxial servo-control loading system, and the Y-loading rate was altered by 0.5μm/s,1μm/s,0.5μm/s and 0.1μm/s in sequence.The observation results show that: (1)quasi-periodic stick-slip always occurred under different loading rates, and the smaller the loading rate,the greater the period and stress drop; (2)low energy AE events increased before faults slid,but high energy AE events appeared as faults slid.AE events distributed near the bends and the upper and lower fault segments which were located by arrival time of AE wave.From the AE location results,AE sources mostly scattered in bend zones,and upper and lower fault segments,and the fault instability appeared first near bend point,then the alternative activities happened between upper and lower fault segments.Large instability took place in the lower fault segment,finally; (3)High strain concentration zone located near bend point and fault segment.And it is significantly different that mean strain and maximum shear strain increment changed alternately at the inside and outside of bend during strain accumulation and release stage; (4)Strain observation results illustrate that mean strain release first occurred near the bend,then released in the whole fault.It would be a critical instability condition for a bend fault.The observation to bend faults is important and helpful to investigate fault activity state.

The stick-slip process of pre-cut bending faults with a 5°angle at bending point between the two fault segments is investigated by use of fault displacement measurement,strain tensor analysis and acoustic emission(AE)technique in the laboratory.The dynamic process and corresponding properties of physical evolution are discussed.The experimental results from bending faults show that: 1)A negative relationship was revealed between the logarithms of the stick-slip cycle and the logarithms of loading rate; 2)Under different loading rate,most of instabilities of bending faults are earthquake doublets,and the interval time between the two sub-events are primarily from 100ms to 200ms; 3)For different observational approaches,even if with the same sampling rate,the differences of the coseismic response were observed,such as the significant strain weakening stage indicated by strain measurements,but there was no significant change in fault displacement before fault instability; and 4)AE sources obviously migrated along faults during fault sliding.More dynamic information about fault instability process is needed to know the mechanism of strong earthquakes and the features of aftershocks.