The spatial distribution and deformation characteristics of the coseismic surface rupture zone are the direct geomorphological expressions of deep fault activities on the surface, which not only record the information of seismic rupture and fault movement but also reflect regional stress and crustal movement. Therefore, prompt investigation on the surface rupture zone after the earthquake is helpful to understand tectonic activities of the seismogenic fault. However, fieldwork is limited by hazardous environments and secondary disasters in the earthquake zone. High-precision geomorphological observation technology can obtain unprecedented high temporal and spatial resolution of the earth's surface features without being restricted by natural conditions, and provide high-quality data for identifying historical earthquake surface ruptures, extracting surface coseismic displacement, and geological mapping of active structures, thus help to understand the rupture processes deeply. The photogrammetric method based on SfM(Structure from Motion)technology provides an effective technical way for fast acquisition of high-resolution post-earthquake topographic data and obtaining 3D geomorphic characteristics in a short time without the limitation of topography. Fuyun Fault is located on the southwest edge of the Altai Mountains. Fuyun M8.0 earthquake occurred in 1931 and produced a coseismic surface rupture zone with obvious linear characteristics. There also developed a large number of right-lateral gully offset, extrusion uplifts, pull-apart basins and a series of tectonic landforms related to strike-slip activities, which are still well preserved after several decades. In this study, the surface rupture zone of the 1931 Fuyun earthquake is selected as the study area. Based on aerial photogrammetry SfM method, a digital elevation model (DEM) with a resolution of 1m is generated, which can reflect micro-structural geomorphology and is suitable for fine geomorphology research in a small area. Combined with the shadow and color change of DEM data, the surface deformation characteristics such as seismic cracks and seismic mole tracks are identified, the surface rupture tracks are drawn in detail, and the surface rupture zone of Fuyun earthquake is segmented through the distribution of its geometric and tectonic geomorphological features. Using gullies as geomorphological markers, the smallest regional offset is regarded as the coseismic offset in the 1931 earthquake. We finally identified the right-lateral horizontal offset of gully along the rupture zone and measured it with software. The results show that the Fuyun earthquake surface rupture zone can be divided into 4 sections from north to south, each of which has different length, connected by compression uplift or pull-apart basin. The main type of surface rupture is shear crack, and there are also transpressional cracks, tension cracks, and tectonic geomorphological expressions such as mole track, ridge, and pull-apart basin. Based on the measurement of the horizontal offset of 194 groups of gullies, it is found that the average coseismic offset in the 1931 earthquake is(5.06±0.13)m, which is equivalent to the coseismic offset produced by similar magnitude earthquake. The area where the local absence or sudden change of coseismic offset occurs also has a good corresponding relationship with the geometry of stepover, which reflects the geometric location of the stepover to a certain extent. The results fill up the gap of the fine morphology of the Fuyun earthquake surface rupture zone and further demonstrate the good application value of high-resolution topographic data in the study of active structures.

With the development and breakthrough of a series of techniques such as the fault surface morphology measurement, the geochemical element determination and Quaternary dating methods, it becomes possible to study paleo-earthquake using information recorded by the bedrock fault surface. At present, more and more scholars domestic and overseas have carried out a large number of paleo-earthquake studies on bedrock fault surfaces in different professional perspectives and have achieved a series of innovative results. This paper systematically introduces the development history, the current situation and the basic principles and applications of paleo-earthquake study on bedrock fault surface. Moreover, after the thorough discussion of the existing problems in paleo-earthquake research of bedrock fault surface, some suggestions for optimizing the current work are proposed. Finally, on the basis of comparison of the characteristics, advantages and disadvantages of various research methods, the prospects and development trends of the bedrock fault paleo-earthquake study are predicted. Lots of weaknesses and limitations in the current study are pointed out in this paper:Firstly, for the method of faullt surface morphology measurement, different morphological expression parameters exist nowadays, however, their advantages and disadvantages are unknown. Secondly, the TCNs method still has a large uncertainty in the age determination of the paleo-earthquake, and the mature cosmogenic nuclides dating methods is too few to meet the dating requirements of different lithologic fault surfaces. Besides, a reliable relationship between relative dating parameters such as morphologicl and physicochemical characteristics and the absolute dating method such as TCNs are not closely established to build a reliable chronology framework. The last but not the least, the lack of mechanical research on the physical and chemical biological processes that the bedrock fault surface experienced before and after the faulting and exposure, and insufficient multi-method comprehensive comparison are also the obstacles for the paleo-earthquake study on bedrock fault surface. It is suggested that in the future study of paleo-earthquakes on bedrock fault surfaces, more attention should be paid to the following aspects:Firstly, strengthen the evaluation of the reliability, applicability and accuracy of the parameters of each morphological model in time and improve the mathematical model of current dating techniques, optimize the mechanism of cosmogenic nuclide production, and introduce new high-precision dating technology timely; Secondly, strive to establish a reliable age framework between relative dating index(X)and absolute dating age(T)regionally; In addition, the morphological structure and mineral compositions of bedrock fault surface are analyzed proactively on the microscopic scale, and the mechanical study is conducted on a series of physical, chemical and biological processes that the fault surface experienced before and after the exposure. At last, comprehensive and comparative research need to be conducted by the multi-disciplinary and multi-method approaches. In conclusion, the paleo-earthquake study on the bedrock fault surface is going through the processes from the qualitative description to the quantitative expression, from the single-disciplinary method to the multi-disciplinary integration, from the exploration of a certain technical index to the comprehensive application of multi-source data technology. The combination of relative dating indicators(X)and absolute dating(T), and putting more emphasis on the mechanical study on the microscopic scale are the development trends of paleo-earthquake study on the bedrock fault surface. The close combination of the paleo-earthquake study of the bedrock fault surface with the traditional method of trenching conducted in the Quaternary sediment region is considered to help more effectively reconstruct a more complete paleo-earthquake sequence and the faulting history on the active fault zone, thus a more reasonable evaluation of the regional seismic hazard can be obtained.