关键词: 高分辨率地形数据, 地震地表破裂带, 富蕴M8.0地震, 同震位移

Abstract:

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.

Key words: high resolution topographic data, earthquake surface rupture zone, Fuyun M8.0 earthquake, coseismic offset