It is crucial to reveal the surface traces and activity of active faults by obtaining high-precision microtopography and three-dimensional shallow geometry. However, limited by the traditional geological investigation methods in the field and geological condition factors, the measurement method on microtopography and shallow geometry of active fault is badly insufficient. In this study, the TLS and GPR are firstly used comprehensively to delineate the microtopography and shallow geometry of the normal fault scarp on the north margin of Maoyaba Basin in Litang. Firstly, the vertical displacements of two landforms produced by the latest two periods of normal faulting and the two-dimensional GPR profiles are obtained separately. Secondly, the three-dimensional measurement method of active fault based on TLS and GPR is preliminarily established. On this basis, three-dimensional model of fault scarp and three-dimensional images of subsurface geometry are also obtained. These data all reveal a graben structure at normal fault scarps. Thirdly, the fusion and interpretation of three-dimensional data from the surface and subsurface are realized. The study results show:1)the vertical displacements of the T₁ and T₂ terraces by the normal fault movement is 1.4m and 5.7m, the GPR profile shows a typical fault structure and indicates the existence of small graben structure with a maximum width of about 40m in the shallow layer, which further proves that it is a normal fault. 2)the shallow geometry of the normal fault scarp can be more graphically displayed by the three-dimensional radar images, and it also makes the geometry structure of the fault more comprehensive. The precise location and strike of faults F₁ and F₂ on the horizontal surface are also determined in the three-dimensional radar images, which further proves the existence of small graben structure, indicating the extensional deformation characteristics in the subsurface of the fault scarps. Furthermore, the distribution of small graben structure on the surface and subsurface is defined more precisely. 3)the integrated display of microgeomorphology and shallow geometry of normal fault scarp is realized based on the three-dimensional point cloud and GPR data. The fusion of the point cloud and GPR data has obvious advantages, for the spatial structure, morphological and spectral features from the point cloud can improve the recognition and interpretation accuracy of GPR images. The interpreted results of the GPR profiles could minimize the transformation of the surface topography by the external environment at the most extent, restore the original geomorphology, relocate the position and trend of faults on the surface and constrain the width of deformation zones under the surface, the geological structure, and the fault dislocation, etc.
In a word, the TLS and GPR can quickly and efficiently provide the spatial data with multi-level and multi-visual for non-destructive inspection of the microgeomorphology and shallow structure for the active fault in a wide range, and for the detection of active fault in the complex geological environments, and it is helpful to improve the accuracy and understanding of the investigation and research on microtopography and shallow geometry of active faults. What's more, it also offers important data and method for more comprehensive identification and understanding of the distribution, deformation features, the behaviors of active faults and multi-period paleoseismicity. Therefore, to continuously explore and improve this method will significantly enhance and expand the practicability and application prospects of the method in the quantitative and elaborate studies of active faults.

Detailed mapping shows that there are two segments of co-seismic surface ruptures on the Litang-Dewu left-lateral strike-slip fault. The north segment is about 25km long, with a strike about 135°NE. The maximum horizontal left-lateral displacement on the north one is~1.8m and located at the high floodplains on the north side of the Wuliang River near Cun'ge village, offsetting the linear ridges that were left behind by human activity. The south segment is about 41km, striking generally about 146°NE. The maximum horizontal left-lateral displacement is located at the piedmont near the north side of the Rongjia mountain pass and the river floodplain scarp here is offset about 3.2m. There is a surface rupture gap about 11km between these two co-seismic surface rupture segments. The distribution of the co-seismic surface ruptures acquired by detailed mapping in the field survey, the earthquake event revealed by the trench, the AMS-¹⁴C dating result, the historical records of earthquakes at least since AD 1729 in the study area and the visiting on the local people, show consistently that the northern co-seismic surface rupture segment is most possibly produced by the 1729 Litang earthquake. The 1948 Litang earthquake was only responsible for the southern surface rupture segment. However, if only according to the 2 sigma calendar calibrated results of ¹⁴C dating, it cannot be excluded the possibility that the north segment maybe was produced by some older large earthquake occurring at some time during the AD 1420 to AD 1690. The moment magnitude(M_W)of the 1729 earthquake is about 6.7 and that of the 1948 earthquake is about 7.0 calculated from the empirical relations between the earthquake magnitude and the rupture length.