|Abstract:The Litang M7.3 Earthquake which happened in 1948 in the western Sichuan, is a major destructive earthquake in Sichuan-Yunan block in the past century. Using differential GPS, seismic fissures are accurately measured and analyzed. It is revealed that the length of surface rupture is 36 km. In consideration of the impact of natural erosion and human activities, the maximum value of 41km obtained from previous investigations is taken as the maximum value, while this measurement is taken as the minimum value. The north end of the surface rupture starts from the north of the Wulianghe river and extends southeastern along the northeastern margin of Zangba basin and the southeastern margin of Dewu Basin to the north of Dewu. There is a 3 km long surface rupture gap near Jiaode, and the surface fracture is divided into two sections, North and south. In the post earthquake investigation, many people mentioned that 2 earthquakes occurred in 2-3 minutes and it is speculated that the Litang earthquake in 1948 had two large rupture activities in three minutes. According to the description of seismic sound, it is inferred that the rupture of seismogenic fault started from the northwest and gradually transferred to the southeast. The statistical results of fracture linear density show that the surface fractures in the south of Dewu section are relatively continuous, with a large distribution density, and the maximum value is 50 at 28km, indicating that the most serious earthquake damage is in Dewu basin. It is speculated that the macro epicenter is located about 5km southeast of Jiaode in the middle of Dewu basin.Riedel shear is the main type of initial rupture of seismic fault, especially when the surface rupture of strike slip earthquake is more obvious. Riedel shear analysis indicates that the Riedel shear structure is mainly composed of R shear in the surface fracture zone of the 1948 Litang earthquake and developed a echelon of extruded bulges, a small amount of R′shear and T fracture, and Y-type shear zone parallel to the main fault plane. The surface fracture zone lacks p-type and X-type shear. R shear fracture is the main fracture, accounting for more than 95%, and its predominant direction is 316° with an angle of 16° between fracture strike Y and R shear fracture. The evolution sequence of Litang seismic fracture zone is established. R and conjugate R" shear occur in the initial stage, and then T shear and Push-up may occur simultaneously. With the gradual development of fracture, Push-up bend and rotate further, forming an anti-S-shaped fold. Based on the comparison results of various seismic and geological parameters, it is revealed that the structural background and formation mechanism of Litang fault zone and Xianshuihe fault zone (Luhuo-Daofu-Kangding section) are similar. It is speculated that the parallel Litang fault zone developed in the later stage of the active block due to the movement of the Sichuan-Yunnan active block and the Xianshuihe fault zone on the boundary. For the clockwise movement characteristics of the Sichuan-Yunnan active block, the Xianshuihe fault zone in the Northeast boundary bears the main left-hand strike slip component, and the Litang fault zone in the interior bears part of the strike slip component.