## 2 Study Area  
The epicenter of the earthquake was in Yangbi County, Dali Bai Autonomous Prefecture in Yunnan Province. Yunnan Province is located on the southeastern edge of the Tibet Plateau, which is at the junction of the Sichuan-Yunnan rhombic block and the western Yunnan block, as illustrated in Figure 1. There are multiple block boundaries and large fault zones that produce destructive earthquakes [8]. The topography of western Yunnan, where the epicenter of the Ms 6.4 earthquake was located, is highly undulating, and there are several active ruptures, mainly distributed in the SN, NE, or NW direction [9], including the NW direction Red River fault, Weixi-Qiaohou fault, Jinshajiang fault, and Longpan-Qiaohou fault near the NS direction [10]. The Red River fault has a total length of over 1000 km and about 600 km of it is in Yunnan. It is an important large-scale strike-slip fault in the Himalayan-Tibet collision zone [11], which has experienced two earthquakes of a magnitude 7.0 or higher in history, and seven earthquakes of a magnitude of 6.0-6.9. The Red River fault is one of the most active faults in Yunnan Province, with a long-term average slip rate of 2-5 mm/a [12; 13; 14]. Yangbi County is located in the northwest of the Red River fault zone and has a complex regional geology and active tectonic development, and there was an Ms 5.4 earthquake in 2017. In addition, the SN trend ruptures in the western part of Yunnan include the Nujiang fault, Lancangjiang fault, Chenghai-Binchuan fault, Xiaojiang fault, Anninghe fault, etc. The Weixi-Qiaohou fault is a branch fault extending in the northwest section of the Red River fault zone, with a total length of about 280 km [15]. The active nature of this fault in the early period was mainly extrusion, and since the Late Quaternary, it was mainly a dextral strike-slip and also had a normal and fractal extension [16]. These faults crisscross and divide the entire western Yunnan area into multiple blocks: Baoshan block, Tengchong block, and the Lamping-Simao block near the location, where an Mw 6.4 Ning'er earthquake occurred in Pu'er in 2007. There are also tectonic units such as the secondary block in central Yunnan [17]. After the Lijiang Ms 7.0 earthquake that occurred in 1996 and the Mojiang Ms 5.9 earthquake that occurred in 2018, this earthquake is the strongest earthquake to occur in the Yangbi area in recent years. The risk of strong earthquakes in the western Yunnan area has always been one of the focuses of attention in the field of seismic geology [18]. Therefore, in this paper, we studied and discuss the characteristics of the deformation field, fault activity, and seismogenic structure of this earthquake.  
Figure 1: Regional tectonic setting of the 2021 Yangbi Ms 6.4 earthquake. The white line in the figure represents the distribution of active faults in the study area. CXB: Chuanxibei Sub-Block; SC Basin: Sichuan basin; YZ: Yangtze Craton; CYB: Central Yunnan Sub-Block; WQF: Weixi-Qiaohou Fault; RRF: Red River Fault; LYF: Lamping-Yunlong Fault; CHF: Chenghai Fault. The dots in the figure represent the magnitude: the purple dots represent earthquakes with a magnitude between Ms 4.0 and Ms 4.9; the green dots represent earthquakes with a magnitude between Ms 5.0 and Ms 5.9; the red star represents the 2021 Yangbi Ms 6.4 earthquake.