## 1 Introduction  
Changes in surface mass (e.g., atmospheric pressure, non-tidal oceans, and hydrological loading) can change the gravitational field and three-dimensional displacement of the elastic crust [1, 2, 3]. Many studies have found that seasonal variations in hydrological loading, which is majorly affected by abundant precipitation, are the main cause of seasonal crustal deformation [4, 5, 6].  
GNSS, which accurately responds to tectonic and non-tectonic movements, is a highly precise multi-dimensional method that can be applied on large scales to observe crustal deformation. It is not only subject to large-scale regional tectonic movements, but is also particularly sensitive to changes in local observation environments; thus, it contains more complex motion information. Related research results show that the Chinese GNSS time series are mainly dominated by linear motion in the horizontal direction with periodic variations [7, 8], while the vertical direction shows complex periodic oscillations and non-linear motion. Moreover, the periodic fluctuations differ among different stations. Gravity Recovery and Climate Experiment (GRACE) satellite (include GRACE and GRACE-FO) observations comprehensively respond to terrestrial gravity field anomalies. Global time-varying gravity data provided by GRACE have been widely used to study terrestrial water changes [9; 10; 11], large watersheds and rivers [12], and changes in the mass of polar ice sheets [13; 14]. According to the elastic load theory [1], the time-varying gravity data of GRACE can not only represent gravity field changes caused by changes in surface mass but can also estimate the corresponding three-dimensional crustal load deformation. However, affected by the resolution, it provides information on large-scale crustal deformation characteristics. Precipitation, as the main influencing factor of hydrological loading change, provides important information for the study of surface quality change. The theoretical displacements obtained by GRACE inversion combined with precipitation information can be used to better analyze seasonal deformation characteristics of GNSS time series from the perspective of seasonal loading studies.  
Sichuan and Yunnan provinces are located on the southeastern margin of the Tibetan Plateau in China, a mountainous plateau terrain, high in the north and low in the south, and most of the region is located in the middle altitude area, mostly hills and mountainous geography. As the Eurasian Plate and Indian Plate collide, these provinces experience some of the strongest tectonic and seismic activities in mainland China. In addition, the region is located in southwestern China and experiences a subtropical monsoon climate with a small annual variation in temperature, a high daily temperature difference, distinct wet and dry seasons, and high precipitation during the rainy season, with a significant annual period (Figure 1). Existing research on the seasonality of this region has mainly focused on vertical features, and annual cycles [15; 16; 17; 18]. Our research found that the time series of GNSS continuous stations not only have significant annual periodic changes in the vertical direction but also in the horizontal direction; additionally, evident semi-annual cycle signals exist. However, studies on the seasonality and horizontal direction of semi-annual cycles are few. To bridge this research gap, this study aimed to comprehensively analyze the GNSS three-dimensional displacement coordinate time series, GRACE inversion modelling results of crustal displacement, and main periodic components of precipitation observation data; moreover, the possible interactions between these factors are discussed.  
Figure 1: Distribution of GNSS stations in the research area. The gray background is the topography of the study area, and the colored background is the average annual precipitation.