# Removing Land Subsidence Impact from GPS Horizontal Motion in Tianjin, China
###### Abstract  
The phenomenon of land subsidence has been demonstrated to exert a considerable influence on GPS observations. However, to date, no study which has successfully removed the impact of land subsidence on GPS horizontal motion has been conducted. We developed an original sequence-to-sequence deep learning model for the elimination of the impact of land subsidence on GPS horizontal motion, employing gated recurrent units. The model is capable of predicting the horizontal motion of the target GPS station with the impact of land subsidence removed by learning the implicit relationship between the horizontal motion and vertical data of the station. A local model was constructed for each GPS station in the Tianjin subsidence area, and the corresponding dataset was generated for the purposes of model training and testing. The vertical data, with the impact of land subsidence removed, were employed as model inputs for the purpose of predicting the horizontal motion of the same station, with the impact of land subsidence similarly removed. The results demonstrate that following the removal of the impact of land subsidence, the dispersion of GPS horizontal motion within the Tianjin subsidence area is markedly diminished, and the horizontal motion trend exhibits greater consistency with that observed at neighboring stations in non-subsidence regions. The impact of land subsidence on GPS horizontal motion exhibits variability across different regions of the Tianjin subsidence area and among disparate stations. 1  
language  
The problem of land subsidence is prominent in the southern part of Tianjin, which is one of the areas with the most extensive groundwater extraction and the most serious land subsidence in the North China Plain [4, 5, 6, 7, 8]. The vertical movement of GPS stations in southern Tianjin clearly records the continuous decline caused by land subsidence [9]. In addition to the traditionally observed subsidence in the vertical direction [10, 11, 12, 13], an increasing body of research has recently highlighted non-tectonic horizontal displacements associated with land subsidence. Within the Shanxi graben zone, GPS stations situated in areas of significant subsidence exhibit notably higher rates of horizontal movement compared with those on either side of the graben, a pattern that does not align with local geological studies [14]. Similarly, in the Beijing Plain region, which shares the northern China subsidence area with Tianjin, GPS stations within zones of subsidence display considerable variability in their horizontal movement rates without a clear directional trend, indicating pronounced heterogeneous characteristics [15, 16]. The horizontal displacements observed at these GPS stations are largely inferred to be influenced by the horizontal effects of ground subsidence, rather than accurately representing genuine surface-level tectonic motion. The aforementioned studies have conducted qualitative analyses on the characteristics and mechanisms of how land subsidence affects horizontal GPS motion, yet they have not provided quantitative outcomes. To date, no methods have been proposed to eliminate the impact of land subsidence on GPS horizontal motion. This situation precludes the use of GPS observational data for obtaining true information about horizontal tectonic surface movements in subsidence areas. In cities such as Xi'an, Shanghai, and Mexico City, the horizontal displacement caused by land subsidence has also potentially led to ground fissures and building deformations [17, 18, 19, 20], thereby impacting the maintainability of housing and public utility infrastructures. Quantitatively acquiring information on the horizontal displacements generated by land subsidence would assist us in more accurately assessing the severity of these risks.