# Removing Land Subsidence Impact from GPS Horizontal Motion in Tianjin, China
## 4 Discussion  
This study develops a sequence-to-sequence model based on GRUs to identify the implicit relationships between horizontal and vertical movement data derived from GPS, thereby mitigating the influence of land subsidence on GPS horizontal movements. The  
Figure 7: Horizontal velocity field without land subsidence in Eurasian reference frame in study area (2014–2023). proposed model is applied to 10 GPS stations in the Tianjin subsidence area, where it effectively alleviates the impact of land subsidence on horizontal movements, allowing for a more accurate interpretation of GPS horizontal displacement data in regions affected by subsidence. The raw horizontal movement data from GPS stations in the Tianjin subsidence area exhibit significant discreteness, a characteristic similar to that observed at stations within the Shanxi Graben zone and the Beijing Plain region. After mitigating the effects of land subsidence, the discreteness of horizontal movements at these Tianjin stations is markedly reduced, revealing a clearer overall trend in horizontal displacements that more closely aligns with the movement patterns of neighboring non-subsiding areas. Our findings provide further evidence supporting the hypothesis that land subsidence induces non-tectonic displacements in the horizontal direction. Moreover, by removing non-tectonic information caused by land subsidence, the GPS-derived horizontal movements we obtained better reflect genuine tectonic activities, thus providing more reliable fundamental data for geodynamics research and seismic hazard analysis in the Tianjin region. Further, after removing the impact of land subsidence, the difference between the predicted horizontal velocity field and the original field in the Tianjin area is not fixed, which indicates that the influence of land subsidence on different areas and different stations in the subsidence zone of Tianjin does not have the same characteristics, which was not revealed by previous studies. The implications of our results indicate that the mechanisms responsible for horizontal displacements associated with land subsidence could differ from one region to another and are subject to the influence of the local environment. This insight facilitates more in-depth investigations into the regularities of surface deformation and the underlying drivers in subsidence-affected areas, thus promoting advancements in the study of geotechnical behavior and subsidence dynamics. Land subsidence poses significant social risks, including the formation of ground fissures and the deformation of built structures. Previous studies have predominantly concentrated on the correlation between vertical deformations and these associated risks [26, 27]. Nevertheless, horizontal displacements resulting from land subsidence are equally intertwined with such risks. The quantitative outcomes derived from our analysis of horizontally induced movements due to land subsidence provide valuable insights that can strengthen the assessment of potential hazards in affected regions. This enhanced understanding is anticipated to foster improved strategies for the prevention and management of these risks, contributing to more effective risk governance in areas experiencing land subsidence. This study has several limitations. One limitation pertains to the temporal coverage of the GPS data utilized in our analysis. Land subsidence in the Tianjin area is a long-standing issue that was noted by researchers as early as the 1960s; however, our study period (2014 to 2024) is constrained by the availability of GPS observations and is relatively brief in comparison. This limitation implies that the results presented herein may not fully represent historical conditions. Incorporating earlier, albeit sporadic and non-continuous, GPS observation data could extend the temporal scope of the study, yet this would necessitate additional efforts for data collection and processing.