Seasonal Hydrological Loading in Southern Tibet Detected by Joint Analysis of GPS and GRACE.

In southern Tibet, ongoing vertical and horizontal motions due to the collision between India and Eurasia are monitored by large numbers of global positioning system (GPS) continuous and campaign sites installed in the past decade. Displacements measured by GPS usually include tectonic deformation as well as non-tectonic, time-dependent signals. To estimate the regional long-term tectonic deformation using GPS more precisely, seasonal elastic deformation signals associated with surface loading must be removed from the observations. In this study, we focus on seasonal variation in vertical and horizontal motions of southern Tibet by performing a joint analysis of GRACE (Gravity Recovery and Climate Experiment) and GPS data, not only using continuous sites but also GPS campaign-mode sites. We found that the GPS-observed and GRACE-modeled seasonal oscillations are in good agreements, and a seasonal displacement model demonstrates that the main reason for seasonal variations in southern Tibet is from the summer monsoon and its precipitation. The biggest loading appears from July to August in the summer season. Vertical deformations observed by GPS and modeled by GRACE are two to three times larger than horizontal oscillations, and the north components demonstrate larger amplitudes than the east components. We corrected the GPS position time series using the GRACE-modeled seasonal variations, which gives significant reductions in the misfit and weighted root-mean-squares (WRMS). Misfit ( divided by degree of freedom) reductions for campaign sites range between 20% and 56% for the vertical component, and are much smaller for the horizontal components. Moreover, time series of continuous GPS (cGPS) sites near the 2015 Nepal earthquakes must be corrected using appropriate models of seasonal loading for analyzing postseismic deformation to avoid biasing estimates of the postseismic relaxation.

在西藏南部，由于印度与欧亚大陆碰撞而产生的持续的垂直和水平运动，由过去十年中安装的大量全球定位系统（GPS）连续观测站和流动观测站进行监测。GPS测量的位移通常包括构造变形以及非构造的、随时间变化的信号。为了更精确地利用GPS估计区域长期构造变形，必须从观测数据中去除与地表负荷相关的季节性弹性变形信号。在这项研究中，我们通过对GRACE（重力恢复与气候实验）和GPS数据进行联合分析，关注西藏南部垂直和水平运动的季节性变化，不仅使用连续观测站，还使用GPS流动观测站。我们发现GPS观测到的和GRACE模拟的季节性振荡非常吻合，一个季节性位移模型表明西藏南部季节性变化的主要原因来自夏季风及其降水。最大的负荷出现在夏季的7月到8月。GPS观测到的和GRACE模拟的垂直变形比水平振荡大两到三倍，北向分量的振幅比东向分量大。我们利用GRACE模拟的季节性变化对GPS位置时间序列进行了校正，这显著降低了不匹配度和加权均方根（WRMS）。对于流动观测站，垂直分量的不匹配度（除以自由度）降低幅度在20%到56%之间，水平分量的降低幅度则小得多。此外，2015年尼泊尔地震附近的连续GPS（cGPS）观测站的时间序列必须使用适当的季节性负荷模型进行校正，以便分析震后变形，避免对震后松弛的估计产生偏差。