Seasonal patterns of 3D crustal motions across the seismically active southeastern Tibetan Plateau

We study the three-dimensional (3D) seasonal crustal deformation and its relationship with local seismicity based on continuous GPS (cGPS), mass loading models and seismicity data covering the period from 2010 to 2018 in the southeastern Tibetan Plateau (SETP). Of the 3D seasonal motions, vertical motion is the most significant, followed by the northward and eastward directions, with average annual amplitudes from cGPS of 9.28 mm, 1.62 mm and 0.72 mm, respectively. The annual motions predicted by the hydrological mass loading (HYDL) model in all directions are about 3-4 times larger than that of the atmospheric mass loading (ATML) model, but smaller than the cGPS measurements. The initial phase difference between the cGPS and loading models can be nearly eliminated by combining the ATML and HYDL models which helps to fit the cGPS results better. The average weighted root-mean-square (WRMS) reduction ratios of the ATML, HYDL model and their combination are 0.30, 0.77 and 0.88, respectively. This indicates that the seasonal vertical crustal motions observed by cGPS in the SETP are dominated by seasonal loading motions from the ATML and HYDL. In four subregions where no M > 4 earthquakes had occurred in the past decade, we observe an anti-correlation between equivalent water height (EWH) from the GRACE (Gravity Recovery and Climate Experiment) and the vertical displacement derived by cGPS. It shows an anti-correlation relationship between the EWH and seismicity in three subregions and positive correlation in one subregion, which underscores the importance of hydrological loading on seismicity in the SETP.