Determination of vertical surface displacements in Sichuan using GPS and GRACE measurements

Seasonal variations of terrestrial water storage (TWS) mainly result in periodical fluctuations of crustal deformation, eliminating the effects from TWS on surface displacement is accordingly conducive to highlight regional tectonic movements. Sichuan Province lies close to the eastern margin of the Qinghai-Tibetan Plateau and has a rising terrain feature from its east to the west. Moreover, abundant river systems, such as Yangtze River system, provide Sichuan with adequate water resource. We combine GPS-derived and GRACE-derived vertical displacement time series to explore the spatial-temporal distribution characteristics of TWS and the crustal tectonic movements in Sichuan. The result shows that the amplitudes of vertical displacements derived by those two ways are accorded very well, but there exist phase delays. The GPS-determined amplitudes show a maximum of 12.7 mm at station HANY and minimum of 1.5 mm at station SCMX, respectively. Due to a low spatial resolution, GRACE-derived amplitudes present comparable magnitude of 3 similar to 4 mm and the largest displacements are in between July and September. However, the time when the largest displacements of GPS happen correspond with topography, the eastern basin and hills, northwestern plateau and southern mountains possess its own largest displacement in July to August, October to November and October, separately. Seasonal TWS changes are relevant to GPS-derived vertical displacement variations. So, in order to illustrate the effect of TWS changes on GPS sites' movements, we use the Modified Ensemble Empirical Mode Decomposition (MEEMD) to extract annual term and a 2-year periodic signal from GPS vertical displacement time series. Consequently, we find that when using MEEMD-derived annual terms to correct GPS time series, the WRMS (Weight Root Mean Square) reduction can decrease roughly 26%, which is superior to least square method, suggesting that the MEEMD method is feasible in dealing with GPS time series.