Long-term gravity changes in Lhasa, Tibet and their implication to hydrology and crust movement

We consider the characteristics of long-term changes in non-tidal gravity and their implication to the local perturbations in water storage and to the local vertical crustal movement using the long-term continuous gravity observations from a superconducting gravimeter (SG) installed in Lhasa, Tibet since December, 2009, together with the co-site measurements from a Global Positioning System (GPS) receiver and an absolute gravimeter FG5. In order to simulate the hydrological effects on gravity measurements, the monitoring well and rainfall data are used to evaluate variations in the soil water content near the SG site via the one-dimensional diffusion equation. The observation results indicate that there are obvious seasonal variations in the long-term gravity changes measured with the SG, which mainly come from the contribution of the local disturbances in water storage. Compared with the measurements of the absolute gravity from the FG5, the long-term drift rate of the SG is determined. From the co-site GPS measurements, it is found that the local crust is slowly uplifting at a rate of about 1.5 mm/a, and the related gravity variations is estimated as -17.6 nms-2/a.