Study on characteristics of long-term gravity changes at Wuhan station

Abstract

We consider the characteristics of long-term changes in non-tidal gravity and their implication to the local perturbations in barometric pressure and water storage and to the local vertical crustal movement using the long-term continuous gravity observations from a superconducting gravimeter (SG) at Wuhan station, together with the co-site measurements from a Global Positioning System (GPS) receiver and an absolute gravimeter FG5. The observation results indicate that there are obvious seasonal variations in the long-term gravity changes measured with the SG. About 70 percent of the whole seasonal changes come from the contribution of the local disturbances in air pressure and water storage, while over 95 percent of the annual changes are attributed to the loading effects of these environmental perturbations. Due to the absence of direct measurements of the local water storage, especially those of the underground water, the global assimilating models of land water LaD (Land Dynamics) and GLDAS (Global Land Data Assimilation System) cannot virtually describe the real hydrologic disturbances around the station. The resulting gravity changes, which are simulated theoretically by means of convolution integration of the loading Green’s functions and water models LaD and GLDAS, show significantly time delay of about 55 days from those measured with the SG. Compared with the measurements of the absolute gravity with the FG5, the long-term drift rate of the SG is determined as about 17.13 nms−2/a. From the co-site GPS measurements, it is found that the local crust is slowly subsiding at a rate of 3.71±0.16 mm/a, and the related gravity variation is estimated as 13.88±0.22 nms−2/a. In other words, the ratio of the changes in gravity and altitude related to the local vertical crustal movement is about −37.41 nms−2/cm. It implies that a considerable mass adjustment may be associated with the local vertical crustal movement, and its dynamic mechanism should be investigated further.