Seasonal variations of gravity and continental water storage
Speaker: Yi-Xiu Lai
Abstract
Superconducting gravimeters (SG) are able to record surface gravity changes on a wide frequency band (periods from a few seconds to secular variations).This presentation focus on the seasonal gravity changes measured by about 20 worldwide SG. modeling all well-known sources of long-term gravity changes, i.e. solid Earth tides, polar motion and length-of-day as well as global atmospheric, tidal and non-tidal ocean loading effects. These corrections lead to gravity residuals characterized by a strong seasonal signal with an amplitude of a few microgals. We compare these residuals with loading estimates from global hydrology (snow and soil-moisture) models. For more than half of the analysed SG, we are able to show a good correlation between the gravity residuals and the estimated continental water storage loading effects. For the other instruments, the discrepancies may be associated with local hydrology effects, which cannot be taken into account in global continental water storage models. We compare this global approach with the plane approximation in terms of height, gravity and tilt changes as a function of the distance from the measurement point to the load. We find that Green's functions for the vertical displacement (resp. horizontal displacement, elastic part of the tilt) agree to within 1% up to ~400 m (resp.2 km, 5 km) from the glaciers. The alpine glaciers of the Mont Blanc region (France) where ice-thickness variations are derived from differential digital elevation model analysis for the period 1979–2003. We show that the rates of ground tilt are well above the limit of detection of up-todate long-base hydrostatic tiltmeters, which, if installed next to the glaciers, could be used to monitor the time evolution of ice thinning. We also show that the topography has a strong influence on the gravity variations near the glaciers.
Reference
Boy, J.P., Hinderer, J., (2006) Study of the seasonal gravity signal in superconducting gravimeter data. Journal of Geodynamics 41, 227–233.
Memin a , A., Rogister, Y., Hinderer, J., Llubes, M., Berthier , E. Boy, J.P., (2009) Ground deformation and gravity variations modelled from present-day ice thinning in the vicinity of glaciers. Journal of Geodynamics 48, 195–203.