GRACEでみた地球重力場の非季節性変動とGPS局の速度場について On the Non-seasonal Gravity Changes From GRACE and Velocities of GPS Stations
Gravity Recovery and Climate Experiment (GRACE) satellite has been producing many scientific results concerning seasonal changes in the Earth's gravity field since its launch in March 2002. On the other hand, global, regional, and local scale deformations have been continuously investigated using Global Positioning System (GPS) over the last two decades. Here we try to compare velocity field from GPS and non-seasonal gravity changes from GRACE. We take two different approaches,i.e. those in the wave number domain and in the space domain. In the first approach we found that it is approximately up to degree 15 in terms of the Stokes' coefficients that secular changes of gravity well exceed measurement errors in the gravity field recovered by GRACE. As for GPS, we found it rather difficult to extract crustal deformation field expressed by spherical harmonics due mainly to the inadequate GPS station coverage, i.e. we could recover such field for only up to degree 4. Correlation between the Stokes' coefficients from the two techniques did not show good significance. Then we tried to compare GRACE and GPS in space domain. From the secular gravity change map up to degree 10 recovered by GRACE, significant regional gravity changes have been observed in numbers of areas, namely Hudson Bay (Canada), Alaska, Greenland, southeastern Africa, Southeast Asia, and in the southern part of South America. For regions with recent and past ice melting, we compared gravity changes and GPS velocities and confirmed that these changes reflect elastic and viscous responses of the solid earth to ice melting. Gravity changes in Southeast Asia were found to reflect the coseismic jump in the gravity field associated with the 2004 Sumatra-Andaman Earthquake rather than climate-driven secular decrease.
測地学会誌 52(4), 297-308, 2006-12-25