Analysis of Possible Global Modes of Earth Deformation from VLBI Measurements

Observed VLBI site position variations contain significant contributions at annual (3-7 mm for the vertical) and semiannual frequencies. These variations lead to apparent global modes of Earth deformation. For instance, a recent paper by Blewitt et al. (Science, 2001) studied the degree-1 spherical harmonic mode of elastic deformation and estimated components of a degree-1 load moment vector from GPS site position time series at annual and semiannual frequencies. Our analysis of VLBI data shows a similar but smaller annual variation. We examine the sensitivity of the VLBI result to the fact that there are more GPS sites and the GPS sites have better global distribution. We also observe a variation of the VLBI terrestrial reference frame length scale of about 0.5 ppb at the annual period and 0.2 ppb at the semiannual period. We have investigated the contribution of several geophysical sources or modeling errors to the observed variations of VLBI site position time series and as explanations of the above global modes. Possible geophysical sources are examined using different models for atmospheric pressure loading, hydrological loading, non-tidal ocean loading, and tidal ocean loading. We also estimate the contribution of possible modeling errors that are specific to VLBI measurements: troposphere delay mismodeling and antenna thermal deformation.