A new global topographic-isostatic model

This is a preliminary report of our on-going research on a global topograph ic-isostatic model. The model comes from a completely new idea - the geoid undulation is the responses of an elastic earth to the topographic mass loa d. Assuming the topography as a condensed surface mass load, we derive expr essions for calculating the vertical displacement, potential and equipotent ial surface changes, based on the load theory proposed in Sun and Sjoberg ( 1996). The modeled geoid is composed of three parts: loading potential, sur face displacement and mass redistribution. The mass redistribution of the e arth compensates to some extent the topography. We mainly calculate and dis cuss the vertical displacements and equipotential surface changes for three depths: the earth's surface, d = 36 km and the core-mantle boundary. Numer ical results show that the displacements at depth 36 km and the earth's sur face have the same distribution pattern and magnitude, while the vertical m ovement of the core-mantle boundary appears much smoother and smaller. The modeled geoid undulations vary between -352 and +555 m. The comparison betw een the modeled and observed geoid undulations shows that there are strong positive correlations between them, but a compensation only by elastic defo rmations is not sufficient to explain the observed undulations because of t he big difference in magnitude between the two geoids. More geodynamic effe cts should be considered to better explain the long-wavelength geoid featur es. (C) 1999 Elsevier Science Ltd. All rights reserved.