Effects of Isostasy On Large‐Scale Geoid Signal‐Ii. Geoid Anomaly Over the Earth

Summary The geoid anomaly due to the oceanic and the continental lithosphere is a second‐order quantity which is sensitive to the definition of isostasy. As the geoid and the compensation level (=200 km depth) are both equipotential surfaces, further potential changes are induced due to infilling of sea‐water above and mantle material below, until equilibrium is maintained. the final geoid is composed of seven second‐order effects. In this work the surface of the earth is divided into 36 times 18 spherical trapezoids, 10°× 10° each, and age is assigned to each trapezoid containing oceanic lithosphere using the map of Larson et al. the geoid anomaly is calculated using the method of rings, in which a set of 18 rings about an arbitrary point (observation point) covers the whole earth. Over most of the earth, the direct dipole effect is dominant relative to the direct mass effect. the final geoid anomaly, after the self‐deformation effect, can be up to twice as much as the initial mass and dipole signals. the calculated geoid follows some major observed anomalies over the earth such as the geoid high in the west Pacific and the low in the Indian ocean; but is poorly correlated with the observed geoid due to large and wide anomalies, presumably caused by flow in the mantle, such as the low over Antarctica. These results suggest, although not statistically significant, that the residual geoid variance reaches a minimum if the reference density under the oceanic lithosphere of 3.28 gcm ‐3 is slightly higher than the 3.26gcm ‐3 under the continental lithosphere.