Relative sea levels in oceanic islands that are situated sufficiently far from glaciated regions were studied in relation to the rheological structure of the earth's mantle. The local hydro-isostatic adjustment associated with the mantle flow from the oceanic side to an inland side depends on the size of the island, the effective upper-mantle viscosity and the elastic plate thickness. The relative sea level has a significant dependence on the upper mantle rheology for islands with a radius larger than 10 km, but it is almost independent of the upper mantle rheology for islands with a radius less than 10 km. In other words, the observed sea-level changes in such small islands follow the global isostatic adjustment which depends strongly on the lower mantle viscosity. In fact, the relative sea levels in small oceanic islands impose important constraints on the lower-mantle viscosity. On the other hand, the effective upper-mantle viscosity and the elastic plate thickness are determined by analyzing the relative sea levels for islands greater than 10 km. The observed sea-level curves in Japan and New Zealand are consistent with the effective viscosity of the upper mantle, equal to 2 to 6 × 1021 poises. The effective elastic plate thickness for Japan and New Zealand is estimated to be less than 50 km and greater than 100 km, respectively. The thickness of the lithosphere obtained by sea-level curves in Japan and New Zealand is compatible with the results obtained by seismological studies.
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