The Contribution of the Upper Crust to Global Dynamic Flattening
The Contribution of the Upper Crust to Global Dynamic Flattening
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发表时间：
2008
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通讯作者：
Liu Yu;Huang Cheng-li
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作者：
Liu Yu;Huang Cheng-li
Global dynamic flattening(H)is an important parameter in the study of the Earth rotation.Precession observations give H_(obs)=0.0032737≈1/305.5.The geometrical flattening (f)profile of the Earth interior is recalculated from potential theory in hydrostatic equilibrium (HSE)state,and the results coincide with that of Denis(1989).In this paper,H expression has been derived to third-order accuracy,and the value of H obtained from PREM earth model is 1/308.5.This coincides with other similar works,in all of which there is a difference about 1% between them and the observed value. In order to study where the difference come from,the homogenous uppest crust and oceanic layers in PREM are replaced by some real surface layers data,such as oceanic layer(ECCO model),topography data(GTOPO30 model),crust data(CRUST2.0 model)and mixed data (ETOPO5 model).According to the depth range of each model,three Earth models have been constructed with real surface data from PREM,ECCO,GTOPO30 and so on.Our attention is focused on the effect from non-uniform distribution of mass in the real surface layer,while the effect from the change of the gravitational field inside the Earth is ignored in our calculation. So,it is simply assumed that the shapes of the equipotential and equidensity surfaces in our new constructed models are as the same as in PREM. From results of the dynamic flattening of each model,the mass of surface layers is less than 0.1% of the whole Earth,but the real surface layers can reduce the global dynamic flattening from 1/308.53 to 1/318.14(about 3%).It is really a large effect and related to the fact that dynamic flattening is associated with not only how much the mass is but also how the mass distributed. The surface layers is the outermost layer of the Earth and plays an important role. If the surface layers are replaced from the surface to 5.615 kilometers under the mean sea level(ECCO),H deviates from that of PREM.When the surface layers are replaced deeper till 10.376 kilometers under the mean sea level(ETOPO5),H deviates more.But when go further till 70.137 kilometers under the mean sea level,the value of H is being enlarged and deviates less than that of above two models.The isostasy theory maybe can explain why this happened. There are several compensation models in isostasy theory.All of them thinks that the mass above the geoid will be counteracted by the loss of mass under geoid with some kinds.There is a compensation level in most compensation models.The compensation level is under geoid and the depth of it is assumed from 30 kilometers to 100 kilometers for different models.When it is replaced that the volume from surface to 5.615 kilometers depth,there is no mass loss under the geoid to counteract the mass above the geoid,so the real lands distribution makes H deviate from H_(PREM).When it goes further to 10.376 kilometers depth,there is still no mass loss under the geoid,because the depth we replaced is not reach the depth of compensation level,things become more serious.But when it reachs 70.137 kilometers depth,mass loss under the geoid and counteracting effect appear,and it drags back H from 1/320.22 to 1/310.70.Although the theory of isostasy can explain the difference among three models,there is still a difference between the calculated value and the observed value.
