利用低阶卫星重力位系数研究下地幔横向密度异常分布  

DISTRIBUTION OF LATERAL DENSITY ANOMALIES IN THE LOWER MANTLE OBTAINED THROUGH LOW ORDER GEOPOTENTIAL COEFFICIENTS

作　　者：王石任 李瑞浩[1] 

机构地区：[1]国家地震局地震研究所,中国武汉430071

出　　处：《地震学报》1991年第1期53-65,共13页Acta Seismologica Sinica

摘　　要：本文试图采用卫星重力资料和一种新的反演方法来研究地幔的横向密度异常分布.先将密度异常△ρ(r,θ,(?))在一个三维正交函数系下进行展开,其展开系数待定.然后,根据密度异常与重力扰动位之间的关系建立观测方程组,其中未知向量由密度异常展开系数组成,重力扰动观测向量由 GEM_(10)B 重力模型中的位系数计算而得,并通过适当选取重力位系数的阶数,对观测向量进行滤波.最后,就下地幔(670km—CM 界面)作了实际计算.计算中,重力扰动位阶数取为2—11阶,密度异常展开式的截断阶数取为 K=4和 L=6,求解观测方程组时采用阻尼最小二乘法.结果表明:密度扰动值在670km 不连续面及核幔界面处达到极大值,且在环太平洋地区存在一高密度带,太平洋中部对应于一低密度区,这些特征与 Dziewonski 得到的下地幔三维波速异常分布特征相一致.但是,在南极地区、大西洋及印度洋部分地区,所得的密度异常分布与三维波速异常分布呈负相关,文章就其原因作了初步分析.In this paper, we attempt to use satellite gravity data and a new inversion method to study the lateral density anomalies distribution in the mantle.First, the density difference Δp(r, θ, (?)) is expanded in terms of a three-dimensional orthogonal function system, the coefficients of the expansion are to be determined. From the relationship between density anomalies and disturbing geopotential, then, a set of observation equations is established, in which the unknown vector contains the coefficients of density anomaly expansion, and the observational vector is obtained by computing geopotential perturbations using the potential coefficients of GEM10B. By suitably selecting the harmonic orders of geopotential, a filtering process is conducted for the observational values. Finally, the lateral density variations in the lower mantle (670 km to the core-mentle boundary) are investigated. In this case, the orders of disturbing geopotential are selected as l = 2-11, the truncated orders of density anomaly expansion are taken as L = 6 and K - 4, and the damping least squares method is used to solve the observation equations.The resulting model shows the high level of density perturbations at the 670 km discontinuity and core-mantle boundary, a high-density zone circumscribing the Pacific and a lower-density region under the center of the Pacific. These features are in agreement with the features of three-dimensional seismic velocity variation obtained by Dziewonski (1984). In the Antarctic region and some parts of the Atlantic and Indian Oceans, however, the resulting density anomalies are negatively correlated with the seismic velocity anomalies. The cause resulting in these phenomena is preliminarily analysed in this paper.

关 键 词：地震 重力扰动 横向密度 下地幔 

分 类 号：P315.2[天文地球—地震学]