地震层析地球内部密度分布横向不均匀研究

SEISMIC TOMOGRAPHY: INTERNAL HETEROGENEITY OF MANTLE AND ITS GEODYNAMICAL INFERENCES

本文使用Woodhouse和Dziewonski等分别用S波和P波层析分析提供的,以670km深度为分界的上地幔和下地幔地震波传播所反映出的横向不均匀的球谐函数系数,假定地幔内部密度与正常水平的偏离分别与P波和S波的速度V_p和V_s成比例(下地幔δρ=γδV_p,上地幔δρ=σδV_S^2),并用一组新的球谐函数系数表示全地幔密度分布模型.计算结果表明:1.比例系数γ=0.199,σ=3.13×10^(-5)时,该全地幔密度模型可以70％恢复Dziewonski提供的下地幔密度模型,50％恢复Woodhouse提供的上地幔密度模型.合成密度分布基本上反映了分别由P波和S波速度异常所提供的地幔密度分布特征;2.所有穿过太平洋中部的剖面均明显地显示出一个低密度异常从地表一直延伸到核-幔边界;3.所有大陆下部均存在一个高密度异常区,而且深入到1200—1300km深度处;4.由密度异常的正、负交替基本上将部分地区地幔分成为3层,其深度依次为:第1层25—1300km左右,第2层1300—2300km左右,第3层2300km,为核-幔边界.

The seismic tomography of the mantle provides a series of new observation data for us to investigate the geophysical and geodynumical problems. In this paper we assume that the density anomalies of the mantle are proportional to the interal velocity hetrogeneity computed by seismic tomography of P and S waveforms. The distributions of density anomalits can be expanded by new seriers of harmonics. Results show that 1. computed mantle density anomalies can mostly recover the density distrioutions calculated by Dziewonski and Woodhouse's work, 2. there is a lower density anomaly from the surface of the earth to the core-mantle boundary in the region of the center of the Pacific Ocean, 3. there are higher density anomalies beneath continents extending to the depth 1 000-1 300 km and there are three density anomaly layears (25-1 300 km, 1 300-2 300 km, 2 300-CMB depth) in the most region of the mantle. These results are probably usefull for us to investigate Geodynamics of tht mant le and understand the nature of the Earth.