Determination and analysis of long-wavelength transition zone structure using SS precursors
Determination and analysis of long-wavelength transition zone structure using SS precursors
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DOI：
10.1111/j.1365-246x.2008.03719.x
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发表时间：
2008-07-01
影响因子：
2.8
通讯作者：
Shearer, P.
中科院分区：
文献类型：
--
作者：
Houser, C.;Masters, G.;Shearer, P.
410公里和660公里不连续地形和过渡带厚度的全球制图已被证明是约束地幔化学、动力学和矿物学的有力工具。大量的地震和矿物物理研究表明，410公里的不连续是橄榄石向水滑石相变的结果，660公里的不连续是环木岩向钙钛矿和镁闪石相变的结果。410公里和660公里不连续面的下侧反射作为SS的前兆到达。随着最近一种确定SS到达的半自动方法的发展，我们已经将Flanagan和Sheeller(1998a)精选SS波形的数据集增加了两倍以上。我们能够通过将波形堆叠在5度而不是10度半径箱中来提高分辨率，并显着增加南半球的数据覆盖率。由此得到的SS-S410S和SS-S660S时间受上地幔速度结构的影响很大。我们对不连续地形和速度非均质性进行了联合反演，并对前兆差分时间进行了简单的速度校正，发现两种方法几乎没有区别。660公里的不连续地形和过渡带厚度与过渡带内的速度相关，而410公里的不连续地形与过渡带的速度无关。此外，410公里的不连续地形与660公里的不连续地形并不像预期的那样相关，而是反相关的，因为它们各自的相变的克拉珀隆坡度有相反的迹象。这些结果表明，660公里不连续面的地形可能是由热效应主导的，而410公里不连续面的地形可能是由成分效应主导的。此外，与以前的研究不同，我们的410公里和660公里的地形具有相似的幅度，而410公里和660公里的不连续面上的地形比660公里不连续上的地形少。
Global mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry, dynamics and mineralogy. Numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite. Underside reflections of the 410 and 660 km discontinuities arrive as precursors to SS. With the recent development of a semi-automated method of determining SS arrivals, we have more than tripled the Flanagan and Shearer (1998a) data set of handpicked SS waveforms. We are able to increase resolution by stacking waveforms in 5 degrees rather than 10 degrees radius bins as well as increasing data coverage significantly in the southern hemisphere. The resulting SS-S410S and SS-S660S times are heavily influenced by upper-mantle velocity structure. We perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods. The 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not. In addition, the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography, rather anticorrelated, as expected due to the opposite signs of the Clapeyron slopes of their respective phase changes. These results suggest that, whereas the topography of 660 km discontinuity could be dominated by thermal effects, the topography of the 410 km discontinuity is likely dominated by compositional effects. In addition, unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity, our 410 and 660 km topography have similar amplitudes.