Collaborative research: Structure and dynamics of the ultralow-velocity zone at the core-mantle boundary

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。地球岩石地幔和金属地核之间的边界是部分熔融结构拼凑的家园。这些构造以尖峰和谷为特征，相对较薄（10 ~ 10 km厚），密度大（比围岩密度大10%），在空间上与下地幔的上升流区域相关。地震波在穿过这些结构时速度会减慢。因此，这些结构的指纹在穿过地球内部深处的地震信号中表现为超低速带（ULVZ）。除了它们独特的地震特征外，ULVZ在化学上也可能与周围的地幔岩石不同。超低震源远不是被动的，它与上覆地幔的剧烈运动紧密相连，这种运动消散了地球内部的热量，推动了板块构造，并导致了地表的火山活动。本研究将通过对地震信号的研究来揭示超空腔内的精细结构，利用流体力学原理模拟超空腔内部分熔融物质剧烈运动产生的结构，并在模拟超空腔结构中产生合成地震信号。这些研究的结果将回答一些关于热和物质在地核-地幔边界上的传输以及超极空间的化学性质和物理性质的基本问题。高分辨率地震ScP和PcP波形数据提供了关于ULVZ局部结构的丰富信息。这种位于地幔底部的薄而高密度的层很可能在确定从核-幔边界产生的地幔柱的位置和稳定性方面发挥重要作用。此外，这个区域也可能是地球外核和地幔之间任何正在进行的质量传递的一个极其重要的阶段。在这个项目中，我们建议对ULVZ的内部结构和动力学进行地球动力学-地震联合调查。我们利用现有的和新的高分辨率ScP和PcP波形迁移和建模，以更好地约束核幔边界底部ULVZ内的波速、层状地形和密度对比。我们将开发一个两阶段的地球动力学模型，首先将极极带建模为在地幔底部扩展的薄的自重力层。利用该重力流模型和观测到的地形，我们可以预测极空区粘度，并推断极空区内的熔化程度。在第二阶段，我们将建立一个岩浆混合和ULVZ内小尺度对流的多相、多组分模型。所得到的模型提供了在极低频区与外核和下地幔的熔体储存和传质的约束条件。所得模型将被构建到二维和三维正演合成地震图模拟中，以测试与观测数据的一致性，并通过识别地表可见的诊断波形效应，更好地指导未来对ULVZ的地震调查。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)The boundary between Earth's rocky mantle and metallic core is home to a patchwork collection of partially molten structures. These structures, characterized by sharp peaks and valleys, are relatively thin (10s of km in thickness), dense (up to 10% denser than the surrounding rocks), and are spatially correlated with regions of upwelling flow in the lower mantle. Seismic waves slow down as they pass through these structures. Consequently, the fingerprint of these structures appear as UltraLow Velocity Zones (ULVZ) in seismic signals traveling through Earth's deep interior. Besides their unique seismic signature, ULVZ are also likely to be chemically distinct from the surrounding mantle rock. Far from being passive, ULVZ are strongly coupled with the vigorous motion in the overlying mantle that dissipates Earth's internal heat, drives plate tectonics, and causes volcanism on the surface. This research will study seismic signals to unravel the fine scale structure within the ULVZ, simulate the structure arising from vigorous motion of the partially molten material within the ULVZ using principles of fluid mechanics, and create synthetic seismic signals traveling through the simulated ULVZ structure. The result of these investigations will answer a number of fundamental questions on heat and matter transport across Earth's core-mantle boundary and the chemical nature and physical properties of the ULVZ.High resolution seismic ScP and PcP waveform data provides a wealth of information regarding the local structure of the ULVZ. Such thin, high density layers at the base of Earth's mantle are likely to play a major role in determining the location and stability of mantle plumes generating from the core-mantle boundary. In addition, this region is also likely to serve as an extremely important stage in any ongoing mass transfer between Earth's outer core and the mantle.In this project we propose to develop a combined geodynamic-seismic investigation of the internal structure and dynamics of the ULVZ. We employ existing and new high resolution migration and modeling of ScP and PcP waveforms to better constrain wave velocities, layer topography, and density contrasts within ULVZ at the base of the core mantle boundary. We will develop a two-stage geodynamic model, beginning by modeling the ULVZ as a thin, self gravitating layer spreading at the bottom of the mantle. Using this gravity current model and the observed topography, we can predict the viscosity of the ULVZ and also infer the extent of melting within the ULVZ. In the second stage, we will develop a multiphase, multicomponent model of magma mixing and small scale convection within the ULVZ. The resulting model provides constraints on melt storage and mass transfer within the ULVZ both with the outer core and lower mantle. The resulting models will be built into 2D and 3D forward synthetic seismogram simulations to test consistency with observed data and to better guide future seismic investigation of ULVZ by identifying diagnostic waveform effects visible at the surface.