Using Dense Seismic Arrays to Map Sharp Features in the Deep Mantle

随着EarthScope的US阵列和其他高密度地震阵列的出现，研究地球深处的地震结构，一直到核心，都有了前所未有的机会。探测地球最深处的结构对了解地球化学和热史的许多方面提供了基本的限制。这笔赠款将资助进一步开发阵列成像技术，以便更详细地调查在如此巨大的深度如何以及为什么会有如此复杂的结构，以及它们如何与地球的演化相联系。这笔赠款将为一名博士后、一名研究生提供支持，并促进在欧洲的持续国际合作。对下地幔地震相采样的观察表明，超低速带(ULVZ)、可能是钙钛矿-后钙钛矿相关的D“层、大的低剪切速度省(LLSVP)以及核幔边界上原始物质的起伏丘陵(CMB)。在开发与深部地幔地震观测相适应的基于动力学和矿物学的地幔模型方面已经取得了相当大的进展，但许多细节仍然缺乏。目前尚不确定ULVZ的密度和速度是与可能的部分熔化有关，还是有明显的化学来源，但回答这个问题所需的物理性质信息很少受到限制。例如，用于成像ULVZ的主要相位SPdKS对密度不是特别敏感，并且速度与结构的形状有很强的权衡。其他地震观测也显示了观测到的D？层：后钙钛矿层在快速区下最厚，在慢速区下消失。然而，地幔底部俯冲大洋岩石圈引起的速度梯度效应与相边界效应很难区分。来自美国阵列和其他阵列的数据集，特别是更大的毕加索(部分由NSF-CD和Career资助)阵列，显示了所有范围内的波形复杂性，覆盖范围显著。这笔赠款将提供资金，以更好地约束地幔结构，并回答以下问题：(1)我们能否解决和区分不同弹性性质的权衡？(2)我们能否确定ULVZ的3D几何形状？(3)除了ULVZ之外，CMB还存在其他类型的低速带吗？(4)推断的板块历史与存在下地幔结构的情况如何？(5)D与D之间的关系是什么？以及深部地幔中的板块？(5)我们能多好地分辨LLSVP的边缘？为了回答这些问题，我们将使用阵列地震学技术、波形建模和新开发的多路径探测器(MPD)来增强当前的层析模型，并产生与观测波形复杂性相匹配的合成地震记录。这些构造在下地幔中的物理尺寸及其推断的地震参数将被用于地幔对流的地球动力学模型，并在CMB条件下的矿物物理实验中进行探索。这些进一步的矿物物理和地球动力学实验将有助于我们更好地理解俯冲板块之间的联系，D？层、ULVZ和LLSVP与深部地幔的动态演化

With the advent of EarthScope's USArray and other dense seismic arrays, there is an unprecedented opportunity for investigating the seismic structures within the deep Earth, all the way to the core. Detecting structures in the very deepest part of the Earth provide essential constraints in understanding many aspects of the chemical and thermal history of the planet. This grant will fund further development of array imaging techniques that will allow for more detailed investigation of how and why there are such complex structures at this great depth and how they link to the evolution of the Earth. The grant will provide support for a postdoc, a graduate student, and foster continued international collaborations in Europe.Observations of seismic phases sampling the lower mantle have suggested Ultra Low Velocity Zones (ULVZs), a possible perovskite to postperovskite related D" layer, Large Low Shear Velocity Provinces (LLSVPs), and rolling-hills of primordial material on the core-mantle boundary (CMB). Considerable progress has been made toward developing dynamical and mineralogical based mantle models compatible with seismic observations at the deep mantle, yet many details are still missing. It is still uncertain whether the densities and velocities of ULVZs are related to possible partial melting or have a distinct chemical origin, but the information about physical properties needed to answer this question are poorly constrained. For example the major phase used in imaging ULVZs, SPdKS, is not particularly sensitive to density and the velocities have strong trade-off with the shape of the structures. Other seismic observations have shown connections between the geographic distribution of the observed D? layer: where the post-perovskite layer is thickest beneath fast regions and disappears beneath slow regions. However, it is difficult to distinguish the velocity gradient effects caused subducted oceanic lithosphere at the base of the mantle from phase boundary effects. The data set from USArray and other arrays, especially the greater PICASSO (partially funded by NSF-CD and CAREER) array, display waveform complexity at all ranges with remarkable coverage. This grant will provide funding to better constrain mantle structures and answer questions such as: (1) Can we resolve and distinguish between the trade-off of different elastic properties? (2) Can we determine the 3D geometry of the ULVZ's? (3) Are there other types of low velocity zones besides ULVZs existing at CMB? (4) How compatible are the inferred slab histories with the presence of lower mantle structures? (5) What is the relationship between D? and slabs in the deep mantle? (5) How well can we resolve the edges of the LLSVPs? To answer those questions, we will use array seismology techniques, waveform modeling and the newly developed Multi-Pathing Detector (MPD) to sharpen current tomographic models and produce synthetic seismograms that match observed waveforms complexities. The physical dimensions of these structures in the lower mantle and their inferred seismic parameters will be used in geodynamic models of mantle convection and explored in mineral physics experiments at CMB conditions. These further mineral physics and geodynamic experiments will allow for improvement in our understanding of the connection between subducted slabs, the D? layer, ULVZs, and LLSVPs and the dynamic evolution in the deep mantle.