Investigation of the earth's mantle plumbing system at the global scale using an advanced seismic imaging approach.

通过地震成像提高地幔结构的分辨率，对于我们理解反映在地表构造中并最终引发地震和火山喷发的深部动力学过程具有重要意义。地震层析成像利用源自天然地震的地震波来照亮地球内部，其原理与医学成像中使用的原理类似。虽然长波、光滑的结构现在得到了很好的记录，但我们努力使图像变得清晰，并解决较小尺度的特征，这些特征将有助于详细了解内部环流如何驱动构造板块的运动，反之亦然，并更好地了解热量如何从深部内部输送到表面，从而不仅导致大洋中脊火山系统，而且还在板块中部火山被称为“热点”，如夏威夷链。由于高于平均温度的区域对应于低地震速度，这一点尤其难以成像，因此直到最近，随着精确计算穿过地幔的地震波场的数值方法的出现，它们的准确成像才成为可能。该项目的主要目标是最终建立更高分辨率(横向可达200-400公里)的全球地幔三维径向各向异性剪切速度模型，该模型基于一种到目前为止仅应用于上地幔的时间域波形反演方法，具有良好的效果。这种波形反演方法建立在基于模式的波场计算的波形层析成像中PI的20年经验的基础上，以及使用长周期基模和泛音波形(周期60s)和数值波场计算(谱元素法)开发的两代全球上地幔剪切速度模型。为了解析下地幔结构，有必要隔离包含体波能量的波包，因此我们建议将我们的数据集的周期范围扩大到30s-这也将进一步提高上地幔的空间分辨率，同时保持计算的可管理性。该方案的进一步目标是：(1)在整个地幔径向各向异性的横向变化中获得更高的分辨率；(2)测试30个S数据集对全球平均S/P速度转换因子的敏感性；(3)收集低至20s的额外的、更短的窗口(起点时间后约30mN)波形，以包含更多的P波能量，并进行额外的迭代，以获得全球下地幔的P速度模型。

Improving the resolution of earth's mantle structure through seismic imaging is important for our understanding of the deep dynamic processes that are reflected in surface tectonics and ultimately give rise to earthquakes and volcanic eruptions. Seismic tomography utilizes seismic waves originating from natural earthquakes to illuminate the earth's interior using similar principles as used in medical imagery. While the long wavelength, smooth structure is now well documented, we strive to sharpen the picture and resolve smaller scale features that will help understand in detail how the internal circulation drives the motions of tectonic plates, and vice-versa, and obtain a better view of how heat is channeled from the deep interior to the surface, resulting not only in the system of volcanoes mid-ocean ridges but also in mid-plate volcanoes called "hotspots", such as the Hawaiian chain. Because hotter than average regions correspond to low seismic velocities, which are particularly difficult to image, their accurate imaging has not been possible until recently, with the advent of numerical methods to accurately compute the seismic wavefield propagation across the earth's mantle. This project builds upon our recent efforts to develop global seismic images of the entire mantle using such computations, and improve the sharpness of these images.The main goal of this project is to finalize the construction of higher resolution (up to 200-400 km laterally) global 3D radially anisotropic shear velocity model of the whole mantle based on a time domain waveform inversion approach that has so far been applied only to the upper mantle, with promising results. This waveform inversion methodology builds upon 20 years of experience of the PI in waveform tomography using mode-based wavefield computations, as well as two generations of global upper mantle shear velocity models developed using long period fundamental mode and overtone waveforms (periods 60s) and numerical wavefield computations (Spectral Element Method). In order to resolve lower mantle structure, isolating wavepackets that contain body wave energy is necessary, therefore we propose to extend the period range of our dataset to 30s - this will also increase spatial resolution further in the upper mantle, while keeping computations manageable. Further goals of the proposal are to (1) obtain higher resolution in the lateral variations of radial anisotropy throughout the mantle, (2) test the sensitivity of the 30 s dataset to inversion for a global averaged S/P velocity conversion factor and (3) collect additional, shorter window (~30mn after origin time) waveforms down to 20s to include more P wave energy and perform additional iterations to obtain a global P velocity model of the lower mantle.