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Constraints from multiple low frequency data on the long wavelength density structure in the deep mantle

多个低频数据对深部地幔长波长密度结构的约束

基本信息

批准号：
1923865
负责人：
Harriet Lau
金额：
$ 59.57万
依托单位：
University of California-Berkeley
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2019
资助国家：
美国
起止时间：
2019-08-01 至 2023-05-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1923865&HistoricalAwards=false
关键词：
Constraints multiple low frequency data

项目摘要

To understand the dynamics of the Earth on a global scale (e.g., how tectonic plates move, resulting in destructive earthquakes and volcanic eruptions), we must understand the nature of the mantle, 3000 km of solid rock that lies beneath the Earth's surface. In particular, two enigmatic dome-shaped structures called the "Large Shear Wave Velocity Provinces" (or LLSVPs) rise from the base of the mantle to around 1000 km above, one beneath Africa and the other beneath the Pacific Ocean. Seismic waves that emanate from earthquakes sometimes travel through these LLSVPs and are slowed down when they enter these structures. It is still unclear why this might be the case: LLSVPs could be hotter (and thus more buoyant) than their surroundings, or they could be chemically distinct (and likely denser) than their surroundings. Both suggest fundamental differences in how the mantle flows over millions of years and therefore how it influences tectonic plate motions and the related natural disasters. The PIs plan to, for the first time, combine information from traditional seismic data sets but also measurements from special types of seismic waves and earth tides to shed new light on the buoyancy of these LLSVPs. The special type of seismic waves are called Stoneley modes and are vibrations trapped along the core-mantle boundary (CMB) and the earth tides are the twice-daily deformation of the solid Earth under gravitational forces from the Sun and Moon. This award will support the training of two graduate students at UC Berkeley and two female PIs, one of whom is an early career scientist and new faculty member at UC Berkeley. The models developed in this project will be made available as part of a webpage specifically developed for this project, as well as through the Incorporated Research Institution for Seismology (IRIS) facility. They will be used in undergraduate workshops at UC Berkeley and at the interdisciplinary CIDER workshop geared towards senior graduate students and early career scientists.Within the last two years, independent studies have presented seemingly contradictory results on the density structure of the large low shear velocity provinces (LLSVPs) imaged by seismic tomography in the earth's deep mantle. In particular, one study used a variety of seismological data (normal mode splitting, seismic travel time and waveform data), and another, semi-diurnal earth tide data, concluding that a significant part of the LLSVPs represent regions of excess density pointing to the source of the LLSVP anomalies being dominated by chemical heterogeneity. In contrast, a study featuring measurements of Earth's Stoneley modes concluded the opposite, implying that the source of LLSVP anomalies are in large part thermal. Both conclusions paint a very different picture of mantle convection: the former implies that the LLSVPs are potentially sluggish upwellings or stabilized piles, while the latter implies a more energetic mode of mantle circulation. In this project, the investigators will combine these different geodetic and seismic datasets - which have different depth sensitivities to elastic and density structure - to establish whether previous results can be reconciled under the hypothesis that the excess density in LLSVPs may be confined to a thin layer (~200 km or less) above the CMB. In doing so, they will investigate further trade-offs with shear and compressional wave speed structure, anisotropy, attenuation, as well as CMB topography and possible structure in the outermost core. First, they will consider a combination of existing global seismic waveform, tide and normal mode splitting data, including Stoneley mode data, and perform inversions for structure in the deepest mantle, using the classical first order mode perturbation formalism for modeling splitting data. They will implement both an optimization and a trans-dimensional Monte Carlo inversion method for this purpose. This, in large part, will act as a guide to prepare for the more advanced inversions to be undertaken in the next stage, while yielding interesting intermediate results. In the second stage, after completing their own dataset of normal mode spectra, they will consider a more rigorous theoretical approach and directly invert mode spectra, together with long period waveform and tide data (complemented by newly acquired data) for density and velocity structure in the vicinity of the CMB.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

为了了解地球在全球范围内的动态（例如，我们必须了解地幔的性质，地幔是位于地球表面之下的3000公里厚的固体岩石。特别是，两个神秘的圆顶状结构称为“大剪切波速度省”（或LLSVP）从地幔底部上升到大约1000公里以上，一个在非洲之下，另一个在太平洋之下。地震产生的地震波有时会穿过这些LLSVPs，当它们进入这些结构时会减慢。目前还不清楚为什么会出现这种情况：LLSVP可能比其周围环境更热（因此更具浮力），或者它们可能比其周围环境化学性质不同（并且可能密度更大）。两者都表明了地幔在数百万年内如何流动以及它如何影响构造板块运动和相关自然灾害的根本差异。 PI计划首次将传统地震数据集的联合收割机信息与特殊类型地震波和地球潮汐的测量结果结合起来，以揭示这些LLSVP的浮力。特殊类型的地震波被称为斯通利波，是沿着核幔边界（CMB）沿着捕获的振动，地球潮汐是固体地球在太阳和月球引力作用下每天两次的变形。该奖项将支持两名研究生在加州大学伯克利分校和两名女PI的培训，其中一人是早期职业科学家和新的教职员工在加州大学伯克利分校。该项目开发的模型将作为专门为此项目开发的网页的一部分，并通过地震学研究所（IRIS）设施提供。他们将被用于本科生研讨会在加州大学伯克利分校和跨学科的CIDER研讨会面向高年级研究生和早期的职业scientists. In过去两年中，独立的研究提出了看似矛盾的结果上的密度结构的大型低剪切速度省（LLSVP）的地震层析成像在地球的深部地幔。特别是，一项研究使用了各种地震学数据（正常模式分裂，地震走时和波形数据），另一个，半日固体潮数据，得出的结论是，一个显着的一部分LLSVP代表区域的过剩密度指向的LLSVP异常的来源是由化学不均匀性占主导地位。相比之下，一项以测量地球斯通利波模式为特征的研究得出了相反的结论，这意味着LLSVP异常的来源在很大程度上是热的。这两个结论描绘了一个非常不同的地幔对流的图片：前者意味着LLSVP是潜在的缓慢上升或稳定的桩，而后者意味着一个更充满活力的地幔环流模式。在这个项目中，研究人员将联合收割机结合这些不同的大地测量和地震数据集--它们对弹性和密度结构具有不同的深度敏感性--以确定在LLSVP中的过量密度可能局限于CMB上方的薄层（约200公里或更少）的假设下，以前的结果是否可以调和。在此过程中，他们将进一步研究剪切波和压缩波速度结构、各向异性、衰减以及CMB地形和最外层岩心中可能的结构的权衡。首先，他们将考虑现有的全球地震波形，潮汐和正常模式分裂数据的组合，包括斯通利波数据，并使用经典的一阶模式扰动形式主义对分裂数据进行建模，对地幔最深处的结构进行反演。为此，他们将实施优化和跨维蒙特卡罗反演方法。这在很大程度上将作为指导，为下一阶段进行更高级的反演做准备，同时产生有趣的中间结果。在第二阶段，在完成他们自己的简正模谱数据集之后，他们将考虑更严格的理论方法，直接反演模谱，结合长周期波形和潮汐数据（补充新获得的数据）该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准。