CSEDI Collaborative Research: 3D Temperature and Composition Structure of the Upper Mantle Using Seismological and Mineral Physics Constraints

这是加州大学伯克利分校的地震学家、矿物物理学家和地球动力学家之间的一项合作研究。其目标是将各自领域的观测约束和理论工具结合起来，直接从地震波形的反演中研究地球上地幔温度和主要元素化学的大范围三维变化。这项研究的核心是这样一种猜想，即在理论考虑的支持下，对地震波形进行成分C和温度T等物理参数的一步反演，应该会产生比首先分别反演速度和衰减结构，然后根据物理参数解释所获得的图的标准方法更稳定的结果。后者目前被3D上地幔衰减Q的低分辨率所抑制，这是由于大部分未建模的但显著的聚焦和散射效应污染了幅度。一步法中方程的耦合特性，再加上振幅对T的强烈敏感性(通过滞弹性项)，应该会抵消现有矿物物理偏导数的近似特性以及由于聚焦理论处理不充分而造成的误差。研究人员将：1)利用最近关于地幔顶部弥散和衰减的实验约束条件，反演T和径向各向异性的全球长周期地震波形集合；2)从(1)中获得的模型开始，他们将测试他们对成分横向变化(以玄武岩亏损表示)的反演能力。团队中的矿物物理学家将使用结合了实验结果和第一性原理计算结果的自洽形式来评估和提供关于C的偏导数及其不确定度。在这一点上，他们还将纳入地球动力学约束，以提高反演成分的分辨率。由于强衰减和相变的存在，各种偏导数对温度的依赖，以及在估计这些偏导数时仍然存在很大的不确定性，因此在反演过程的每一步都需要迭代程序。该项目包括资助两名学生和一名博士后，他们将接受跨学科方法的培训。

This is a collaborative study between seismologists, mineral physicists and geodynamicists at UC Berkeley and at the Univ. of Michigan, the goal of which is to combine observational constraints and theoretical tools from their respectivefields and investigate large scale 3D variations in temperature and major element chemistry in the earth's upper mantle directly from the inversion of seismic waveforms. At the heart of this proposed study is the conjecture, supported by theoreticalconsiderations, that the one step inversion of seismic waveforms for physical parameters such as composition C and temperature T, should yield more stable results than the standard approach of first inverting for velocity and attenuation structure separately, and then interpreting the obtained maps in terms of physical parameters. The latter is currently inhibited by the poor resolution in 3D upper mantle attenuation, Q, due to contamination of amplitudes by largely unmodelled, butsignificant, effects of focusing and scattering. The coupled character of the equations in the one step approach, combined with the strong sensitivity of amplitudes to T (through anelastic terms) should offset the approximate character of the available mineral physics partial derivatives as well as the errors due to inadequate theoretical treatment of focusing.The investigators will: 1) invert a global collection of long period seismic waveforms for T and radial anisotropy, using recentexperimental contraints on dispersion and attenuation in the uppermost mantle; 2) starting from the model obtained in (1), they will test their ability to invert for lateral variations in composition (as expressed in terms of basalt depletion). Partial derivatives with respect to C and their uncertainties will be evaluated and provided by the mineral physicists in the team using a self-consistent formalism which incorporates results from experiments and first principles calculations. At this point, they will also incorporate geodynamical constraints to increase resolution of the compositional component of the inversion. An iterative procedure will be required at each step in the inversion process, because of the dependence of various partial derivatives on temperature, due to the presence of strong attenuation and phase changes, as well as the large uncertainties remaining in the estimation of these partial derivatives. The project includes funding for two students and a post doc, who will be trained using a cross-disciplinary approach.