Simulations of dislocation-mediated deformation, attenuation and dispersion in the Earth's mantle

要了解地球的结构和行为，既需要地球物理观测，也需要研究被认为构成地球内部深处的物质的性质。地震学是观察内部的一种特别有效的方法，但是要提取地幔和地核的结构、温度和组成，必须知道矿物在极端温度和压力下的弹性行为，这就突破了实验的极限。为了避免这个问题，现在使用的许多数据实际上是由计算机模拟原子尺度上的矿物产生的，这种方法近年来取得了惊人的成功。然而，这些方法只考虑了矿物的弹性反应，而忽略了地震波穿过地球时能量耗散的事实。计算纯弹性响应时也没有提供第二个相关因素，即不同频率的地震波以不同的速度传播。为了理解这两种效应，即衰减和色散，我们必须研究非弹性，即地震波通过时的延时响应。非弹性是矿物结构中存在缺陷或缺陷的结果。这些缺陷也是导致地幔长期流动的因素，这一过程导致了板块构造、山脉的形成、火山活动和许多形成我们今天所看到的地球的过程。该项目的关键是能够模拟含有缺陷的晶体，特别是一种称为位错的缺陷。通过研究原子尺度上的位错，该项目将允许地震学家从观测数据中提取更多信息，并将增强我们对地球内部的了解。对衰减和色散的原因有了扎实的了解，就有可能从观测中提取更多关于地球内部性质的信息。这项研究在位错改变材料性能的其他领域也有价值。例如，药片生产过程中药品变形的问题可以通过研究地球内部的方法来解决。此外，用于计算机芯片、催化剂、燃料电池和电池的材料中的位错会改变这些材料的行为，而这些方法可以用来解决所有这些问题。最后，晶体表面出现的位错允许在表面发生反应，这可能很重要的一个环境例子是上层大气中云层中的污染物和冰之间的相互作用。

Understanding the structure and behavior of the Earth requires both geophysical observation and studies of the properties of the materials which are believed to comprise the deep interior. Seismology is a particularly powerful way to observe the interior, but to extract the structure, temperature and composition of the mantle and core the elastic behavior of minerals must be known to extreme temperature and pressure, pushing the limits of experiment. To sidestep this problem much of the data now used is actually generated by computer simulation of minerals performed at the atomic scale and this approach has been phenomenally successful in recent years. However, these approaches have only considered the elastic response of the minerals - the fact that energy is dissipated as seismic waves pass through the Earth is ignored. A second related factor is also not provided by calculating the pure elastic response, the fact that seismic waves with different frequencies travel at different speeds. To understand these two effects, which are known as attenuation and dispersion, we must study anelasticity - the time delayed response to the passage of the seismic wave. Anelasticity is the result of the presence of imperfections or defects in the mineral structure. These imperfections are also the agents responsible for long term flow in the mantle, a process which results in plate tectonics, growth of mountain ranges, volcanism and many of the processes that make the Earth we see today. Key to the project is the ability to simulate crystals containing defects and especially a type of defect called dislocations. By studying dislocations on the atomic scale the project will allow seismologists to extract more information from the observed data and will enhance our understanding of the Earth's interior. With a solid understanding of the causes of attenuation and dispersion it will be possible to extract much more information about the nature of the Earth's interior from observation. The research will also be valuable in other fields where dislocations alter the properties of materials. For example, the problem of deformation of pharmaceuticals during the production of tablets could be addressed by the methods developed for the study of the Earth's interior. Furthermore dislocations in materials used in computer chips, catalysts, fuel cells and batteries alter the behavior of these materials and the methods can be used to tackle all these problems. Finally, dislocation emerging at the surface of crystals allow reactions to take place on the surface, one example of an environment where this could be important is the interaction between pollutants and ices in clouds in the the upper atmosphere.