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

要了解地球的结构和行为，既需要进行地球物理观测，也需要研究据信构成地球内部深处的材料的性质。地震学是观察地球内部的一种特别强大的方法，但要提取地幔和地核的结构、温度和成分，矿物的弹性行为必须在极端温度和压力下已知，从而突破实验的极限。为了避免这个问题，现在使用的大部分数据实际上是通过在原子尺度上对矿物进行计算机模拟而产生的，这种方法在最近几年取得了惊人的成功。然而，这些方法只考虑了矿物的弹性响应--忽略了地震波穿过地球时能量耗散的事实。第二个相关因素也没有通过计算纯弹性响应来提供，即不同频率的地震波以不同的速度传播的事实。为了理解这两种效应，即衰减和色散，我们必须研究非弹性--对地震波传播的时间延迟响应。滞弹性是矿物结构中存在缺陷或缺陷的结果。这些不完美也是地幔长期流动的原因，这一过程导致了板块构造、山脉的增长、火山作用和我们今天看到的许多形成地球的过程。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。通过在原子尺度上研究位错，该项目将使地震学家能够从观测数据中提取更多信息，并将加强我们对地球内部的了解。对衰减和弥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质的其他领域也将具有价值。例如，药物在片剂生产过程中的变形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，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，这一点可能很重要的环境的一个例子是高层大气中云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.