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

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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.