Thermal & compositional structure of Earth’s inner core using free oscillations
European Research Council Starting Grant
October 2008 to September 2014
Description
The core, comprising the innermost parts of the Earth, is one of the most dynamic regions of our planet. The inner core is solid, surrounded by an outer core of a liquid iron alloy. Inner core solidification combined with motions in the fluid outer core drive the geodynamo which generates the Earth’s magnetic field. Solidification of the inner core also supplies some of the heat which drives mantle convection and subsequently plate tectonics at the surface of the Earth. The thermal and com- positional structure of the inner core is thus key to understanding the inner workings of our planet.
No direct samples can be taken of the Earth’s outer and inner core and our knowledge of its thermal and compositional state relies on seismology, which is the only technique that can ‘see through’ the Earth and measure its elastic parameters and density. Ray theoretical studies using short period body waves provide the most commonly used seismological data; these have led to a large range of veloc- ity models of the Earth’s inner core, including concepts such as anisotropy, layers and hemispherical variations. However, body waves only sample a few small regions of the inner core due to uneven station and earthquake distribution, and therefore the robustness and global distribution of these fea- tures are still controversial. Long period seismic free oscillations, or normal modes, on the other hand, are able to provide global constraints and have been used to study elastic anisotropy. In addi- tion, only normal modes can provide constraints on density structure, which is essential to determine the core’s thermal and compositional state. Unfortunately, lack of appropriate theory has prevented more complicated structures and regional variations from being studied using normal modes. As a results, many fundamental questions remain unanswered, including: Which light elements are present and explain the density of the core? What causes inner core anisotropy? Which high pressure phase of iron is stable in the inner core? And how does inner core structure relate to the magnetic field?
Here, I propose (i) to develop such theories, using fully coupled normal mode synthetics for the first time, and (ii) to apply the new techniques to develop a comprehensive model of inner core structure using data from large earthquakes, such as the Sumatra-Andaman event of 26 December 2004. My theoretical expertise in normal mode coupling, and my previous research on observation and interpretation of seismological data, as well as the fluid dynamics expertise available in Cambridge, place me in a unique position to carry out this project. Using this novel combination of sub-disciplines, I will apply mineral physics and fluid dynamics to interpret the thermal and compositional nature of the structures found at the centre of our planet, which in turn are fundamental to understand its geodynamo and magnetic field.
