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Thermal and compositional state of the Earth's inner core from seismic free oscillations

Final Report Summary - EARTH CORE STRUCTURE (Thermal and compositional state of the Earth's inner core from seismic free oscillations)

The innermost parts of our planet, the Earth's solid inner core and fluid outer core, form a very dynamic region. Inner core solidification combined with fluid motions in the outer core drive the geodynamo generating Earth’s magnetic field. Some of the released heat also provides the energy for mantle convection and plate tectonics. Thus, the structure of the Earth's core is key to understanding the inner workings of our planet. While we have a good understanding of regional variations at the Earth's surface, only a few hints of regional variations had been discovered in the inner core. In this project, we developed novel theoretical techniques and used these to image the Earth's inner core and determine its regionally varying structure and their dynamic and compositional origins.

We illuminated Earth's deep structure using seismic waves from earthquakes, similar to making brain scans. Our special interest is in using whole Earth oscillations, which are generated by large earthquakes like the devastating 2011 Japan Tohoku earthquake and are similar to the 'ringing of a bell'. Their advantage over the commonly used body waves is that they do not suffer from limited earthquake and seismometer locations, which becomes especially problematic for imaging the inner core. We developed pioneering theoretical tools, using 'resonance' between different oscillations, which allowed us to focus on specific parts of our deepest planet which had not been possible before due to lack of appropriate theory. Our new methodologies will continue answering many unresolved issues in global seismology, well beyond the duration of our ERC project. I have also made a new catalog of normal mode observations. Among its different uses, this catalog formed, for example, one of the three seismic data sets in a collaborative effort to develop a new global mantle model and also has been the main backbone for my inner core research.

Our work shows that the top of the inner core is divided into two hemispheres, which have very sharp boundaries and are so different they might be the equivalent of the continental and oceanic regions at the Earth's surface. These observations limit the amount of inner core super rotation to be much slower than previously thought. Our new methodologies revealed that the anisotropy displays more complicated regional variations than a simple Eastern versus Western hemispherical pattern. The inner core is divided into parts, like the pieces of an orange, with very strong anisotropy located in a narrow wedge located right under North and South America, indicating that the magnetic field may be important in shaping inner core anisotropy.