Attenuation of the uppermost inner core viewed by tomography

Abstract:

The inner core of the Earth remains one of the research puzzles within deep Earth sciences. While it has been extensively probed with various seismic methods, ranging from observing differential rotation to travel-time anomalies and attenuation tomography of its uppermost part, there is still no consensus on the structure and dynamics of the inner core that could explain all observations. A hemispherical structure of the inner core was the prevalent theory that explained seismic travel-time anomalies and the attenuation estimates, for the past two decades. The boundaries of the hemispheres were defined in a pioneering study by Tanaka and Hamaguchi (1997) and accepted by the scientific community in some of the later studies. Indeed, the hemispherical heterogeneity was observed in both isotropic velocity and attenuation, with some observations of high attenuation correlating with high velocity, and vice-versa. Various geodynamical models have also been proposed to account for the observed hemispherical dichotomy. However, over the last few years recent studies have yielded results that depart from this traditionally-accepted view of the inner core. Our work focuses on attenuation tomography, which complements travel-time tomography in that it should be able to better resolve temperature variations in the top of the inner core. We performed the tomography using both traditional approaches and Transdimensional Bayesian Inversion on a spherical surface, the first such approach applied to a problem in global seismology. This type of inversion does not require an explicit parameterization or regularization of the model. Bayesian solution provides a whole ensemble of models and allows for more comprehensive assessment of model parameters and their uncertainties. Our results show high attenuation in what was previously considered the quasi-Eastern hemisphere, but what was previously considered the quasi-Western hemisphere is more complex. In fact, the quasi-Western hemisphere can be divided into two regions: one with low and the other with high attenuation. These, and more recent, results depart significantly from the purely hemispherical heterogeneity. We will discuss these observations in the context of recently-developed geodynamical models and the inner core connection with the outer core and mantle.