Attenuation Tomography of the Inner Core

Abstract:

We perform attenuation tomography of the Earth's uppermost inner core (~300 km below the inner core boundary) by means of linear inversion. By parameterising the uppermost inner core into blocks, each of which has a certain fixed value of quality factor Q, we performed synthetic tests assuming different attenuation models. For this we use a dataset of 51 global events. Synthetic tests showed that using current dataset and block parameterisation of 30 and 45 degrees it is possible to recover the underlying attenuation anomaly pattern in linear inversion. Attenuation models used for these tests include north-south hemispherical model with boundary at the equator, hemispherical model with boundaries defined by Tanaka and Hamaguchi (1997), a checkerboard model and various combinations of meridional and zonal sections of interchanging Q. All of these models are successfully recovered in linear inversion. Using the simulated annealing algorithm we obtain t* measurements from 51 events and use these observed data to perform tomography of the uppermost inner core.

However, the limitation of block parameterisation is imposing strict boundaries in the model. We wish to avoid subjective parameterisation of the boundary between low and high Q values in the inner core and for this reason we employ different inversion methods. In this way we hope to resolve some of the ongoing debates regarding the attenuation structure of the inner core and mechanisms thereof, with a stress on recent observations pointing to a degree-two heterogeneity (Attanayake et al, 2013; Iritani et al, 2014) opposing the previous view of degree-one heterogeneity. By expanding our dataset including more global events we also aim to improve the resolution of our results.