Seismic structure of the Earth’s inner core using free oscillations
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Author||Deuss, Arwen; Irving, Jessica|
|Holding Date||03 September 2008|
The solidity of the Earth’s inner core was inferred in 1971 from normal mode observations. However, since then there has been little further proof that the inner core is solid and observations of PKJKP body wave arrivals are strongly dependend on the assumed inner core shear wave velocity. At the same time, many fundamental questions about the Earth’s inner core remain unanswered. For example, it is unclear which high pressure and high temperature phases of iron, and its alloys, are stable in the inner core, and which light elements are present to explain the core’s density. A key parameter required to answer these questions is the seismic inner core shear wave velocity. Free oscillation data are the most reliable source of seismic inner core shear wave velocity estimates; these data have been used here to determine the average compressional and shear wave velocity structure and density of the Earth’s inner core and place tight constraints on their values. The new data reinforce the argument for a solid inner core, and the best data fit is obtained for average inner core velocities close to the PREM reference model: v s = 3.55 +/- 0.05 km/s, vp = 11.15 +/- 0.05 km/s. These values limit the time windows in which inner core shear waves, like PKJKP, may be observed. Inner core velocities for hcp and bcc iron at inner core conditions from ab initio molecular dynamics simulations and diamond anvil cell experiments are found to be incompatible with the seismological data, in particular for the shear wave velocity. When light elements are taken into account, the misfit is worse than for a fluid inner core model. The discrepancy between seismic and mineral physical values for the inner core shear wave velocity may be explained by an inner core temparature in excess of 6500K, the existence of fluid inclusions in the inner core, the effect of viscoelastic weakening or crystal defects.