Normal mode constraints on shear and compressional wave velocity of the Earth's inner core

Abstract

Free oscillation data are used to determine the average compressional and shear wave velocity structure 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: υ s = 3.55 ± 0.05 km/s, υ p = 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 temperature in excess of 6500 K, the existence of fluid inclusions in the inner core, the effect of viscoelastic weakening or crystal defects.