Structure of the inner core inferred from observations of seismic core modes

Core modes are spheroidal modes of the Earth's free oscillations whose energy is dominantly partitioned into the inner core as shear energy. To detect low-frequency core modes a new autoregressive technique of spectral analysis, which is called the Sompi method, has been used. It is shown that the Sompi method can unambiguously recover core-mode signal from noise-containing synthetic seismograms. IDA records are then analysed to detect the core modes 2S2, 6S2 and 7S3, all of which have been observed as single multiplet peaks. The acceleration amplitudes of the observation are in fair agreement with their theoretical values. The eigenfrequency data indicate that the shear velocity in the inner core increases with depth. The Q values are consistently high, indicating that the inner core is least dissipative within the Earth for low-frequency shear waves. The variation of the Q values between the modes suggests that the shallower part of the inner core is more dissipative than the deeper part.