Study of Earth's inner core using synthetic seismograms calculated by Spectral-Element method

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We use broadband seismograms recorded at an antipodal station, where the epicentral distance is more than 179 degrees, and show that the amplitude of a PKIIKP phase (single reflection within the inner core at the inner core boundary) can be modeled by the existence of thin (20km) slow P-wave velocity (-10% at the inner core boundary from PREM) layer at the bottom of the outer core. We calculate theoretical seismograms for fully three dimensional Earth model using the Spectral-Element method on the Earth Simulator. We found that the current Earth model cannot reproduce the amplitude of PKIIKP phase. Because the amplitude of PKIIKP phase is sensitive to seismological impedance (product of the density and the seismic wave velocity) contrast at the inner core boundary, it is suggested that the reduction of P-wave velocity within this layer should not accompany a reduction of density.We also have calculated synthetic seismograms with aisotropic inner core model for several earthquakes and compared with the synthetics which are calculated for isotropic inner core model. Preliminary comparison shows that the travel time differences between anisotropic inner core model and isotropic core model for PKPab phases are at most a few seconds. There seems to be no significant differences in waveforms of PKP phases. These differences in travel times may help us to improve inner core fine structure by comparing these synthetics with observation.At present we could calculate synthetic seismograms that are accurate up to 3.5 sec by using the 507 nodes (4056 CPUs) of the Earth Simulator. It is desirable so that we can calculate up to 1 sec accuracy, which represents dominant period of body wave. We could estimate that we need at least 16 times larger machine than the current Earth Simulator to realize computation of synthetics which have 1 sec accuracy.