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Figure 2. Inner Core Anisotropy: Dependence of the wave speed on the angle the wave makes with the rotation axis (right) for the central 300 km (red curve) and overlying inner core (green curve). The two layers of the inner core correspond to the green and red regions on the left plot. They are quite small in comparison to the outer core (pale blue) and mantle shown with velocity variations.
The Earth’s inner core, buried beneath the crust, mantle, and outer core, is a difficult target to study. Nevertheless, large collections of inner-core sensitive data at IRIS and other institutions are allowing for improved imaging of this deepest structure. We have investigated the anisotropic property of the inner core using normal-mode data obtained from seismograms available at the IRIS Data Management Centre, as well as body-wave information from IRIS and elsewhere (Figure 1).
To obtain an inner-core model that is compatible with long-period free oscillations and short-period body-wave data, we invert the normal-mode, absolute and differential travel-time measurements simultaneously for a model of inner-core anisotropy. This model predicts a difference in wave speed of about 0.2 km/s with fast wave propagation along the Earth’s rotation axis and slow wave propagation along the equatorial plane (Figure 2).
Although arrival times of PKP-DF (or PKIKP) from epicentral distances between 120° and 173° are consistent with such anisotropy, PKP-DF observations from nearly antipodal distances (between 173° and 180°) deviate significantly from the predictions based upon the joint inversion. The antipodal data correspond to the central 300 km of the inner core, and they require a difference in wave speed of about 0.8 km/s. Furthermore, even though the fast propagation direction is the same as in the overlying layer, the direction of slow propagation is ~45° from the rotation axis. These observations suggest that the inner core consists of at least two layers (Figure 2).
Date Taken: January 29, 2009 Photographer / Contributor: Miaki Ishii • Scripps Institution of Oceanography; Adam M. Dziewonski • Harvard University
