Equatorial anisotropy of the Earth’s inner inner core from autocorrelations of earthquake coda

Abstract:

The anisotropic structure of the inner core seems complex with significant depth and lateral variations. An innermost inner core has been suggested with a distinct form of anisotropy, but it has considerable uncertainties in its form, size, or even existence. All the previous inner-core anisotropy models have assumed a cylindrical anisotropy with the symmetry axis parallel (or nearly parallel) to the Earth’s spin axis. In this study, we obtain inner-core phases, PKIIKP² and PKIKP² (the round-trip phases between the station and its antipode that passes straight through the center of the Earth and that is reflected from the inner-core boundary, respectively), from stackings of autocorrelations of earthquake coda at seismic station clusters around the world. The differential travel times PKIIKP² – PKIKP², which are sensitive to inner-core structure, show fast arrivals at high latitudes. However, we also observed large variations of up to 10 s along equatorial paths. These observations can be explained by a cylindrical anisotropy in the inner inner core (IIC) (with a radius of slightly less than half the inner core radius) that has a fast axis aligned near the equator and a cylindrical anisotropy in the outer inner core (OIC) that has a fast axis along the north-south direction. The equatorial fast axis of the IIC is near the Central America and the Southeast Asia. The form of the anisotropy in the IIC is distinctly different from that in the OIC and the anisotropy amplitude in the IIC is about 70% stronger than in the OIC. The different forms of anisotropy may be explained by a two-phase system of iron in the inner core (hcp in the OIC and bcc in the IIC). These results may suggest a major shift of the tectonics of the inner core during its formation and growth.