Equatorial anisotropy in the Earth's inner-inner core: evidence from seismic interferometry at low latitudes

Anisotropy of Earth's inner core plays a key role on understanding the evolution and dynamics of the core. The previous inner core anisotropy models have assumed a cylindrical anisotropy with the symmetry axis (nearly) parallel to the Earth's spin axis. Recently, by using autocorrelations from earthquake's coda, we found the equatorial anisotropy of the inner inner core (IIC) with a fast axis near the Central America and the Southeast Asia, in apparent contrast to the polar anisotropy of the outer inner core (OIC). To exclude possible contaminations from large Fresnel zone of the PKIKP2 and PKIIKP2 phases at low periods, we processed the coda (10,000 40,000 s after Mw>=7.0 earthquakes) from stations at low latitudes (<=±35°) during 1990 2013. By imposing an automatic grouping strategy, the standard deviation normalization and a selection filter, we extracted more stable empirical Green's functions from 52 arrays. All arrival times were automatically picked by waveform cross correlation. Large variation (up to 10.9 s) of the residuals near the equator cannot be explained by the fast axis along the north-south direction. The interpolation of observed residuals shows a similar pattern with previous global dataset and the prediction of OIC+IIC model, including the fast axis and two low-velocity open rings. Even considering the influence of the Fresnel zone, our results support the IIC has an equatorial anisotropy. From the IIC to the OIC, large transition of the fast axis may shed light on the growth process of the inner core.