Anisotropy of Earth's inner core provides a key role to understand its evolution and the Earth's magnetic field. Recently, using autocorrelations from earthquake's coda, we found an equatorial anisotropy of the inner-inner core (IIC), in apparent contrast to the polar anisotropy of the outer-inner core (OIC). To reduce the influence of the polar anisotropy and reduce possible contaminations from the large Fresnel zone of the PKIKP2 and PKIIKP2 phases at low frequencies, we processed coda noise of large earthquakes (10,000–40,000 s after magnitude ≥7.0) from stations at low latitudes (within ±35°) during 1990–2013. Using a number of improved procedures of both autocorrelation and cross-correlation, we extracted 52 array-stacked high-quality empirical Green's functions (EGFs), an increase of over 60% from our previous study. The high-quality data allow us to measure the relative arrival times by automatic waveform cross correlation. The results show large variation (∼10.9 s) in the differential times between the PKIKP2 and PKIIKP2 phases. The estimated influence of the Fresnel zone is insignificant (<1.1 s), compared to the observed data variation and measurement uncertainty. The observed time residuals match very well previous IIC model with a quasi-equatorial fast axis (near Central America and the Southeast Asia) and the spatial pattern from the low-latitude measurements is similar to the previous global dataset, including the fast axis and two low-velocity open rings, thus providing further support for the equatorial anisotropy model of the IIC. Speculations for the shift of the fast axis between the OIC and the IIC include: change of deformation regimes during the inner core history, change of geomagnetic field, and a proto-inner core.
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Physics and Astronomy (miscellaneous)
- Space and Planetary Science