Weak Inner Core Anisotropy Along Polar Paths Under the Western Pacific Ocean

Abstract

The Earth's inner core may have a nearly isotropic, "quasi-eastern" hemisphere and an anisotropic, "quasi-western" hemisphere, but there is still no consensus on the precise boundaries of these hemispheres or on the exact magnitude of the anisotropy in each. This paper examines seismic anisotropy along polar paths in the quasi-eastern hemisphere, where most previous studies have used equatorial paths. I hypothesized that the quasi-eastern hemisphere would show regional anisotropy. Seismic waves generated by earthquakes that occurred in the northern Pacific Ocean were recorded at two seismic stations in Antarctica, and two methods were used to find PKIKP and PKPab travel times: handpicking, and cross correlation of each seismogram with its Hilbert transform. Observed PKPab-PKIKP travel time residuals were compared to PKPab-PKIKP travel time residuals from the velocity model ak135. The observed difference between the PKPab and PKIKP travel times is on average greater than the predicted difference, which was confirmed by vespagram analysis. ak135 inner core compressional velocities were increased by a constant factor to create a one-dimensional velocity model with larger PKPab-PKIKP travel time differences to match the observations. The observed data indicate very weak to no anisotropy in the inner core under the western Pacific Ocean. However, the travel times of the PKPab and PKIKP phases generated by the easternmost earthquake event studied may indicate slightly stronger anisotropy. This longitudinal anisotropy dependence supports the theory that the inner core has quasi-hemispheres of differing anisotropy, with one boundary between them in the eastern Pacific Ocean.