Regional seismic variations in the inner core under the North Pacific

An asymmetry between a nearly isotropic, faster 'eastern' hemisphere and an anisotropic, slower 'western' hemisphere in Earth's inner core has been revealed by previous seismic studies. However, it remains unclear if division of the inner core into just two hemispheres is too simplistic. Here, we carry out regional-scale tomography using a new body wave data set to study the hemisphere boundary region beneath the northern and central Pacific Ocean and North America. If anisotropy is not considered, then a hemispherical pattern seems to be present in the study region, though the hemisphere boundary appears to be irregular. However, once the presence of anisotropy is permitted we find that this region cannot be simply separated into an anisotropic western hemisphere and an isotropic eastern hemisphere; instead the strength of the anisotropy varies regionally. The global hemispherical pattern is not observed here, instead the strongest anisotropy is observed in the centre and south west of the study region. Some of the strongest anisotropy appears to be in the -eastern' inner core, while part of the inner core assumed to be in the western hemisphere shows weaker anisotropy. Thus, this part of the inner core displays complex variations in anisotropy which differ from a simple hemispherical division. We suggest that a long-lived global heterogeneity, such as uneven heat flow through the core-mantle boundary over a period of hundreds of millions of years, may be responsible for the observed pattern of inner core anisotropy.