Hemispheric dichotomy in seismic structures of the Earth's inner core

In the past several years, we have made effort to understand the lateral and depth extent of the inner core hemispheric dichotomy. In this presentation, we will review some peculiar characteristics of the inner core hemispheric differences that have been recently discovered. These major findings include: 1) along equatorial paths, the east-west hemispheric dichotomy in seismic velocity, with the velocity in the eastern hemisphere (40°E - 180°E longitude) being higher than that in the western hemisphere (180°W - 40°E) by about 1%, extends larger than 400 km of the inner core. Seismic velocity and attenuation structures in the eastern hemisphere appear to be complex: an anomalously small velocity gradient in the top 235 km of the inner core, a gradual velocity transition at depths of 235-375 km, and a PREM-like velocity gradient in the deeper portion of the inner core. The attenuation structure in the eastern hemisphere has an average Q value of 300 in the top 300 km and an average Q value of 600 in the deeper portion of the inner core. Seismic velocity and attenuation structures in the western hemisphere appear to be simple: a constant gradual velocity gradient and an average Q value of 600 in the top 400 km of the inner core; 2) magnitude of the inner core anisotropy is larger in the western hemisphere (about 3% - 4%) than in the eastern hemisphere (about 0.5%), but the presence of anisotropy appears to be shallower in the western hemisphere (about 0 - 100 km) than in the eastern hemisphere (about 200 km); 3) a correlation between high attenuation and high velocity is ubiquitously observed in the seismic data. Along equatorial paths, the eastern hemisphere has high attenuation (a Q value of 300) compared to the western hemisphere (a Q value of 600). The attenuation structure along polar paths in the western hemisphere has even higher attenuation (a Q value of about 200-250) compared to that along equatorial paths. Such a polar-equatorial difference in attenuation corresponds to a polar-equatorial difference in velocity with the velocity along polar paths being higher than that along equatorial paths by 1.3% - 2.8%; 4) near the east-west border at 20 ° W - 40 ° E longitude (beneath Africa), seismic anisotropic structures appear to be complex within the uppermost 80 km of the inner core: a laterally undulating isotropic layer increases from 0 km beneath eastern Africa to 50 km beneath central Africa and the velocity anisotropy in the deeper portion of the inner core varies from 1.6%-2.2%. The above modeling results suggest that the east-west hemisphericity and the polar-equatorial anisotropy can be explained by different alignments of the anisotropic hcp iron crystals, under the hypothesis that the axis of high velocity corresponds to that of high attenuation. We begin to systematically search for the waveform doublets for the PKIKP sampling the inner core to study temporal change of inner core properties.