L. Wen $^{a}$ and F. Niu $^{b}$
$^{a}$ Department of Geosciences, State University of New York, Stony Brook, NY, USA. $^{b}$ Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC, USA.
We collect PKP data recorded at a distance range between 130-142 degrees from the Global Seismic Network (GSN) and many regional seismic networks to study the seismic velocity and Q structures at the top of the inner core. The PKIKP and PKiKP observations show different characteristics between those sampling the "eastern" hemisphere ($40^{\circ}$E - $180^{\circ}$E) of the inner core and those sampling the "western" hemisphere ($180^{\circ}$W - $40^{\circ}$E). PKIKP phases 1) arrive about 0.4 second earlier than the theoretical arrivals based on PREM for those sampling the "eastern" hemisphere of the inner core, and about 0.3 second later for those sampling the "western" hemisphere; 2) bifurcate at smaller epicentral distances for those sampling the "eastern" hemisphere, compared to those sampling the "western" hemisphere; and, 3) have smaller amplitudes for those sampling the "eastern" hemisphere. The waveform and differential travel times have no dependence on both ray azimuth and ray angle to the polar direction. There also exists a strong lateral variation of the differential travel time for PKP phases sampling the top of the inner-core beneath the Caribbean sea. Waveform modeling of these observations suggests two different types of models for the "two hemispheres" of the top of the inner core, with a model in the "eastern" hemisphere having a P velocity increase of 0.765 km/sec across the inner core boundary, a small radial velocity gradient of 0.000055 (km/sec)/km, and an average Q structure of 250 and, a model in the "western" hemisphere with a P velocity increase of 0.633 km/sec across the inner core boundary, a radial velocity gradient of 0.000533 (km/sec)/km and an average Q value of 600. The difference of seismic structures between the "two hemispheres" may be explained by different geometric inclusions of melt and/or different alignments of iron crystals with anisotropic properties in both velocity and attenuation. We speculate that this large-scale pattern of seismic heterogeneities may be caused by a large-scale heat flow anomaly at the bottom of the outer core and/or convection within the top of the inner core.