Description: [1] We study seismic velocity and attenuation structures in the top 400 km of the Earth's inner core based on modeling of differential traveltimes, amplitude ratios, and waveforms of the PKiKP-PKIKP phases observed at the epicentral distance range of 120°–141° and the PKPbc-PKIKP phases observed at the distance range of 146°–160° along equatorial paths. Our data are selected from the seismograms recorded in the Global Seismographic Network from 1990 to 2001 and many regional seismic networks. The observed PKiKP-PKIKP and PKPbc-PKIKP phases exhibit distinctive “east-west” hemispheric patterns: (1) At the distance ranges of 131°–141° and 146°–151°, PKIKP phases arrive about 0.3 s earlier than the theoretical arrivals based on the Preliminary Reference Earth Model (PREM) for the PKIKP phases sampling the “eastern hemisphere” (40°E–180°E) of the inner core and about 0.4 s later for those sampling the “western hemisphere” (180°W–40°E). At the distance range of 151°–160°, PKIKP phases arrive about 0.7 s earlier than the predicted arrivals based on PREM for those sampling the eastern hemisphere and about 0.1 s later for those sampling the western hemisphere. (2) Amplitude ratios of the PKIKP/PKiKP phases at the distance range of 131°–141° and of the PKIKP/PKPbc phases at the distance range of 146°–151° are, in general, smaller for the PKIKP phases sampling the eastern hemisphere than for those sampling the western hemisphere. At distances greater than 151°, the PKIKP/PKPbc amplitude ratios become indistinguishable for the two hemispheres. These observations can be best explained by two different types of seismic velocity and attenuation models along equatorial paths, one for each hemisphere, in the top 400 km of the inner core. For the eastern hemisphere, the velocity structure has a velocity increase of 0.748 km/s across the inner core boundary (ICB), a small velocity gradient of 0.0042 (km/s)/100 km in the top 235 km, followed by a steeper velocity gradient of 0.1 (km/s)/100 km extending from 235 km to 375 km, and a velocity gradient of 0.01 (km/s)/100 km in the deeper portion of the inner core; the attenuation structure 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. For the western hemisphere, the velocity structure has a velocity increase of 0.645 km/s across the ICB and a velocity gradient of 0.049 (km/s)/100 km in the top 375 km; the attenuation structure has an average Q value of 600 in the top 375 km of the inner core. Our results suggest that the inner core hemispheric variations in velocity extend deeper than 375 km below the ICB and the top 235 km of the inner core in the eastern hemisphere is anomalous compared to the rest of the inner core in having a small velocity gradient, high velocity, and high attenuation.

Description: We report complex seismic anisotropy in the top 80 km of the Earth's inner core beneath Africa. The anisotropy in the top 80 km of the inner core is constrained using differential travel times, amplitude ratios, and waveforms of the PKiKP-PKIKP phases sampling Africa along various directions. The differential PKiKP-PKIKP time residuals (relative to the Preliminary Reference Earth Model [PREM]) along the polar paths are larger than those along the equatorial paths by 0–1.4 s, indicating the presence of seismic anisotropy in the top 80 km of the inner core. Furthermore, the observations along the polar paths show complex regional variations beneath Africa: the differential PKiKP-PKIKP travel time residuals vary from 1.2 s beneath eastern Africa, to −0.1 s beneath central Africa, and to −0.2 to 0.8 s beneath western Africa. A correlation between small PKIKP/PKiKP amplitude ratios and large differential PKiKP-PKIKP travel time residuals is observed. The waveform data are spatially binned into six groups to constrain the regional dependence of velocity and attenuation anisotropy in the top 80 km of the inner core. Overall, the seismic data can be explained by an isotropic upper inner core (UIC) overlying an anisotropic lower inner core (LIC) in the top 80 km of the inner core across Africa. The thickness of the isotropic UIC varies from 0 to 50 km, and the P velocity transition from the isotropic UIC to the anisotropic LIC is sharp, with velocity increases laterally varying from 1.6% to 2.2%. The attenuation structure along the polar paths has a Q value of 600 for the isotropic UIC and Q values varying from 150 to 400 for the anisotropic LIC. The complex seismic anisotropy in the top of the inner core is found in a region where a rapid change of the inner core boundary (ICB) between 1993 and 2003 was discovered (Wen, 2006) and may be explained by complex alignments of iron crystals, resulting from a localized anomalous solidification of the inner core.