We constrain seismic velocity structure in the Earth's outer core by analyzing differential travel times, waveforms, and amplitude ratios of several pairs of core phases. We use differential travel times and waveforms of PKPbc‐PKIKP at 144°–148°, PKiKP‐PKIKP at 120°–141°, PKiKP‐PKPBdiff at the PKP caustics distance range (141°–145°), and differential travel times and amplitude ratios of PKPab‐PKPbc at 146°–159°. To avoid the influence of the inner core anisotropy in velocity, we only use the PKiKP‐PKIKP and the PKPbc‐PKIKP observations whose PKIKP phases sample the inner core along the equatorial paths (paths with their ray angles being larger than 35° from the Earth's rotation axis). These observations show the following characteristics: (1) both the observed PKPbc‐PKIKP and PKiKP‐PKIKP differential travel times show a distinct “east‐west” hemispheric pattern. PKIKP phases arrive about 0.7 s earlier for those sampling the “eastern” hemisphere (40°E–180°E) than those sampling the “western” hemisphere (180°W–40°E); (2) the observed differential PKiKP‐PKPBdiff travel time residuals also exhibit a hemispheric difference. PKiKP‐PKPBdiff differential travel times are about 0.9 s larger for those sampling the western hemisphere than those sampling the eastern hemisphere; and (3) both the observed PKPab‐PKPbc differential travel times and PKPbc/PKPab amplitude ratios show scatter. Overall, these observations can be best explained by two one‐dimensional P velocity models, one for each hemisphere, at the bottom of the outer core. The seismic data sampling the eastern hemisphere can be explained by PREM at the bottom of the outer core, while those sampling the western hemisphere can be explained by a lower velocity gradient at the bottom of the outer core, which has reduced velocities relative to PREM linearly increasing from 0% at 200 km above the inner core boundary (ICB) to −0.35% at the ICB. Different velocity gradients at the bottom of the outer core indicate that there may exist a compositional difference and/or a large‐scale temperature difference there and that inner core formation processes may be different between the two hemispheres. Different inner core formation processes may produce different geometric inclusions of melt in the top of the inner core, and thus may provide an explanation to the hemispheric variation of seismic velocity and attenuation in the top of the inner core. The seismic data also suggest that the variation of seismic velocity in the tangential cylinder of the outer core, if it exists, is less than 0.1%. We use a compressional wave tomographic model to study the travel time delays caused by the seismic heterogeneities in the mantle. The tomographic model is unable to explain the scatter of the travel time data, suggesting the existence of unknown strong small‐scale seismic heterogeneities in the mantle.

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.

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