Knowledge of the seismic velocity structure at the top of the Earth's inner core is important for deciphering the physical processes responsible for inner-core growth. Previous global seismic studies have focused on structures found 100 km or deeper within the inner core, with results for the uppermost 100 km available for only isolated regions. Here we present constraints on seismic velocity variations just beneath the inner-core boundary, determined from the difference in travel time between waves reflected at the inner-core boundary and those transmitted through the inner core. We found that these travel-time residuals-observed on both global seismograph stations and several regional seismic networks-are systematically larger, by about 0.8 s, for waves that sample the 'eastern hemisphere' of the inner core (40 degrees E to 180 degrees E) compared to those that sample the 'western hemisphere' (180 degrees W to 40 degrees E). These residuals show no correlation with the angle at which the waves traverse the inner core; this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0.8% higher than that of the western hemisphere.

S U M M A R YWe investigate the forces involved in driving long-term large-scale continental deformation in western North America, and quantify the vertically averaged deviatoric stress field arising from internal buoyancy forces and the accommodation of relative plate motions. In addition, we investigate the ability of regional models to resolve the level of tractions acting at the base of the lithosphere. We directly solve force-balance equations for vertically averaged deviatoric stresses associated with differences in values of 1/(lithospheric thickness) times the gravitational potential energy per unit area (GPE). The GPE values are inferred using both the ETOPO5 topographic data set and the CRUST2.0 crustal thickness model. Deviatoric stresses associated with basal tractions are calculated globally, with inputs determined from an isoviscous upper mantle (η = 10 21 Pa s) 3-D large-scale convection model in which mantle density variations were inferred from tomographic data and the history of subduction. In a 211-parameter iterative inversion we then solve for a stress field boundary condition by fitting stress field indicators (i.e. the directions and relative magnitudes of the principal axes of kinematic strain rates). Magnitudes of the total vertically averaged deviatoric stress field (sum of GPE solution with the boundary condition solution) range from 5 to 10 MPa within a 100-km thick lithosphere. These magnitudes are calibrated by the GPE differences, along with the spatial variation in deformation style. There is a trade-off between the scaling of the basal traction deviatoric stress field and the boundary condition solution. However, the combined boundary conditions plus basal traction solution is robust (in both magnitude and style), and when added to the contribution from GPE differences provides a global minimum of misfit between the total deviatoric stress solution and the stress field indicators. GPE variations account for ∼50 per cent of the deviatoric stress magnitudes driving deformation, while boundary condition stresses account for the remaining ∼50 per cent of deviatoric stress magnitude. By comparing possible end-member strength profiles with our vertically averaged deviatoric stresses we infer that the bulk of the strength within the lithosphere in western North America lies within the brittle seismogenic layer.

We collect a global data set of PKIKP and PKiKP phases recorded by the Global Seismic Network and many regional seismic arrays to study seismic structure in the top of the Earth's inner core. The PKIKP and PKiKP observations show different characteristics between those sampling the “eastern” hemisphere (40°E–180°E) of the inner core and those sampling the “western” hemisphere (180°W–40°E). PKIKP phases (1) arrive about 0.4 s earlier than the theoretical arrivals based on Preliminary Reference Earth Model (PREM) for those sampling the eastern hemisphere of the inner core and about 0.3 s later for those sampling the western hemisphere (131°–141°); (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. 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/s across the inner core boundary, a small radial velocity gradient of 0.000055 (km/s)/km, and an average Q value of 250, and a model in the western hemisphere with a P velocity increase of 0.633 km/s across the inner core boundary, a radial velocity gradient of 0.000533 (km/s)/km and an average Q value of 600. The hemispherical difference of seismic structures 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 different vigorousness of convection in the top of the inner core between the two hemispheres.