Regional Variations Of The Velocity And Attenuation Structures In The Inner Core

Inner core solidification releases latent heat that,combined with motions in the fluid outer core,drives the geodynamo,generating Earth's magnetic field.Solidification of the inner core also supplies some of the heat that drives mantle convection and subsequently plate tectonics at Earth'surface.Thus,the inner core is key to understanding the inner workings of our planet,and details of its anisotropy and heterogeneity may provide constraints on Earth's geodynamic and compositional structure.Firstly,we systematically collect seismic data from global,regional and temporary seismic networks from year 1991 to 2014,and by using the differential travel times and amplitude ratios of PKPBC and PKIKP to study the velocity and attenuation anisotropy at the top 300km of inner core beneath Australia,Africa and central Pacific.Our results show regional variations of the inner core velocity,with no anisotropy underneath Australia,and stronger anisotropy underneath Africa than that in the central Pacific region.Meanwhile,the average velocity is 0.5%faster underneath Australia than AK135 model,however,the average velocity is similar to reference model underneath Africa and central Pacific.For inner core attenuation structure,we have the following results:1)In equatorial directions,attenuations in these regions all show depth dependence.The highest attenuations in Australia and Africa are all around 200-250km below the inner core boundary,although the average attenuation in Australia is stronger than that of Africa.However,the attenuation in the central Pacific increase monotonically with depth and its average attenuation is stronger than that in Africa but lower than that in Australia.2)Attenuation in Australia shows no/weak anisotropy,but shows obvious anisotropy in Africa and central Pacific region.Besides,the attenuation anisotropy in Africa is stronger than central Pacific,which is consistent with the velocity anisotropy in these regions.3)Finally,we find that the velocity and attenuation in all three areas have good correlation,with fast/slow velocity corresponds to high/low attenuation.Considering the locations of three regions,we conclude that inner core attenuation may exhibit regional variations rather than simply hemispherical pattern.These results suggest core-mantle boundary heterogeneities and different deformation may be coupled with the inner core growth,and these processes should also be responsible for different iron crystal alignment in different regions,which causes anisotropy in the inner core.Secondly,by analyzing the differential travel times residuals of PKiKP-PKIKP and the amplitude ratios of PKIKP/PKiKP,we inverted both the velocity and attenuation in the top 100 km of the inner core beneath the central Pacific region.Our results find clear longitudinal variations in this region,with the boundary of east and west Pacific be to between 180~o to 190~oE.The west Pacific demonstrates weak velocity and attenuation anisotropy,with a 0.23%velocity perturbation relative to PREM and higher attenuation(Q=200);On the other hand,the east Pacific show strong velocity and attenuation anisotropy.The amplitude of velocity anisotropy is about 3.3%,and Q value changes from 400 to 150 from equatorial to polar directions.Moreover,the velocity and attenuation show a positive relationship in equatorial directions,and also in anisotropy amplitude in east Pacific region.We suggest that the strong correlation between velocity and attenuation anisotropy,as well as those in equatorial direction,may be related to regional variations of inner core solidification.This solidification process may be coupled with the heterogeneity in the core-mantle boundary,which affect the heat flow and deformation of the inner core,and result random or preferred orientations of iron crystals.Finally,according to the distribution of PKiKP-PKIKP and PKPBC-PKiKP travel time residuals and we choose the eastern Pacific region as our study region.Here,we use 3D ray tracing,cubic spline interpolation and a large amount of PKP differential travel time residuals to non-linear invert the anisotropy variations with depth at the top360 km of the inner core.The anisotropy velocity models obtained has following main characteristics:(1)With depth increasing,the anisotropy gradually decreases;(2)With depth increasing,the difference between various velocity model for different ray angle gradually decrease.This may imply that anisotropy is mainly influenced by the structure of inner core boundary,and this effect gradually decreases with depth increasing.Based on the study of the velocity,attenuation and anisotropy in small scale regions,it is suggested that the growth of the core may be related to the heterogeneity structure of the core-mantle boundary.The heat flow at the core-mantle boundary couples the inner core,makes the thermal heterogeneity of the core-mantle boundary influence on the thermochemical convection,heat flow and solidification/melting rate of the uppermost inner core,and resulting different growth processes and deformation,leading to different alignment of iron crystal direction,further causing the heterogeneity of velocity,attenuation and anisotropy.Due to the influence of the core-mantle boundary,such heterogeneity may exist only at the top of the inner core.With depth increasing,the effect diminishes.