HR: 09:45h
AN: S31B-08    [PDF]
TI: Three-dimensional structure of inner core anisotropy from PKP differential travel times and waveforms
AU: * Song, X
EM: xsong@uiuc.edu
AF: Department of Geology, University of Illinois, Urbana, IL 61801 United States
AB: Discussion with Prof. Don Anderson was one of the most inspiring and stimulating experiences in my science career when we were searching for evidence for inner core anisotropy a little over ten years ago. At the time, the hypothesis of inner core anisotropy was not fully accepted. Now models of 3D inner core anisotropy have been proposed. Recent studies suggest that the uppermost inner core is nearly isotropic and western and eastern hemispheres of the inner core are different in both velocity and anisotropy. Yet differential PKP travel times at near antipodal distances show convincingly that significant anisotropy exists in the bulk of the inner core at both hemispheres. Recently Xinlei Sun and I assemble differential PKP travel-time measurements at distances of 145 to 180$\rm ^o$, obtained by us and others over the years, to investigate the 3-D structure of the inner core anisotropy. These differential times reduces the biases from earthquake mislocations and heterogeneous upper mantle. We invert for 3D inner core anisotropy by dividing the inner core into four layers in radius and six sectors in longitudes. The top 100 km of the inner core is assumed to be isotropic. Our inversion suggests that at depth of 100-500 km below the inner core boundary, the quasi-eastern hemisphere (40 to 160$\rm ^o$E) is nearly isotropic (less than 1% of anisotropy) while the western hemisphere is strongly anisotropic (3 to 5% anisotropy). However, at depth of 500-900 km, both hemispheres are anisotropic (about 3% anisotropy). At the innermost 300 km of the inner core, the form of anisotropy seems distinctly different. One of the most dramatic manifestations of changes in anisotropy is the observation of seismic triplication of inner core waves; a type of multipathing effect due to heterogeneity that Don challenged to think about even in the early days. The best example is the seismic triplication observed from South Sandwich Islands earthquakes recorded at stations in Canada and Alaska.
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
SC: Seismology [S]
MN: 2003 Fall Meeting