HR: 16:57h
AN: T54A-05    [Abstracts]
TI: Is There an Innermost Inner Core?
AU: * Sun, X
EM: xsun@uiuc.edu
AF: Geology Department, University of Illinois at Urbana-Champaign, 1301 W. Green st., Urbana, IL 61801 United States
AU: Song, X
EM: xsong@uiuc.edu
AF: Geology Department, University of Illinois at Urbana-Champaign, 1301 W. Green st., Urbana, IL 61801 United States
AB: Ishii and Dziewonski (2002) suggests that the inner core has a simple constant anisotropy except the innermost inner core (IMIC, about 300 km or so in radius), which possesses a different form of anisotropy. The primary data constraining the innermost inner core are PKP(DF) arrival times from the ISC. Although abundant, absolute arrival times have large picking errors and are subject to contamination of heterogeneous upper mantle. There are also strong evidence for significant lateral variations in anisotropy in at least top 500 km of the inner core. Because of the extremely small volume of the innermost inner core, a relatively small uncorrected structure of mantle and the rest of the inner core can be falsely mapped into this region. Here we examine this issue using a large collection of high-quality differential PKP AB-DF and BC-DF travel times. We examine our data in three different ways and our preliminary results are generally consistent: the form of anisotropy in the IMIC indeed seems different. However, our biggest difference between the IMIC and the shallower depth is at the EW direction where the IMIC is faster by about 3% in the innermost 400 km or so; near $45\deg$ from the EW direction, where Ishii and Dziewonski (2002) shows the biggest difference, our velocity is comparable. The three different data processing methods used are as follows. (1) We invert for 3D anisotropic structure by dividing the inner core into layers and sectors. (2) We correct the AB-DF data at distances 173 to $180\deg$ for the average anisotropic models inferred from the data at slightly smaller distances. (3) We construct the ''travel-time curves'' (differential PKP residual vs. distance) of equatorial paths with similar ray directions (within 10 or $15\deg$). Because the lateral variation of the inner core velocity at equatorial directions is small (except perhaps the top 100 km of the inner core), the velocity change with depth would be indicated directly by the change of the curvature of the travel-time curve. We observe an change in the slope of the travel-time curve at distance greater than about $160\deg$ for rays with less than $10\deg$ from the EW direction, a smaller change in the slope for rays around $20\deg$ from the EW direction, and little change in the slope for rays around $40\deg$ from the EW direction, suggesting a change in anisotropy in the IMIC, rather than the inaccuracy of the 1D radial reference model. So far we have not found evidence of waveform triplication for EW paths at near antipodal distances.
DE: 7260 Theory and modeling
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting