Test of Innermost Inner Core Models Using Broadband Absolute PKIKP Travel Time Residuals

It has been suggested that in the central part of the inner core, the character of anisotropy is different from that in the surrounding shell (Ishii and Dziewonski, 2002(ID02), Beghein and Trampert, 2003 (BT03)). Three different Innermost Inner Core (IMIC) models have recently been proposed, each with different radii and different orientation and strength of anisotropy. In order to test these IMIC models, we have systematically downloaded broadband vertical component seismograms from the IRIS Data Management Center (DMC) for events with M_w >6.0, in the epicentral distance range 150° to 180°, and for the time period 1990- 2003, for which the relocated EHB event catalog is available. Our new dataset is thus based on ~ 1,000 absolute PKIKP arrival time residuals measured from those high quality global broadband seismograms. Also, PKIKP bottoming points and ray paths in the inner core are well distributed in the east-west or north-south hemispheres We invert this dataset for two layer models of anisotropy in the inner core and compare the fits obtained with those predicted from the existing IMIC models. Our results show that, if there is an IIC, its radius is most likely around ~ 550 km rather than the ~ 300 km originally proposed by ID02, and in better agreement with the inference of Cormier and Stroujkova(2005) on layering in the inner core based on PKIKP waveform modeling. BT03 and ID02 proposed two different anisotropic models in the upper part of the inner core: depth-dependent anisotropy (BT03) and constant anisotropy (ID02). When inverting for the IMIC anisotropic parameters, we used these two models in the upper portion, respectively. The resulting IMIC structures are noticeably different. The depth-dependent anisotropy in the upper inner core fits our observations better than the constant anisotropy assumption.