We have assembled and analysed 275 PKP data from vertical broadband records of the IRIS network. In particular, we measured differential travel times of two phases; PKP(DF) that passes through the inner core and PKP(AB) that bottoms in the uppermost part of the outer core. All events deeper than 550 km with Mw larger than 6.0 from 1997 to present day were considered. In order to improve global coverage limited by uneven distribution of deep and large events, we also considered a number of shallower events with high quality data, for which were we able to distinguish PKP(DF) and PKP(AB) phases from depth phases such as pPKP(DF) and sPKP(DF), pPKP(BC) etc. Some very large events (Mw > 7.0) were omitted in this analysis due to their complex source time function. The differential travel times PKP(AB-DF) were measured both by hand picking and overlapping of traces and by crosscorrelation from digital records after Hilbert transform had been applied to PKP(DF) phase. The PREM Earth model and ellipticity corrections have been used to form differential travel time residuals (observed differential travel time minus theoretically predicted travel time). The differential travel time technique reduces biases from uncertainties in the hypocentral time, source location, receiver structure and the upper mantle heterogeneities. We use the PKP(AB) and PKP(DF) phases, because for the epicentral distance range within which the PKP(AB) appears, the PKP(DF) samples the deepest portion of the inner core.
Figure 25.1a shows the already recognized anisotropic pattern of the inner core, with faster polar paths (smaller angle between paths and Earth's rotation axis) and slower equatorial paths. PKP(AB-DF) travel time residuals are plotted on the vertical axis, whereas the angle between paths and Earth's rotation axis is plotted on the horizontal axis. Figure 25.1b represents the same data set with residuals plotted on the vertical axis and epicentral distance ploted on the horizontal axis. Data for quasipolar paths (for which the angle between paths and Earth's rotation axis is smaller than 48o) are marked by triangles. Even though data are fairly scattered when plotted in this way, the PKP(AB-DF) residuals from quasipolar paths are systematically larger. In figure 25.2, we divide data into two datasets; one for raypaths that bottom in the quasieastern hemisphere (defined longitudinaly from 43oE to 177oE), and another one for raypaths that bottom in the quasiwestern hemisphere (Tanaka and Hamaguchi, 1997). The quasieastern hemisphere shows a clearer anisotropic trend, whereas the data are much more scattered for the quasiwestern hemisphere. One possibility is that such a geographical pattern could be a consequence of spatial heterogeneity in the inner core. In collaboration with A. Souriau (Toulouse, France), we are going to expand our dataset with the measurements done for GEOSCOPE network and also supplement our analysis by amplitude ratio measurements in order to continue with attenuation studies (Souriau and Romanowicz, 1997).
Souriau, A. and Romanowicz, B., Anisotropy in inner core attenuation: a new type of data to constrain the nature of the solid core, Geophys. Res. Lett 23, 1-4, 1997.
Tanaka,S. and Hamaguchi, H., Degree one heterogeneity and hemispherical variation of anisotropy in the inner core from PKP(BC)-PKP(DF) times, J. Geophys. Res., 102, 2925-2938, 1997.