The presence of anisotropy in the inner core seems today widely accepted, although its exact strength and distribution remains the subject of debate. Unfortunately, due to imperfect path coverage, a large portion of the inner core remains unsampled by quasi-polar PKPdf, which are used in body-wave studies, and models of anisotropy often rely on a small number of measurements for rays quasi-parallel to the earth's spin axis. In the light of our work on the presence of large heterogeneity at the base of the mantle (Bréger and Romanowicz, 1998), we recently argued that the contamination of differential residuals by deep mantle structure for those paths could have been underestimated, yielding unrealistically large magnitude of inner core anisotropy (Bréger et al. [1999,2000]). It has become critical to seek new observables that will provide additional constrains on the seismic structure of the deepest shell of the earth, and here, we present some preliminary results obtained using P'P' waves.
Because of its two legs in the inner core, the P'P'df phase is very sensitive to structure below the Inner Core Boundary (ICB). Earthquakes in the Aleutians recorded at the NORSAR array in Norway, and in the South Sandwich Islands recorded at station CASY, correspond to paths for which the epicentral distance is between 50 and 65o, which is an appropriate distance to observe P'P', and for which the average angle between the two legs of P'P'df in the inner core and the earth's spin axis is on the order of 16o. Models of inner core anisotropy derived from travel times of body waves have been so far based on datasets with very few observations for smaller than about 25o. This particular source geometry is therefore well suited to provide critical additional constrains on the inner core anisotropic structure.
In Figure 28.1, we present an example of short-period vertical component data
for an earthquake near Kodiak Island recorded at 23 stations of the NORSAR array.
Three distinct arrivals are clearly visible around the predicted arrival times of P'P'df, bc, and ab and can be attributed to P'P'df, bc, and ab for several reasons.
There is no other large enough teleseismic event that could be responsible for these arrivals. There is very little seismicity around the NORSAR array, and a small local event would generate
waves with very different slownesses. The average observed amplitude ratio between P'P'bc and
P'P'df and P'P'bc and P'P'ab are compatible with theoretical values, and, as expected, P'P'df and P'P'bc on the one hand, and P'P'ab and the Hilbert transform of P'P'df on the other hand, have
Measuring travel time residuals on individuals seismograms with an accuracy less than 1s can be problematic, because P'P' first motions are usually hidden by noise. In order to reduce as much as possible the uncertainty on travel time measurements, we used a standard Nth root stacking method with N=3.
We were also able to measure P'P'df and P'P'bc - P'P'df residuals for an event in the South Sandwich Islands recorded at station CASY (-66.28oN,+110.53oE), which corresponds to an average angle of about 14.6o.
Our P'P' observations corresponds to PKPdf legs that regions of the inner core previously identified as anisotropic, and in Figure 28.2 we compare our observations with two simple axisymmetric models of inner core anisotropy, with strength of 1.5 and 3.5%, respectively. Both models clearly overpredict the P'P'bc - P'P'df observations.
The apparent discrepancy between P'P' and earlier observations could stem from an exceptionally complex inner core structure, with very sharp anisotropic gradients and lateral contrasts (Bréger et al., 1999), and possibly produced by a complex pattern of convection associated with a strong intrinsic anisotropy. Following some of our earlier studies, we believe that a strong contribution of D" could also explain this discrepancy.
To conclude, P'P' seems a very promising tool to investigate the structure of the inner core, and preliminary results confirm the results that we obtained using PKP(bc-df) differential travel times (Bréger et al., 1999).
We are very grateful to the NORSAR team, and in particular to Drs Johannes Schweitzer and Joergen Torstveit for providing us with their data. We also thank Johannes Schweitzer and two anonymous reviewers for helpful comments. This work was partially supported by IGPP/LLNL grants # 99-GS013 and 00-GS010. It is BSL contribution 00-05.
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