next up previous contents
Home: Berkeley Seismological Laboratory
Next: Modelling of D" by Up: Ongoing Research Projects Previous: BDSN Surface Wave Magnitude

New Constraints on the Structure of the Inner Core from P'P'

Ludovic Bréger, Barbara Romanowicz and Sébastien Rousset

Introduction

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.

Data

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 $\xi $ 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 $\xi $ 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

  
Figure 28.1: (a) 2s highpassed vertical short-period velocity records for the September 12, 1986 Alaska Peninsula magnitude 6.1 event at 23 NORSAR stations. (b) Comparison between P'P'ab and the Hilbert transform of P'P'df for station NAO01. (c) Comparison between P'P'bc and P'P'df for station NAO02. Note the similarity between waveforms in (b) and (c) respectively.
\begin{figure}
\begin{center}
\epsfig{file=breger00_1_1.ps, width=7cm}\end{center}\end{figure}

similar waveforms (Figure 28.1b-c). For an event of this type (Mb=6.1 Depth=17.3 km), the combination of a source duration of several seconds and of depth phases is expected to lead to a complex P coda. P'P'df should not critically affect P'P'bc because of the large P'P'bc/P'P'df ratio, which implies that the first arrival of P'P'bc has an amplitude which is well above that of the late P'P'df coda. However, P'P'bc could potentially perturb the P'P'ab waveform, yielding larger uncertainties in the measurements of P'P'ab travel times. We also verified that arrival slownesses measured using cross-correlation were consistent with theoretical values.


  
Figure: (a) Observed travel time residuals with respect to reference model ak135 ( Kennett et al., 1995) plotted as a function of epicentral distance (gray diamonds). For comparison, we plotted the residuals predicted by a uniform axisymmetric anisotropy of 3.5% (black triangles) and 1.5% (white circles). The average observed P'P'df are slow while the models of inner core anisotropy predict arrivals which should be early by at least 2.5s. (b) Same as (a) for residuals plotted as a function of the average angle $\xi =(\xi _1+\xi _2)/2$, where $\xi _1$ and $\xi _2$ are the angles between the earth's spin axis and the first and second PKPdf inner core branch of P'P'df.
\begin{figure}
\begin{center}
\epsfig{file=breger00_1_3.ps, width=7cm}\end{center}\end{figure}

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 $\xi $ of about 14.6o.

Results and discussion

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).

Acknowledgements

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.

References

Bréger, L., and B. Romanowicz, Three-dimensional structure at the base of the mantle beneath the central Pacific, Science, 282, 718, 1998.

Bréger, L., B. Romanowicz, B, and H. Tkalcic, PKP(BC-DF) travel time residuals and short scale heterogeneity in the deep earth, Geophys. Res. Lett., 26, 3169, 1999.

Bréger, L., H. Tkalcic, and B. Romanowicz, The effect of D" on PKP(AB-DF) travel time residuals and possible implications for inner core structure, Earth Planet. Sci. Lett., 175, 133, 2000.

Kennett, B.L.N., E.R. Engdahl, and R. Buland, Constraints on seismic velocities in the Earth from travel times. Geophys. J. Int., 122, 108, 1995.


next up previous contents
Next: Modelling of D" by Up: Ongoing Research Projects Previous: BDSN Surface Wave Magnitude

The Berkeley Seismological Laboratory, 202 McCone Hall, UC Berkeley, Berkeley CA 94720
Questions and comments to www@seismo.berkeley.edu
Copyright 2000, The Regents of the University of California.