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The Pacific plume as seen by S, ScS, and SKS

Ludovic Bréger, Christine Ng and Barbara Romanowicz

Introduction

It is critical to resolve the strength, shape and extent of seismic velocity heterogeneity in the lower mantle in order to understand how it relates to the physical and chemical processes that take place in this region (see Lay et al., 1998 for a review). The deep mantle beneath the Central Pacific is thought to experience some vigorous convection, and may be responsible for the largest hotspot observed at the surface of the earth. Tomographic models of this region consistently show a large slow anomaly that extends several hundred kilometers above the core-mantle boundary (CMB) (e.g. Li and Romanowicz, 1996). However, while these models give a reasonable description of the position of anomalous domains, they do not fully explain travel times sensitive to structure in that region (e.g. Bréger et al., 1998). Bréger and Romanowicz (1998) showed that it was possible to use tomographic studies as a starting point for forward modeling of travel times, and proposed a revised model of the lowermost mantle beneath the Central Pacific characterized by a large slow S-velocity anomaly ($\sim-4\%$) extending above D", and adjacent, on its eastern side, to a smaller domain of very fast S velocities (faster by up to 4 to 5%). They used S-SKS and SKKS-SKS differential residuals, which have little sensitivity to heterogeneity in the mid-mantle, and thus poor vertical resolution. In the present study, we apply the forward modeling approach of Bréger and Romanowicz (1998) to an extended dataset which includes both S-SKS and ScS-S differential residuals (Figure 25.1) in an attempt to verify our model and constrain it better at mid-mantle depths.

Data, Modelling and Results

We use the $S_{diff}-SKS$ differential travel time residuals of Garnero and Helmberger (1993), as well as the measurements of Vinnik et al. (1998), and the ScS-S differential travel time dataset of Russell et al. (1998). Residuals are computed with respect to PREM reference model (Dziewonski and Anderson, 1981). ScS and S arrivals are measured on the transverse component and SKS ones on the radial component. The uncertainty on measurements is on the order of 1 to 2s. Dispersive effects are expected to affect measurements of Sdiff travel times so that this uncertainty should be higher at larger epicentral distances. The compiled dataset that we use here provides a particularly good coverage of the lowermost mantle beneath the Central Pacific (Figure 25.2). We adopt the approach of Vinnik et al. (1998), Bréger et al. (1998), and Bréger and Romanowicz (1998), and analyze the variations of the residuals as a function of epicentral distance when the station or the event is fixed, along narrow linear "corridors".

Figure 25.1: Raypaths discussed in this study.
\begin{figure}\begin{center}
\epsfig{file=barbara01_2_fig1.eps, bbllx=111,bblly=385,bburx=520,bbury=609,width=8cm}\end{center}\end{figure}

In an attempt to foward model the observed residual variations, we first computed the residual values predicted by several recent tomographic S-velocity models, using the 1D raypaths predicted by PREM. The model that gave the closest match to both S-SKS and ScS-S observations (SKS12, Liu and Dziewonski, 1994) was chosen as a starting model. This initial model was progressively perturbed in two ways: (1) by keeping the shape of the heterogeneity and increasing its strength, and (2) by applying small vertical and horizontal shifts of no more than 200km. The perturbations which were applied to the model are obviously non-unique. There are some trade-offs, particularly between the shape, position, vertical extent, and S-heterogeneity in this domain. In fact, our approach leads to a whole family of models rather than one single `best' model. The features which are common to all the models can be considered as robust, and are discussed in what follows (Figure 25.2).

We found that the data require a large slow domain extending from the CMB to several hundred kilometers above the CMB, where the S-velocity anomaly is strong and reaches -2 to -4%, which is significantly larger than predicted by SKS12 (Figure 25.2). Predicted residuals fit the observations better when the model is modified up to 1200 km above the CMB, that is, when the slow anomalous domain extends well above the top of D". This does not preclude the possibility of explaining the observations by a more complex structure localized in the last 300 to 500 km of the mantle. However, such a scenario seems unlikely because it would imply a structure that is less compatible with existing tomographic models. On the other hand, we note that differential residuals appear to rapidly lose their sensitivity as heterogeneity gets shallower, so that we cannot preclude that the anomalous domain extends even higher above the CMB.

A few S-SKS profiles for a fixed earthquake show a sudden increase at small epicentral distance (86 to 90$^o$), followed by a decrease of the residuals at distances from about 90 to 96$^o$ (Bréger and Romanowicz, 1998). Increasing residuals can be easily explained by the fact that S waves spend more time in the slow region with increasing distance, and therefore accumulate an increasingly large delay. However, it is impossible to explain decreasing residuals with the slow structure alone, and introducing a fast region becomes necessary. This region needs to be localized in the last 400 km of the mantle in order to start affecting the S-wave at epicentral distances larger than about 90$^o$. The exact strength, position, and velocity anomaly of this region are difficult to determine precisely considering that it is sampled by only a few profiles, but we estimate that it is a fairly localized domain, with lateral dimensions on the order of 500 km by 500 km, and extending vertically to about 400 km above the CMB respectively, and corresponds to a S-velocity anomaly on the order of 3 to 4%.

Figure 25.2: (a) Surface projections of wavepaths analyzed here (grey lines). Sources and stations bounce points are indicated by light blue stars and triangles, respectively. The S-velocity heterogeneity at a depth of 2800km predicted by a model that explains S-SKS and ScS-S travel time residuals reasonably well has been plotted as background. (b) Cross-section through the original model SKS12 (Liu et al., 1994) and examples of S, ScS, and SKS rays, for the Kermadec Islands event of June 25, 1992 (lat.=-28.17$^o$, long.=-176.22$^o$, depth=21 km) and the Fiji Islands event of January 19, 1994 (lat.=-17.37$^o$, long.=-178.28$^o$, depth=561 km), and stations ARC (thin lines) (Berkeley Digital Seismic Network) and INK (thick lines) (Canadian National Seismic Network). Subhorizontal light blue lines outlines the 410km-discontinuity, the 670km-discontinuity, the top of D", and the CMB. The model is only represented where it is sampled by rays. (c) Same as (b), but for an example of modified model that explains the observed residuals significantly better than the original SKS12.
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\epsfig{file=barbara01_2_fig2.eps,bbllx=82,bblly=75,bburx=305,bbury=486, width=10cm}\end{center}\end{figure*}

References

Bréger, L., and B. Romanowicz, Three-dimensional structure at the base of the mantle beneath the Central Pacific, Science, 282, 718-720, 1998.

Bréger, L., B. Romanowicz, and L. Vinnik, Test of tomographic models of D" using differential travel time data, Geophys. Res. Lett., 25, 5-8, 1998.

Dziewonski, A. M., and D. L. Anderson, Preliminary Reference Earth Model, Phys. Earth Planet., Int 25, 297-356, 1981.

Garnero, E., and D. V. Helmberger, Travel times of S and SKS: implications for three-dimensional lower mantle structure beneath the central Pacific, J. Geophys Res., 98, 8225-8241, 1993.

Lay, T., Q. Williams, and E.J. Garnero, The core-mantle boundary layer and deep Earth dynamics, Nature, 392, 461-468, 1998.

Li, X.-D., and B. Romanowicz, Global mantle shear velocity model developed using nonlinear asymptotic coupling theory, J. Geophys Res., 101, 22245-22272, 1996.

Liu, X.-F., W.-J. Su, and A. M. Dziewonski, Improved resolution of the lowermost mantle shear wave velocity structure obtained using SKS-S data (abstract), Eos. Trans. AGU, 75, 232, 1994.

Russell, S.A., T. Lay, and E. J. Garnero, Seismic evidence for small-scale dynamics in the lowermost mantle at the root of the Hawaiian hotspot, Nature, 396, 255-258, 1998.

Vinnik, L., L. Breger, and B. Romanowicz, Anisotropic structures at the base of the Earth's mantle, Nature, 393, 564-567, 1998.


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