Ecole Normale Superieure, Paris
We have developped an a priori model of the upper mantle (Nataf H-C. and Y. Ricard, 3SMAC : an a priori tomographic model of the upper mantle based on geophysical modeling, Phys. Earth Planet. Inter., 95, 101-122, 1996). This model is available at :http://geoscope.ipgp.jussieu.fr/logiciels/3SMAC The idea was to put in all information available on topography, sediment thickness, crustal thickness, and lithospheric temperatures, in order to predict the variations in "observable" parameters such as P and S-velocities. We could show that such a model can explain many of the features, and the overall amplitude variation, of seismological data sets that are most sensitive to upper mantle structure such as SS-S (and PP-P) mapped at the bounce point of SS (or PP). A more detailed comparison was performed with surface wave data sets (Ricard Y., H-C. Nataf and J-P. Montagner, The three-dimensional seismological model a priori constrained : confrontation with seismic data, J. Geophys. Res., 101, 8457-8472, 1996). It showed that the amplitude of observed variations can well be accounted for by "predicted" temperature variations. It also demonstrated that usual tomographic techniques do not permit to recover the shorter wavelength content of the model (and of the Earth). We also show that a large part of the long-wavelength component of tomographic models is due to lithospheric signal. This is true even when one considers low frequency normal modes, usually thought to reveal structure in the transition zone.
At the REM workshop, I will summarize the main features of the 3SMAC model. I will show various recent uses of the models, and comparisons with actual data. I will discuss what can be taken from this model to be used in REM. Reprints of the two papers mentionned above will be available.