THE 3-DIMENSIONAL SEISMOLOGICAL MODEL A-PRIORI CONSTRAINED - CONFRONTATION WITH SEISMIC DATA

We compare the predictions of an a priori model of the upper mantle wi th seismic observations of surface waves and eigenmodes. The S-Dimensi onal Seismological Model A Priori Constrained (3SMAC) has been develop ed by Nataf and Ricard [1996]. It is based on the interpretation-by ge odynamicists of the near surface layers of the Earth; on distributions of temperature, pressure, and composition as a function of depth; and then on estimates of seismic parameters (density, velocities, attenua tions) from solid state laboratory measurements as a function of tempe rature and pressure. The 3SMAC predictions are confronted with observa tions consisting of phase velocities for Love and Rayleigh waves in th e period range of 70-250 s [Montagner and Tanimoto, 1990]. We first sh ow that tomographic inversions applied to 3SMAC synthetics induce a st rong smoothing of the heterogeneities. This casts doubt on the meaning of the spectra of mantle heterogeneities revealed by tomography We th en show that most of the Love and Rayleigh fundamental mode observatio ns for periods less than 200 s are satisfactorily predicted by 3SMAC. The major differences come from the seismic velocities under the Red S ea and Southeast China, which are much slower than what is estimated f rom 3SMAC, as well as those under Greenland, which are not as fast as the other cratonic areas. Because the lithosphere is thinner than 100 km under oceans and thinner than 300 km under continents in 3SMAC, we suggest that the existence of deeper lithospheric anomalies as propose d in many tomographic models is mostly due to a spurious effect of the inversion rather than implied by surface wave data. Half of the varia nce of the degree 2 anomaly mapped by low-degree eigenmode observation s can be explained by lithospheric velocity structures. The other half is highly correlated with the distribution of deep slabs, but its amp litude is a factor of 3 or 4 larger than that predicted by 3SMAC. The lithospheric anomalies present a degree 6 pattern well correlated with the distribution of hotspots even when the thermal anomalies that cou ld be associated with plumes are not included in 3SMAC. Our results em phasize the importance of giving very close attention to ''surface cor rections'' in tomographic models.