HR: 0830h
AN: S21E-0374    [PDF]
TI: Viscosity and S-Wave Conversion Factor for the Earth's Mantle Based on CHAMP Gravity Data and New Tomography Models
AU: * Marquart, G
EM: marquart@geo.uu.nl
AF: Faculty of Earth Sciences, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AB: The viscosity structure and the internal buoyancy forces of the Earth's mantle are essential for understanding the convective flow pattern. These buoyancy forces are directly related to density inhomogeneities. With the use of seismic tomography it is possible to determine the seismic velocity distribution in the Earth - but the conversion factor between the seismic velocity and density is still not properly known. This is mainly due to uncertainties in the chemical cmposition of the mantle. If the origin of lateral heterogeneities in seismic wave velocity is purely thermal, the conversion factor should be around 0.2 to 0.4 as deduced from mineral physics. However, recent studies to relate mantle dynamics and gravity data reported even negative values for the conversion factor in the uppermost and lowermost mantle. For the uppermost mantle this finding has been explained by depletion in iron during partial melting. In the presented study the equation of motion is solved for an incompressible 6-layer shell model and the response function for geoid, dynamic topography, and (poloidal) surface velocity is determined. The internal load is derived from 4 different tomography models (sb4l18, Masters et al., 1999; s362d1, Gu et al., 2001; s20rts, Ritsema \& van Heijst, 2000; saw24b16, Megnin \& Romanowicz, 2000). In a large forward search we determined models with a correlation $>$ .85 between the synthetic geoid and the CHAMP hydrostatic geoid for L$<$16 and correlation $>$ .6 for gravity and surface velocity. The findings for the various tomographic models are quiet similar: If only the geoid fit is considered, successful models show a negative conversion factor between 100 and 300 km depth. However, if the fit to gravity and surface velocity is also taken into consideration, the conversion factor is small, but remains positive. In the deeper part of the mantle the conversion factor is reduced to values of about 0.1 between 700 and 1200 km depth, and otherwise roughly constant with values around 0.28. The viscosity is slightly reduced (compared to the scaling value of 10\textsuperscript{21} Pa s) in the asthenosphere and even stronger decreased in the mantle transition zone between 410 and 670 km where major changes in mineral geometry occur and the release of water was recently proposed in the upper part of the transition zone. Resolution for both, viscosity and conversion factor, is poor below the transition zone down to about 1500 km, but well confined in deeper parts of the mantle, where a viscosity between 30 to 40 10\textsuperscript{21} Pa s and a conversion factor of 0.28 to 0.32 is found.
DE: 1025 Composition of the mantle
DE: 1212 Earth's interior--composition and state (8105)
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 7207 Core and mantle
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting