Zeitliche Änderungen des globalen Schwerefeldes durch Mantelströmungen: Detektion durch die Satellitenmission GRACE
Zusammenfassung der Projektergebnisse
One of the aims of the project was to test whether time variations of tectonic or geodynamic processes in the earth's mantle may be detectable by the time-dependent geoid measurements of the satellite missions. According to our results, the time-dependent geoid signals produced by mantle-dynamic processes may only give the weak possibility to reach the resolution limits for temporal geoid changes of the 5 – year GRACE mission. Global temporal geoid variations estimated by advecting density heterogeneities from our set of successful models exhibit varying spatial patterns which are, however, as small as 2 - 5 m/a. In comparison, variations due to ocean circulation, redistribution of water and biological masses, post glacial rebound or massive volcanic processes are of the order of al variability itself (O(10-3) m/a). During the ongoing Grace mission it became evident that the postulated temporal resolution of O(10-6 m/a) would not be reached. Dynamic topography variations were obtained to be about one order of magnitude larger O(0.04 mm/a), but of smaller wavelength. The focus has then been shifted to laterally variable viscous flow in the Earth's mantle. Due to the temperature and stress dependence of the viscosity large lateral variations of viscosity (LVV) are expected in the mantle. Global mantle flow models usually assume only radially varying viscosity variations. Therefore we extended our analytical mantle flow code for laterally variable viscosity. We found that inclusion of LVV necessitates a modification to our previously optimized, merely radially stratified, viscosities: While the latter displayed no clear preference for the location with depth of the lowest viscosities, models with LVV seem to favor the occurrence of a soft layer just below the lithosphere over one in the transition zone. The misfit between predicted and constraining surface observables varies little between the cases radially and laterally variable viscosity, while differences are more pronounced between input tomographies themselves. Focusing on only radially varying viscosity within the earth's mantle statistical optimization methods have been developed and applied to models of global viscous mantle flow models. In contrast to previous approaches optimization of adjustable parameters is used in terms of probabilities based on measures of information. The ensemble of 8 different seismic tomography models has been taken and translated into density, to solve for the mantle flow field for several synthetic viscosity depth and scaling factor distributions within the mantle. This gives synthetic surface observables, which then can be used to test the statistical optimization and parameter search algorithms. The tested algorithms are the “neighborhood algorithm” and the “downhill simplex algorithm”. First an extensive study has been carried out showing under which conditions synthetic structures are found. Interestingly, in some cases the search algorithms seem to converge towards viscosity distributions with pronounced but spurious features in the lower mantle, thus such inversion could lead to severe misinterpretations. Applied to real rather than synthetic observables a huge number of different model combinations have been tested including free slip, kinematic boundary conditions, inverting for geoid, gravity, dynamic topography, surface velocities and heat flow, different initial viscosity and scaling factor profiles, number of layers, different regularizations, with and without flow barriers etc.. Applied to observed geoid and other surface observables, a mid mantle low viscosity layer may be inferred for a high degree of regularization.
Projektbezogene Publikationen (Auswahl)
- Geodynamic interpretation of temporal geoid variations, 8th EUROPEAN WORKSHOP ON NUMERICAL MODELING OF MANTLE CONVECTION AND LITHOSPHERIC DYNAMICS, September 13 - 18, 2003, Castle of Hruba Skala, Czech Republic
Niehuus, K., Schmeling, H.
- Geodynamische Interpretation zeitlicher Geoidvariationen. DGG-Tagung, Berlin 2004 (GDP01)
Niehuus, K., Schmeling, H.
- Temporal geoid variations due to global mantle dynamics. Poster, Geodynamik-Workshop 2004, Universität Hamburg. (Frankfurt) 2004.
K. Niehuus
- Geoid, its Temporal Variation and Dynamic Topography as Constraints in Global Geodynamics. DGG Tagung Graz 2005.
Niehuus, K., H. Schmeling,
- Models of Laterally Variable Viscous Flow in the Earth's Mantle With Constraints From Mineral Physics and Surface Observations. AGU. 2005.
Niehuus K., Schmeling H.
- On the Effect of a Low Viscosity Asthenosphere on the Temporal Change of the Geoid - A Challenge for Future Gravity Missions. Journal of Geodynamics, Vol. 39. 2005, Issue 5, pp. 493–511.
Marquart, G., B. Steinberger, K. Niehuus
(Siehe online unter https://dx.doi.org/10.1016/j.jog.2005.04.006) - Temporal geoid variations as constraint in global geodynamics, abstr. , 9th International Workshop on Numerical Modeling of Mantle Convection and Lithospheric Dynamics, 8-14 Sept. 2005, Erice, Italy,
Niehuus, K., Schmeling, H.
- Temporal Geoid Variations as Constraint in Global Geodynamics. EGU05 (Session GD15: Potential Fields in Geostatics and Geodynamics) SRef-ID: 1607-7962/gra/EGU05-A-09190, Geophysical Research Abstracts, Vol. 7, 09190, 2005.
Niehuus, K.
- Statistical Inference on Models of Global Viscous Mantle Flow With Constraints From Mineral Physics and Surface Observations. AGU Fall meeting, 2006.
Niehuus, K., H. Schmeling
