Density within the Earth? Mike Warner Earth Science & Engineering Imperial College London
Outline Earth structure Methods Accuracy Recommendations
Earth Structure crust vs mantle lithosphere vs asthenosphere ocean vs continent
thermal boundary layer convecting adiabatic interior depth ~ 10 – 50 km highly heterogeneous, chemically differentiated crust mantle (rather) homogeneous & chemically uniform chemical composition less accurate more accurate
thermal boundary layer convecting adiabatic interior temperature depth ~100 km lithosphere heterogeneous, strong, complicated “asthenosphere” highly viscous, convecting at high Raleigh number well-mixed, homogeneous, simple less accurate more accurate
Oceans vs Continents Oceans young simple thin crust mantle density related to cooling Continents long history complicated thick crust no simple density relationships – more accurate – less accurate
Methods gravity seismology others
Gravity low resolution non-uniqueness density heterogeneity much improved if boundaries are known from another technique
Seismology high spatial resolution good absolute accuracy determines boundaries does not measure density directly elastic properties correlated with density more money –> better results
Other methods Density varies with porosity composition temperature pressure Measure or estimate each of these
Accuracy absolute densities –conversion from seismics –determination of composition –determination of temperature boundaries –base of sediments –base of crust –base of lithosphere
Profile length European ~2500 km Trans-continental ~9000 km
European 2500 km lithosphere
European - continental crust
European - oceanic crust
Errors in density location length“a priori”“best” Continental 2500 km4.7%2.9% Oceanic 2500 km2.6%1.7% Continental 9000 km2.0%1.7% Oceanic 9000 km1.8%1.5% Errors are ~2 sigma (errors not really Gaussian)
Recommendations If at all possible: source and receiver on oceanic crust maximum profile length beneath ocean use maximum profile length custom seismic experiments
Recommendations Avoid: Alps central Europe thick crust (e.g. Fenoscandia) Europe to Japan
Recommendations Best profiles: Western Europe to Eastern US Atlantic Islands (Canaries, Maderia, Azores) to Portugal, western Spain, NW France, southern Ireland, western England
Errors in density location length“a priori”“best” Continental 2500 km4.7%2.9% Oceanic 2500 km2.6%1.7% Continental 9000 km2.0%1.7% Oceanic 9000 km1.8%1.5% Errors are ~2 sigma (errors not really Gaussian)