Constraints on transition zone discontinuities from surface wave overtone measurements Ueli Meier Jeannot Trampert Andrew Curtis

Non-linear inversion of probability density functions of phase velocity maps Visser et al. 2008

The neural network (NN) approach: Bishop 1995, MacKay 2003 A neural network can be seen as a non-linear filter between any input and output The NN is an approximation to a non-linear function g where d=g(m), forward or inverse A training set is used to calculate the coefficients of the NN by non-linear optimisation

1) Generate a training set: D={d n,m n } Network Input: Observed or synthetic data for training Neural Network dndn p(m|d n,w) Network Output: Conditional probability density 2) Network training:3) Forward propagating a new datum through the trained network (i.e. solving the inverse problem) d obs p(m|d obs,w*)

Topography (Sediment) Moho Depth 220 km Discontinuity Vpv, Vph, Vsv, Vsh,  Vs (Vp,  linearly scaled) % Prem +- 5 % Prem 8 layers 3 layers Model Parameterization 9 layers 24 layers down to 1500 km below Prem 400 km Discontinuity 660 km Discontinuity 8 layers Vpv, Vph, Vsv, Vsh,  20km +- 5 % Prem 20km 30km

Example of a model realisation

Data At each geographical point, we use local dispersion curves and their Gaussian uncertainties (Visser et al. 2008) Periods between 35 s and 175 s Rayleigh waves up to 6 th overtone Love waves up to 5 th overtone

Mean dlnVs model

Discontinuities

Trade-offs between parameters

Interpretation Mixture of olivine, wadsleydite and melt Surface waves see top of discontinuity SS underside reflections see bottom of discontinuity ~30 km ~0.8wt% water The nature of the average 410km discontinuity 394 km 418 km

Interpretation dlnVs and dhTZ  dT and dOH Romanowicz and Gung 2002

Interpretation Average nature of discontinuities: 660-km relatively sharp: surface waves and SS underside reflection are consistent 410-km is ~30km broad: corresponds to an average of 08wt% of water

Interpretation Lateral variations in the transition zone: Temperature variations agree with tectonic expectations Water variations show a remarkable correlations with maps of global attenuation Slabs unlikely to transport water to the TZ Water is likely a remnant of deep magma ocean  water filter model