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Undergraduate students:
Wardah Mohammad Fadil
Graham Gerdes

  Graduate students:
Sam Haugland (PhD)
Ross Maguire (PhD)
Russel Wilcox-Cline (PhD) (with E. Hetland)
  Postdocs:
Carlos Chavez

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Seismic tomography

 

Model S40RTS [Ritsema J, Deuss A, van Heijst HJ, Woodhouse JH, Geophys. J. Int., 2011] and SP12RTS [Koelemeijer P, Ritsema J, Deus A, van Heijst HJ, Geophys. J. Int., 2015] are our most recently developed tomographic models of the mantle based 20 million Rayleigh wave dispersion, 500,000 shear-wave Traveltime, and 1100 normal-mode Splitting function measurements.

A gzipped TAR file that includes the model, codes to read the model coefficients, and GMT scripts for plotting cross-sections and maps can be downloaded. Since we compute the exact inverse, it is straightforward to construct models based on different damping parameters (and hence different number of resolved unknowns), Contact me or Paula Koelemeijer to discuss how to best transfer these files (they are too large to post here). Tomographic models from other research groups can be accessed via the IRIS EMC pages.

(right) Maps of shear velocity perturbations (in %) at 100 km, 600 km (the transition zone), 1000 km and 2850 km (the core-mantle boundary region) depth in the mantle, according to model S40RTS. The shear velocity is low (high) compared to PREM in regions shaded red (blue). .Shear velocity variations of more than 15% in the uppermost 100 km of the mantle are due to the ocean/continent variations and plate tectonics. High velocity anomalies in the transition zone indicate the position of slabs of subducted oceanic lithosphere. Broad low shear velocity structures beneath Africa and the central Pacific in the lower mantle are likely hot, but relatively dense (and stable) thermo-chemical piles.

 

 

3D wave propagation – scattering

Using AxiSEM (Nissen-Meyer et al. 2014) simulations, we investigate whether thermochemical convection in the mantle explains the global characteristics of wave traveltime distribution and scattering. The example on the left shows the expected signal in short-period P-wave recordings of scattering from fragments of eclogite that are distributed in the mantle after 4 billion years of ridge spreading and lithosphere recycling. In our work we aim to constrain the thermodynamic parameters that control the distribution of subducted crust in the mantle using the amplitude and arrival time of PKIKP precursors

(left) AxiSEM waveform simuations of PKIKP precursors due to PKP scattering off small-scale compositonal heterogeneity in Earth's mantle formed by thermochemical convection (according to Brandenburg et al. [2007]. The seismograms in green show PKIKP precursors due to scattering of small-scale structure, shown as black tracers, in the cross-section labeled "Composition" (adapted from Haugland et al. [2015]).

 

3D wave propagation – traveltimes

 
Using SES3d (Fichtner et al. 2008) simulations, we investigate the expected waveform perturbations for hypothetical mantle structures auch as plumes and slabs. We construct these geological structures by numerical convection simulations and by relating temperature and compositional heterogeneity to wave speed variations using mineral physics constraints. The example on the right shows the expected (frequency-dependent) traveltime delay of S waves propagating through the tail of a thermal plume in the lower mantle.

(right) (a) Temperature (left half) and shear velocity (right half) structure of a hypothetical thermal mantle plume. (b) Ray geometry of shear waves propagating through this plume. (c) Predicted traveltime delays as a function of distance and signal frequency (adapted from Maguire et al.[2015]).

 

 

3D wave propagation – reflections

Using SpecFEM3d (Komatitsch et al. 2002) simulations, we investigate the effect of wave speed heterogeneity in the mantle on the traveltimes and amplitudes of SS precursors. The example on the left shows the expected traveltime variation in SS-S660S and S660-S410S for shear wave speed heterogeneity as in S20RTS. Using hypothetical models of the upper mantle heterogeneity and 3D sensitivity kernels we estimate the resolution of undulations on the phase transitions in the upper mantle transition zone.

(left) Estimate of the SS-S660S and the S660S-S410S traveltimes in a mantle with horizontal 660-km and 410-km phase transitions and volumetric shear velocity heterogeneity according to S20RTS (adapted from Bai et al. [2012]).

 

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