A comprehensive earth model across the scales: a global full waveform update, and methodological developments

We present the current state of the ‘Comprehensive Earth Model’ (CEM), a solver-independent multi-scale model of the global distribution of density and visco-elastic parameters. The overall goal of this project is to produce a model that represents the Earth on all seismically accessible scales; which contains high resolution sub-models where data and computational concerns allow, and which presents a low wavenumber Earth in regions yet to be probed in detail. To accomplish this, we have designed the model to be independent of any particular forward solver. This allows the usage of a wide variety of forward and inverse techniques, each of which may contribute updates within their respective regimes of validity.

In order for these updates to be included in future releases of the CEM, they must satisfy a global reference dataset. This dataset is currently being constructed, drawing from waveform, traveltime, and normal mode catalogues, and incorporating both earthquake and ambient noise sources.

To support future multiscale inversions, we report on methodological developments surrounding the project, including specific interfaces with forward solvers and a suite of tools for processing gradient-based model updates. Advances in forward modelling codes, such as the porting of the spectral element solver SPECFEM3D to heterogeneous computing clusters, allows for the efficient and fully numerical calculation of sensitivity kernels on the global scale. Taking advantage of these developments, we present a global-scale transversely isotropic mantle-and-crust update to the CEM, with a misfit criterion based on waveform phase differences, and iterative nonlinear model perturbations found via adjoint techniques. Additionally, regional scale updates from both traveltime and waveform tomography are presented and discussed.

An open source software package has been developed, which aims to ease the processing of model updates, and which exists independently from any particular forward solver. By interpolating sensitivity kernels onto an external, regular grid, a generic collection of processing and smoothing techniques are used. This flexibility allows the package to be included in a variety of inversion workflows, from regional scale traveltime tomographies, to global scale full waveform inversions.