Glossary

2.5D: see Axisymmetric.

Axisymmetric: Here, a model or method which does not have any longitudinal variation; i.e., where any variation in parameters or physics is confined to within a single plane (‘2D slice’) of the planet. What this means is that any 2D slice through the Earth is identical to all the others. Note that in a 2.5D method, any structure must wrap all the way around the axis: e.g., a high velocity circular province in the 2D plane will become a torus in the 2.5D version.

AxiSEM: The AxiSymmetric Spectral Element Method: the 2.5D version of the code we are using here.

AxiSEM3D: The AxiSymmetric Spectral Element Method 3D: the 3D version of the above.

Geometric model: A 3D model which involves the radial deformation of some interface to change its depth/radius at different latitudes and longitudes. An example is the Moho in Crust 1.0.

Green’s Function: In this case, the response of the simulation domain to a near impulsive (quasi delta function) source. Note that we say near impulsive because in a computational method of this type the source must have some width (here, a few time steps). In general this is so much less than the seismic period that it is close enough to a delta function for most purposes.

Jacobian: Computations on individual elements in AxiSEM3D are mapped back to a reference square element, which increases computational speed. As most elements are not square to begin with, the Jacobian (and its inverse) associated with each coordinate transform must be calculated.

Pseudospectral Method: A method of solving the wave equation which uses basis functions to represent key quantities in the equations of motion. See for more detail.

Spectral Element Method: A fast and efficient way of solving the wave equation, see for more detail.

Volumetric model: A 3D model which represents a change (with respect to the 1D case) in seismic properties (e.g., Vp or Vs), such that they vary with latitude and/or longitude and/or depth/radius.