Physical Constraints

When designing AxiSEM3D simulations, or running them, it is worth bearing in mind what the limitations of the code are and what they might mean for this project. We will briefly detail those relevant here:

Boundary conditions

These need to be consistent along the outer surface. What this means is that you cannot have arbitrarily alternating patches of solid and fluid boundaries (ocean/land), or alternating bits of absorbing and reflecting – at least at the moment. Instead, your boundary condition needs to be the same across the length of any azimuth.

Note that we have worded this rather carefully: it should, in theory, be possible to create a ‘part-ocean’ Earth, where the ocean occupies a ‘trench’ in a ring around the planet (imagine excavating everything between 10° N and 20° N to a depth of 3 km, and filling it with water). This might be useful if you are interested in things like quakes at subduction zones or mid ocean ridges – but you would have to edit the mesh file manually in Python, and we have not tried this. It should not be too difficult though. If you try this, take care with the geometry: you will need to rotate your crustal models and source-receiver pair to represent the angles correctly.

If the previous paragraph seems unnecessarily complicated, you can (and probably might want to?) stick to using an all-solid or all-fluid boundary condition. If you need to, you can account for the weight of the water column in a non-uniform way across the whole surface using the ‘ocean load approximation’ or by deforming the physical water layer as described in the particle relabelling section below.

Discontinuities

AxiSEM3D allows you to insert any discontinuity you might want into your ‘base’ (1D) structural model: the Moho, the CMB, 410/660 km, the seafloor, etc. You can also undulate these boundaries [Fernando et al., 2020, Leng et al., 2019] to represent the variation in depth of a particular boundary with location.

However, this undulation remains subject to a very important constraint - the undulated configuration must remain homeomorphic to the original, non-undulated one. This is a fancy way of saying that all boundaries must remain smooth and well-defined – the Moho cannot jump from 8 km depth to 15 km depth at a plate boundary (for example), unless you smooth this transition out across some number of elements. Similarly, you cannot have two Mohos at different depths beneath a particular point on the surface.

This is not normally important on a global scale, as you would not resolve a subduction zone finely enough for these details to be important. In the case of some projects (e.g., simulating a subducting slab), it might be important – and you may have to think carefully about how to incorporate the correct Moho (or seafloor) configuration.