# [aspect-devel] internal velocity boundary conditions

Magali Billen mibillen at ucdavis.edu
Tue May 12 17:43:01 PDT 2015

```Hello Everyone,
Unfortunately I’m not going to make it to Hackathon to ask this question in person, but this way everyone
can read (ignore?) and hear or contribute to the response.

The short form of my question is: Is it possible to define “internal” velocity boundary conditions in Aspect: that is
can I fix the velocity at nodes inside the model domain? If the answer is yes, can someone comment on the basic steps
that would be needed (e.g., need to flag these nodes as “boundary conditions” so they get handled properly during assembly of the solution matrices, then assign velocities,…).

Here is the background for why I ask this question:

One of the projects we are starting to develop in Aspect in my group is instantaneous models for a specific
subduction zones. The key issue with these models is that we need to define a starting thermal structure
that is based on the observed geometry of the subducted plate (e.g., from seismicity). There are different
was to do this, and I’ve done several of them for previous models completed using Citcom.

Based on that experience, and given the AMR capabilities of Aspect, I think the best (most accurate and easiest) way to
define the starting thermal structure is to run model in which you have defined the surface of the plate
INSIDE the model (going down into the mantle) and then define fixed velocities associated with this surface.

Note it is not necessary for the elements to conform to this surface (no distortion of the grid), we can use refinement of the grid to get accurate enough for our purposes.

Once the velocity conditions inside the mesh are defined together with the normal external boundary conditions and an initial temperature structure for the plates at the actual top of the mesh, then we would run this model forward in time to kinematically “subduct” the plate. This will allow us to create a smooth 3D starting temperature models for our instantaneous dynamically-driven models that follows the observed shape of the slab.

In addition to using this capability for the purpose describe above, this would allow Aspect to also run what is commonly referred to as “mantle wedge thermal models”, in which the subducted plate and overriding plates are really used as boundary conditions on the flow/temperature in the mantle between them. These models are commonly used to look at the detailed thermal structure and melting in the mantle wedge.

Cheers,
Magali

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Professor of Geophysics & UCD Chancellor Fellow
Earth & Planetary Sciences Dept., UC Davis
Davis, CA 95616
2129 Earth & Physical Sciences Bldg.
Office Phone: (530) 752-4169
http://mygeologypage.ucdavis.edu/billen/
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