# Step 8: Slip on Two Faults and Power-law Viscoelastic Materials

% Metadata extracted from parameter files.
```{include} step08_twofaults_powerlaw-synopsis.md
```

## Simulation parameters

In this example we replace the linear, isotropic Maxwell viscoelastic bulk rheology in the slab in Step 7 with an isotropic powerlaw viscoelastic rheology.
The other parameters remain the same as those in Step 6.
The parameters specific to this example are in `step08_twofaults_powerlaw.cfg`.

```{code-block} cfg
---
caption: Power-law viscoelastic bulk rheology parameters for Step 8.
---
[pylithapp.problem.materials]
slab.bulk_rheology = pylith.materials.IsotropicPowerLaw

[pylithapp.problem.materials.slab]
db_auxiliary_field = spatialdata.spatialdb.SimpleDB
db_auxiliary_field.description = Maxwell viscoelastic properties
db_auxiliary_field.iohandler.filename = mat_powerlaw.spatialdb

bulk_rheology.auxiliary_subfields.power_law_reference_strain_rate.basis_order = 0
bulk_rheology.auxiliary_subfields.power_law_reference_stress.basis_order = 0
bulk_rheology.auxiliary_subfields.power_law_exponent.basis_order = 0
```

## Running the simulation

```{code-block} console
---
caption: Run Step 8 simulation
---
$ pylith step08_twofaults_powerlaw.cfg

# The output should look something like the following.
 >> /software/unix/py39-venv/pylith-debug/lib/python3.9/site-packages/pylith/meshio/MeshIOObj.py:44:read
 -- meshiopetsc(info)
 -- Reading finite-element mesh
 >> /src/cig/pylith/libsrc/pylith/meshio/MeshIO.cc:94:void pylith::meshio::MeshIO::read(pylith::topology::Mesh*)
 -- meshiopetsc(info)
 -- Component 'reader': Domain bounding box:
    (-100000, 100000)
    (-100000, 0)

# -- many lines omitted --

25 TS dt 0.2 time 4.8
    0 SNES Function norm 5.602869611772e-06 
    Linear solve converged due to CONVERGED_ATOL iterations 262
    1 SNES Function norm 6.656502817834e-08 
    Linear solve converged due to CONVERGED_ATOL iterations 117
    2 SNES Function norm 9.645013365143e-10 
  Nonlinear solve converged due to CONVERGED_FNORM_ABS iterations 2
26 TS dt 0.2 time 5.
 >> /software/baagaard/py38-venv/pylith-debug/lib/python3.8/site-packages/pylith/problems/Problem.py:201:finalize
 -- timedependent(info)
 -- Finalizing problem.
```

As in Step 7, the simulation advances 26 time steps.
With a nonlinear bulk rheology, the nonlinear solver now requires several iterations to converge at each time step.

## Visualizing the results

The `output` directory contains the simulation output.
Each "observer" writes its own set of files, so the solution over the domain is in one set of files, the boundary condition information is in another set of files, and the material information is in yet another set of files.
The HDF5 (`.h5`) files contain the mesh geometry and topology information along with the solution fields.
The Xdmf (`.xmf`) files contain metadata that allow visualization tools like ParaView to know where to find the information in the HDF5 files.
To visualize the data using ParaView or Visit, load the Xdmf files.

In {numref}`fig:example:reverse:2d:step08:solution` we use ParaView to visualize the y displacement field using the `viz/plot_dispwarp.py` Python script.
First, we start ParaView from the `examples/reverse-2d` directory.
Before running the `viz/plot_dispwarp.py` Python script as described in {ref}`sec-paraview-python-scripts`, we set the simulation name in the ParaView Python Shell.

```{code-block} python
---
caption: Set the simulation and exaggeration in the ParaView Python Shell.
---
>>> SIM = "step08_twofaults_powerlaw"
>>> FIELD_COMPONENT = "X"
```

:::{figure-md} fig:example:reverse:2d:step08:solution
<img src="figs/step08-solution.*" alt="Solution for Step 8. The colors indicate the magnitude of the x displacement, and the deformation is exaggerated by a factor of 1000." width="100%"/>

Solution for Step 8 at t=100 years.
The colors of the shaded surface indicate the magnitude of the x displacement, and the deformation is exaggerated by a factor of 1000.
The undeformed configuration is show by the gray wireframe.
Our parameters for the power-law bulk rheology result in much less viscoelastic relaxation in this case compared to Step 7.
:::