# Common Information

In addition to the finite-element mesh, PyLith requires files to specify the simulation parameters.
We specify parameters common to all simulations in a directory in `pylithapp.cfg`, which contains numerous comments, so we only summarize the parameters here.

## Metadata, Mesh, and Output

The `pylithapp.metadata` section specifies metadata common to all simulations in the directory.
We control the verbosity of the output written to stdout using `journal.info`.
We set the parameters for importing the finite-element mesh in `pylithapp.mesh_generator`.

:::{important}
We specify the geographic projection that we used when we constructed the finite-element mesh.
This georeferences the model and enables us to specify points in other geographic coordinates system in spatial databases, which may be more convenient.
:::

```{code-block} cfg
---
caption: Parameters for the mesh input file and the model coordinate system.
---
[pylithapp.mesh_generator]
reader = pylith.meshio.MeshIOPetsc
reader.filename = mesh_tri.msh

# Model coordinate system is UTM zone 11
reader.coordsys = spatialdata.geocoords.CSGeo
reader.coordsys.crs_string = EPSG:32611
reader.coordsys.space_dim = 2
```

## Physics

These quasi-static simulations solve the elasticity equation and include a fault, so we have a solution field with both displacement and Lagrange multiplier subfields.
%
\begin{gather}
\vec{s} = \left(\begin{array}{c} \vec{u} \quad \vec{\lambda} \end{array}\right)^T \\
\boldsymbol{\nabla} \cdot \boldsymbol{\sigma}(\vec{u}) = \vec{0}
\end{gather}
%
We use the default `TimeDependent` problem and solution field with a single displacement subfield of basis order 1.

```{code-block} cfg
---
caption: Parameters for the solution.
---
[pylithapp.problem]
solution = pylith.problems.SolnDispLagrange
```

We use the same material properties in all of the simulations in this directory, so we specify them in `pylithapp.cfg` to avoid repeating the information in the file with parameters for each simulation.
We have one material with uniform material properties.

```{code-block} cfg
---
caption: Material parameters common to all simulations in this directory.
---
[pylithapp.problem]
materials = [elastic]

[pylithapp.problem.materials.elastic]
# We use the default material (elasticity) and rheology
# (isotropic, linearly elastic).
#
# `label_value` must match the tag for the physical group in the Gmsh Python script.
description = Elastic material
label_value = 1

# The properties are uniform within the material, so we use a UniformDB.
db_auxiliary_field = spatialdata.spatialdb.UniformDB
db_auxiliary_field.description = Elastic properties xneg
db_auxiliary_field.values = [density, vs, vp]
db_auxiliary_field.data = [2500.0*kg/m**3, 3.00*km/s, 5.29*km/s]

# The properties are uniform, so we use a basis order of 0, corresponding
# to uniform properties within a cell.
auxiliary_subfields.density.basis_order = 0
bulk_rheology.auxiliary_subfields.bulk_modulus.basis_order = 0
bulk_rheology.auxiliary_subfields.shear_modulus.basis_order = 0

# We discretize the displacement field with a basis order of 1
# so the stress and strain computed from the displacement field
# will have an accuracy of one order lower. Consequently, we use
# a basis order of 0.
derived_subfields.cauchy_strain.basis_order = 0
derived_subfields.cauchy_stress.basis_order = 0
```

Similarly, we set the fault parameters common to all simulations in the directory.
For these simulations, we have three faults that intersect at a common point.

```{code-block} cfg
---
caption: General fault parameters common to all simulations in this directory.
---
[pylithapp.problem]
interfaces = [main_fault, west_branch, east_branch]

## main fault
[pylithapp.problem.interfaces.main_fault]
# The `label` and `label_value` correspond to the name and tag of the
# physical group in the Gmsh Python script.
label = fault_main
label_value = 20

edge = fault_main_ends
edge_value = 30

# Output `slip` and `traction_changes` on the fault.
observers.observer.data_fields = [slip, traction_change]


[pylithapp.problem.interfaces.west_branch]
# The `label` and `label_value` correspond to the name and tag of the
# physical group in the Gmsh Python script.
label =  fault_west
label_value = 21

edge = fault_west_ends 
edge_value = 31

# Output `slip` and `traction_changes` on the fault.
observers.observer.data_fields = [slip, traction_change]


[pylithapp.problem.interfaces.east_branch]
# The `label` and `label_value` correspond to the name and tag of the
## physical group in the Gmsh Python script.
label = fault_east
label_value = 22

edge = fault_east_ends
edge_value = 32

# Output `slip` and `traction_changes` on the fault.
observers.observer.data_fields = [slip, traction_change]
```

All simulations in this directory use Dirichlet boundary conditions on the boundaries of the domain with zero displacements in which we constrain the displacement component perpendicular to the boundary.

```{code-block} cfg
---
caption: Dirichlet boundary conditions common to all simulations in this directory.
---
[pylithapp.problem]
bc = [bc_south, bc_east, bc_north, bc_west]
bc.bc_south = pylith.bc.DirichletTimeDependent
bc.bc_east = pylith.bc.DirichletTimeDependent
bc.bc_north = pylith.bc.DirichletTimeDependent
bc.bc_south = pylith.bc.DirichletTimeDependent

# The `label` and `label_value` correspond to the name and tag of the
# physical group in the Gmsh Python script.
#
# We constrain the displacement component normal to each boundary.
# We use the specialized `ZeroDB` to specify zero values for the Dirichlet
# BC. We will override this parameter in some of the .cfg files to specify
# nonzero values.
[pylithapp.problem.bc.bc_south]
label = boundary_south
label_value = 10
constrained_dof = [1]
db_auxiliary_field = pylith.bc.ZeroDB
db_auxiliary_field.description = Dirichlet BC on south boundary

[pylithapp.problem.bc.bc_east]
label = boundary_east
label_value = 11
constrained_dof = [0]
db_auxiliary_field = pylith.bc.ZeroDB
db_auxiliary_field.description = Dirichlet BC on east boundary

[pylithapp.problem.bc.bc_north]
label = boundary_north
label_value = 12
constrained_dof = [1]
db_auxiliary_field = pylith.bc.ZeroDB
db_auxiliary_field.description = Dirichlet BC on north boundary

[pylithapp.problem.bc.bc_west]
label = boundary_west
label_value = 13
constrained_dof = [0]
db_auxiliary_field = pylith.bc.ZeroDB
db_auxiliary_field.description = Dirichlet BC on west boundary
```