Priorities for PyLith software development, such as new features and enhancements. This a draft for community comment (May 19, 2017).

This plan attempts to balance meeting short-term objectives of delivering high priority, new features and meeting long-term objectives of extending the code to solve a broader range of scientific problems.

1. Multiphysics [30%]
   • Implement modular approach for specifying governing equations and computing residuals and Jacobians.
   • Incompressible elasticity via a pressure field [20%]
2. Higher order basis functions [50%]
   • Allow user to select order of basis functions independent of the mesh (which defines the geometry). This permits higher resolution for a given mesh.
3. Switch to using PETSc time-stepping (TS) algorithms. [75%]
   • Replace simple Python-based time-stepping implementations with PETSc time-stepping algorithms that provide support for higher order discretization in time and real adaptive time stepping.
4. Update user manual
   • Convert from LyX to LaTeX for ease of maintenance and editing. [100%]
   • Reorganize for multiphysics implementation. [10%]
   • Reorganize examples. [5%]
     • Focus on demonstrating the range of physics and features beginning with simple cases and building towards more complex cases.
     • Include ParaView Python scripts for plotting results.
     • Consider moving examples to Jupyter notebooks; export to PDF files for “print” documentation.

1. Improve fault formulation for spontaneous rupture [10%]
   • Removes inner solve associated with updating Lagrange multipliers. This will significantly accelerate the nonlinear solve.
2. Allow full specification of the initial conditions (solution and state variables) [0%]
3. Add additional multiphysics implementations and rheologies
   • Poroelasticity [5%]
4. Convert to Python 3 and Pyre 1.0.

1. Add additional multiphysics implementations and rheologies
   • Drucker-Prager bulk rheology with relaxation to yield surface
   • Elasticity + heat flow
2. Reorganize output for time-dependent Green's functions and adjoints
3. Multilevel nonlinear solve
4. Radial basis functions for spatial databases
5. Use threading to accelerate integrations on multi-core machines.

1. Earthquake cycle modeling
   • Same mesh for dynamic and quasi-static parts (dynamic → quasi-static, quasi-static → dynamic, complete cycle)
2. Create strain hardening/softening 2-D and 3-D Drucker-Prager elastoplastic models.
3. Moment tensor point sources via equivalent body forces [5%]
   • Moment tensor point sources provide a mesh independent deformation source that is better suited for Green's function calculations than slip on a fault surface via cohesive cells.

• Major features
  1. Earthquake Cycle Modeling
     • Different meshes for dynamic and quasi-static parts
       • Requires interpolation of fields between different meshes/discretizations and may require extrapolation of solutions when quasi-static problems span a larger domain than the dynamic problems.
  2. Data assimilation
     • Use flexibility of multiphysics implementation to support inclusion of data assimilation

• Minor features
  1. Begin implementation of data assimilation capabilities via adjoint equation.
  2. Combined prescribed slip / spontaneous rupture fault condition
     • Use fault constitutive model to control slip on fault except during episodes of prescribed slip. Need some way to describe when to turn on/off prescribed slip.