General issues related to running !PyLith.

=== Nondimensionalization ===

It is '''VERY IMPORTANT''' to make sure that the scales used in
the nondimensionalization are appropriate for your
problem. !PyLith can solve problems across an extremely wide
range of spatial and temporal scales if the appropriate scales
are used in the nondimensionalization.

Due to roundoff errors and convergence tolerances in the
iterative solvers, !PyLith relies on reasonable scales in the
solution in constructing the friction criterion and
preconditioning the system. Failure to set appropriate scales in
the nondimensionalization will cause the solution to be garbage.

  * Quasi-static problems
{{{
  Default values:
    relaxation_time = 1.0*year
    length_scale = 1.0*km
    pressure_scale = 3.0e+10*Pa
  Recommended values:
    relaxation_time = TIME_STEP
    length_scale = DISCRETIZATION_SIZE or DISPLACEMENT_MAGNITUDE
    pressure_scale = SHEAR_MODULUS
}}}
  * Dynamic problems
{{{
  Default values:
    shear_wave_speed = 3.0*km/s
    density = 3000.0*kg/m**3
    wave_period = 1.0*s
}}}
{{{
  Recommended values:
    shear_wave_speed = MINIMUM_SHEAR_WAVE_SPEED
    density = DENSITY
    wave_period = MINIMUM_WAVE_PERIOD
}}}
=== Fault output ===
    
Slip and traction vectors are output in the fault coordinate
system (along strike, up-dip, and opening). The direction of the
slip vector corresponds to the direction of motion on the
"negative" side of the fault, which is defined by the origin of
the fault normal vector. To convert to the global coordinate
system, request that the fault orientation be included in the
fault output via:
{{{
  vertex_info_fields = [strike_dir,dip_dir,normal_dir]
}}}
With this information it is easy to rotate the slip or traction vector
from the fault coordinate system to the global coordinate system. This
is usually done in a Python script with HDF5 output or within !ParaView
using the calculator. The expression for the slip in global
coordinates is:
{{{
  (slip_X*strike_dir_X+slip_Y*dip_dir_X)*iHat+(slip_X*strike_dir_Y+slip_Y*dip_dir_Y)*jHat+(slip_X*strike_dir_Z+slip_Y*dip_dir_Z)*kHat
}}}
=== Spontaneous (Dynamic) Rupture in Quasistatic Simulations ===

Use of the !FaultCohesiveDyn object for spontaneous (dynamic) rupture in quasistatic simulations requires careful selection of solver parameters. See Session V of the 2013 Crustal Deformation Modeling tutorial for a detailed discussion.