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DOMAIN
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-4 km <= x <= +4 km
-4 km <= y <= +4 km

Mesh is tri3 cells with nominal 1 km discretization size.

Problems involve rigid body motion with prescribed slip of 2.0 m along
faults.

FAULTX: See faultx_soln.py
  Fault along x-axis.

FAULTY: See faulty_soln.py
  Fault along y-axis.

FAULTXYP: See faultxyp_soln.py
  Fault along xy line.

FAULTXYN: See faultxyn_soln.py
  Fault along -xy line.
