[CIG-SHORT] traction_perturbation

[翁辉辉]

dear developers：
 
  I'm confused about the traction_perturbation module.As a example in examples/3d/hex8/step13.cfg ,I add the code in it as follow  :
[pylithapp.timedependent.interfaces.fault]
traction_perturbation = pylith.faults.TractPerturbation
[pylithapp.timedependent.interfaces.fault.traction_perturbation]
db_initial.label = Initial fault tractions
db_initial = spatialdata.spatialdb.UniformDB
db_initial.values = [traction-shear-leftlateral,traction-shear-updip,traction-normal]
db_initial.data = [0.0*MPa,0.0*MPa,0*MPa]

The earthquake shear stress change on the center of the fault is about 2*MPa,5.83MPa to 4MPa.If I change the x_pos initial displacement which produce the fault normal stress(1*m equivalent to 10*M) from -1*m to 0*m and the traction-normal in above code from 0*MPa to 10*MPa, then the earthquake shear stress change is still the same. If I make the traction-normal be 20*MPa and the x_pos initial displacement be 0*m,the expected stress drop shall be 4*M(12*MPa to 8*MPa) using the static-coefficient(0.6) and dynamic-coefficient(0.4),but the actual shear stress change is 9*M (17.3*MPa to 8*MPa). how can this happen?

thanks 
huihui