The change of viscosity during the simulation

Hi everyone,
Recently I wanted to see if the effect of gravity would affect the viscosity of the low-viscosity layer in crust to weaken further . When modeling, I give the initial viscosity, can I see the viscosity at different time steps? Or the viscosity is calculated by the following formula :
I can get the Cauchy stress and the total strain in the X and Y directions, and can I use this formula to calculate the viscosity. If so, what are the values of the spring constant of the linear maxwell viscoelastic model in pylith?