I am trying to understand the postprocessor output, maximum horizontal compressive stress. The output is a vector, the direction that is perpendicular to the compressive stress direction is extension. I want to get the horizontal tensile stress magnitude. Is the horizontal tensile stress magnitude same as the horizontal compressive stress magnitude. sigma_c= -sigma?

I want to get the horizontal tensile stress magnitude. Is the horizontal tensile stress magnitude same as the horizontal compressive stress magnitude. sigma_c= -sigma?

Yes, if the model is incompressible then the trace of the of the stress tensor should sum to 0, so that logic follows. To be sure, I would test this with a simple analytical model (ex: square geometry, outflow on sides, inflow on top/bottom to confirm. As a second check, you can also recalculate the stresses and stress directions by hand following the workflow in the principal stress post processor.