Method for prediction of mechanical properties used in the description of large-strain plastic deformation for any subsurface rock strata of interest. The method is not specific to any geographic region, geological environment or subsurface depth interval, nor is it constrained to a specific lithology or plastic mechanical property. Predictive algorithms for triaxial compressive strength and rock compressibility are derived (23) from a mechanical properties database (21), and used to predict subsurface variation in mechanical properties such as rock strength or rock compressibility, with petrophysical properties generated from, for example, geophysical wireline logs as input to the algorithms (26).