IF you look at the stresses the shear stresses gets bigger to 45 degrees then gets smaller after 45 degrees. So why doesn't it follow the angle with seemingly the most stress. See Page: Why 30 degrees instead of 45 degrees Fracture Angle with sigma 1 Nature wants the minimum normal stress possible with the maximum shear stress possible and at 45 degrees we aren't in that sweet spot were the normal stress is as relatively small as possible and the shear spot is as relatively high as can been and this is at about 60 degrees this is why the faults will crack in the same orientation in nature.