Provided is a diagnostic wellbore testing method that includes forming a diagnostic wellbore in a subterranean zone including one or more formations. The wellbore extends through a portion of a formation having a maximum horizontal stress extending in a first direction and a minimum horizontal stress extending in a second direction perpendicular to the first direction. The method also includes forming a pair of notches on a circumference of the wellbore. The pair of notches are formed diametrically opposite each other. The method also includes applying fluidic pressure to the wellbore at the pair of notches in the second direction while avoiding applying fluidic pressure in the first direction. Fractures propagate from the pair of notches responsive to the fluidic pressure. The method also includes measuring a closure pressure of the wellbore, and providing the closure pressure as the maximum horizontal stress of the formation.