Strong nonlinear responses originating mainly from sliding on microfractures and open grain boundaries occur when rocks are strained above a threshold near 10−6 (1). We report here that introducing artificial tensile fractures in a brittle rock such as granite produces even stronger nonlinearities. By applying sufficient normal stress perpendicular to the fracture, the nonlinear response can be reduced to that typical of the intact rock. Our measurements are made by driving the sample in torsion at low frequency near 1 Hz. The nonlinear effects we observe are analogous to those caused by partially open fatigue cracks in metals, which generate harmonics when insonified at ultrasonic frequencies (2).