Stick slip experiments were performed in a direct shear apparatus using large Inada granite blocks. The dimensions of the pre-existing fault surface were 100 cm (length) 10cm (width). By applying heterogeneous normal stress using three actuators of which forces could be independently controlled, two asperities with different strengths were formed on a fault plane. Under a certain normal stress distribution, single asperity failure and double asperity failure occurred alternately. The single asperity failure refers to a stick slip event in which a weaker asperity ruptures alone, without triggering the rupture of a stronger asperity. The double asperity failure is an event in which a rupture of a weak asperity triggers the rupture of a strong asperity, resulting in a failure of the whole fault. For triggering of a rupture, it is necessary that stress at the strong asperity is accumulated to a certain level just prior to the failure of the weak asperity.