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 by 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. ¿ 2001 American Geophysical Union