To investigate failure and damage mechanisms in rock, a moment tensor inversion procedure named the SiGMA (Simplified Green's function for Moment tensor Analysis) was applied to acoustic emission (AE) data measured during uniaxial compression and three-point bending tests. In addition, the relative crack volume estimated from the moment tensor analysis was used as the damage index. This paper reports the applicability of the moment tensor analysis to estimate failure planes in rock, as well as to investigate rock failure mechanisms.