We used a new rapid data acquisition system to examine the role of pre-existing crack density in determining the mechanical properties of crystalline rock. Two similar coarse-grained samples. Tsukuba granite (TG) and granitic porphyry (GP), were used in triaxial compression tests. The main difference between the two rocks is that TG contains main large pre-existing cracks, whereas GP is almost crack-free. In our tests, the two rocks showed significantly different behaviors. In GP, before the fault nucleation, AE activity was low and showed increasing b-values with increasing stress. But in TG, a large number of AEs were observed and showed short-term b-value anomalies-mutual fluctuations on a decreasing background. The short-term fluctuations of b-values are closely correlated with those of the spatio-temporal clustering of AE locations. Our results demonstrate: 1) pre-existing cracks are the most dominant factor of all heterogeneities that govern the fault nucleation process in laboratory rock samples; 2) failure predictability is dependent on the pre-existing crack density. ¿ 2000 American Geophysical Union