Source parameters of acoustic emission (AE) generated by crack generation in a granite sample are determined using a spectral model for the crack. To make a quantitative spectral analysis of AE possible, a relatively large sample having a size of 32×32×82cm³ is used, and a sensor consisting of a coil and a magnet is newly developed. In the spectral analysis, it is assumed that the rupture velocity amounts to 0.9V_s, where V_s is the S-wave velocity. Although no mechanism solutions of cracks are determined directly, it is deduced from the configuration of a fault zone in the sample that the mode of cracks is tensile.
AE's having an amplitude that is sufficiently large for the present analysis are obtained under the ultimate stress of this experiment, which is about 60% of the compressional fracture strength of sampled rocks. Linear dimension r of cracks are estimated as 1-3cm. A quantity M₀, which is defined by the rigidity multiplied by the crack volume, amounts to an order of 10⁶-10⁷ dyn·cm. The stress drops which are calculated from r and M₀ are several bars and are relatively low compared to the tensile fracture strength of the sampled rock, which is about 170bar.