Fractal Structure of the Hypacenter Distributions and Focal Mechanism Solutions of Acoustic Emission in Two Granites of Different Grain Sizes

Abstract

Coarse-grained Inada granite (grain size 5 mm in average) and fine-grained Oshima granite (grain size < 2 mm) samples were deformed in triaxial compression experiments. Acoustic emission (AE) was monitored using 20 transducers in real time. In each experiment, hypocenters of some thousands of events were determined using anisotropic velocity model based on measured data. The spatial distributions of AE hypocenters in both rocks had fractal structure. Their fractal dimensions were 2.3 and 2.7 in average for Inada granite and Oshima granite, respectively. Focal mechanisms of AE showed an important difference between two granites. In Inada granite, type-S (emitted by shear fracturing) was dominant throughout the fracturing process. However, in Oshima granite, fracture types were dependent on stress levels. At lower stress stage, type-C (assigned to implosive fracturing) was dominant (although only few in number). At the stress level below 80% of the fracture strength, type-T (emitted by tensile fracturing) was dominant, whereas above this stress level, type-S gradually became dominant.