Using acoustic emission data obtained from laboratory double torsion tests,
we have analysed the fractal nature of a series of 29 granite microfractur
ing processes in time. The data represent a wide variety of timescales, str
ess environments (increasing load with a constant displacement rate, relaxa
tion, creep), soaking conditions [air, water, dodecyl trimethyl ammonium br
omide (DTAB), polyethelene oxide (PEO)], and material anisotropy. We find t
hat the time distribution of rock microfracturing displays fractal and mult
ifractal properties. In some cases, it has a single fractal or a multifract
al structure. In other cases, it changes from a single fractal structure in
to a multifractal structure as the system evolves dynamically. We suggest t
hat the heterogeneity of the rock, the distribution of joints or weak plane
s, the stress level, and the nature of the microfracturing mechanism lead t
o these multifractal properties. Whatever the fractal structure of the syst
em, a lower fractal dimension is generally produced at near-failure of the
rock due to an increased clustering. This result concerning the fractal-dim
ension decrease is consistent with the conclusion drawn from the spatial di
stribution of rock microfracturing. Therefore, from the vantage point of ob
servation of the time distribution of rock microfracturing, the decrease of
the fractal dimension has a potential use as a rock failure predictor.