Fig. 1

Complex fracture pattern representing a cloud-fracture network, produced by hydraulic fracturing of granite at 450 °C under true triaxial stress, adapted from Watanabe et al. (2019). The fractured granite impregnated with the fluorescent resin (a). Even blue fluorescence over cross-sections of both the σ₁–σ₂ and σ₁–σ₃ planes of the sample (b, c), where σ_1, σ_2, and σ₃ denote the maximum, intermediate, and minimum principal stresses, respectively. Optical microphotographs of a thin section of the σ₁–σ₂ plane cross-section near and far from the borehole, using uncrossed polarized, crossed polarized and UV light (d–l)