Acoustic emission time-frequency-spatial evolution characteristics and source mechanisms during hydraulic fracturing of rock mass under true triaxial stress conditions

Hydraulic fracturing is widely used in subsurface engineering, yet quantifying fracture propagation remains challenging. We conducted true-triaxial hydraulic fracturing experiments on sandstone with acoustic emission (AE) monitoring and propose a weighted hybrid double-difference/Geiger location algorithm with moment-tensor inversion to resolve the spatiotemporal and time–frequency evolution. The tests reveal four stages. During the main-propagation phase (Stages B–C), a rapid pressure rise coincides with intense AE dominated by waveforms below 100 kHz indicating tensile opening. In the terminal stage (D), pressure declines and low-amplitude AE near 160 kHz indicate fluid–rock friction and breakthrough. Localized AE clouds reproduce the observed fracture geometry. Source mechanisms evolve from mixed opening/closing shear in Stages A–B to predominantly opening–tensile in Stage C, with crack-mode angles of 13 to 59 degrees and magnitudes from -8.5 to -8.0 in A–B, and 12 to 22 degrees and -7.0 to -6.5 in C. Fracture volume grows fastest in Stage C and is mainly tensile; the total tensile volume is about one and a half times the shear volume. The integrated framework offers quantitative, laboratory-validated diagnostics for nondestructive evaluation under complex stress fields.