Stress evolution caused by hard roof fracturing and associated multi-parameter precursors

To reveal the mechanism and precursors of the 1.21 rockburst induced by hard roof fracturing in the No. 1 working face of the Junde coal mine (JCM), the inversions including P-wave velocity, velocity gradient, stress concentration and accumulated energy were elaborately conducted by seismic tomography, and the static high stress concentration and dynamic stress transfer were vividly demonstrated. Simultaneously, the multi-parameter characteristics including microseism (MS), electromagnetic emission (EME) and acoustic emission (AE) before and after the rockburst were comprehensively investigated, and then the corresponding precursors were summarized. The main conclusions are as follows: (1) the intrinsic factor of the 1.21 rockburst is owing to the original static high stress concentration in tailentry ahead of the No. 1 working face, and the external factor is ascribed to the dynamic stress transfer and superimposition with energy input generated by roof fracturing; (2) before roof fracturing, the region where sources congregate shows higher velocity, velocity gradient and stress concentration, accompanied with energy accumulation. After fracturing, the region manifests abnormal low velocity. However, the surrounding and far-field rockburst destruction regions appear higher velocity due to dynamic stress transfer; and (3) an obvious transform from dominantly hybrid to dominantly low-frequency of MS events will occur as roofs experience micro-fissure initiation, propagation, convergence, and coalescence, and thus it is concluded that the gradual transform of MS dominant spectrum from high to low frequencies represents a precursor for warning roof fracturing. The findings may put forward a certain reference for warning the type of rockburst triggered by roof fracturing.