Inversion of stress field evolution consisting of static and dynamic stresses by microseismic velocity tomography

•In micro-fissures region, P-wave characterizes with low velocity and high gradient.•Large-scale and high velocity gradient as a precursor of macroscopic caving of roof.•Prior to rockburst, high velocity gradient region gradually evolves into a very small area.•Source of rockburst locates in the peak zone of velocity gradient, rather than velocity.•The larger P-wave velocity gradient is, the higher the rockburst dangerous level is.

In order to reveal the stress precursors of the “3.15” rockburst disaster that occurred in the No. 1 working face of Junde coal mine, the combined stress field inversion including static and dynamic stresses was conducted by using P-wave velocity tomography, and the static high-stress concentration regions were further designated. Simultaneously, the evolution of dynamic stress generated by seven periodical caving events of primary roof was analyzed, and the fracturing characteristics were described in detail. Ultimately, the stress precursors of the “3.15” rockburst for early warning were clearly revealed. The main conclusions are as follows: (1) the micro-fissures inside coal and rock mass produce the significant reflection and refraction on the propagation of microseismic (MS) waves, and thus P-wave overall characterizes with lower velocity and higher velocity gradient; (2) after the slightly fracturing of primary roof on March 9, 2013, the maximum stress concentration factor suddenly rises to 20. This sign can be viewed as an early warning precursor of rockburst; (3) until March 12, 2013, the large-scale micro-fractures begin to form inside primary roof, and thus the corresponding high velocity gradient regions are generated. This phenomenon can be regarded as an effective precursor of macroscopic fracturing of primary roof; and (4) before the “3.15″ rockburst, the scope of P-wave velocity gradient in the No. 1 working face does not change significantly. The above findings may put forward a certain reference for accurately warning the stress precursors of rockburst by P-wave velocity tomography.