Evidence for universal rupture behavior during coal and gas outbursts

Every catastrophic coal and gas outburst begins as a tiny coal failure at almost a point, followed by violent ejection of enormous amounts of gas and pulverized coal. Whether the initial rupture of a large outburst produces observable signatures different from those of smaller events is a fundamental question associated with the potential for outburst prediction. Although outburst processes have been investigated both experimentally and theoretically, the underlying mechanism responsible for outburst rupture behavior is still poorly understood. Here, we performed a series of physical simulations and recorded the infrasound during each outburst. We found that a sharply high-amplitude (~300 Pa) pressure wavelet followed by long (~1 s) low-pressure acoustic waves is a common feature of outburst-generated infrasounds. These pulse-like waveform signatures have been observed during explosive volcanic eruptions, suggesting that outburst dynamics can be characterized as gas-driven eruptions. Furthermore, we used waveform cross-correlation analysis and a simple method to demonstrate that all outbursts in the investigated size range (relative intensity of outburst < 38.5%) share a universal initial rupture behavior and discuss ensuing implications for physical rupture processes. The results of this study provide evidence of the identical onset of large and small events. Energy analysis also further showed that the final size of an outburst lies in the energy contribution of gas desorption and thus can be independent of its initial conditions. If our results hold true for natural outbursts, a size dependency of the rupture onset is unlikely, and consequently, estimating the destructiveness of a future outburst event would be difficult.