Earthquake-induced unusual gas emission in coalmines — A km-scale in-situ experimental investigation at Laohutai mine

In the present study, a km-scale in-situ experiment was carried out at Laohutai coalmine in Fushun city, China, to investigate the possibility of earthquake-induced unusual methane gas emission in coalmines. The motivation of this study was attributed to the observation of a series of coalmine gas explosion accidents in China and the incidental strong earthquakes that seemed to be related to these accidents. Laohutai coalmine was chosen as the experiment base because of its high methane content in the coal body and its past and current active rockburst activities. The mechanisms of rockbursts and earthquakes are the same, i.e., both are violent failure of rock masses and geological structures due to stressing or straining. Detailed microseismic event and gas content monitoring programs were carried out at the mine site and combined with field inspection by experienced engineers, a large amount of data were collected and analyzed. It is revealed that there is a good correlation between rockburst and high methane gas emission events at the mine site. High gas emissions have been observed before, during, and after rockburst events. Therefore, an analogy can be made from the findings of this study, i.e., unusual gas emission can happen in coalmines before, during, and after an earthquake. When gas content is high and all boundary conditions are met, both rockbursts and earthquakes can trigger unusual gas emission, and sometimes the gas gushes are violent enough to fit into the category of gas outbursts. On the other hand, gas gushes can also trigger rockbursts in coalmines. This type of rockburst, which is called gas outburst rockburst, is triggered by the coupling between the unloading of the porous material and the gas storage structures and the expansion due to desorption of methane gases. A fractal relationship between seismic energy and influence radius is derived. It is suggested that methane gas emission warnings could be provided to coalmines located within the influence radius when releasing earthquake risk warnings.