Study on movement law of extremely thick strata and triggering mechanism of mine earthquakes

Mine earthquake are unnatural seismic activities during mining. In the Ordos mining area in North China, the Jurassic coal seam is commonly overlain by the extremely thick cretaceous sandstone strata. It is easy to trigger strong mine earthquakes when the fracture and slip of the extremely thick strata occur caused by the coal extraction. The movement characteristics and the dynamic response of the extremely thick strata are the basis for mine earthquake prevention and control. Based on Vlasov’s theory, this paper used data from surface boreholes, surface subsidence and seismic monitoring to investigate the structure evolution law and movement characteristics of the extremely thick strata in fully-mechanized caving mining of an Ordos coal mine, aiming to reveal the triggering mechanism of the mine earthquakes. The results showed that in the retreating period with no goaf nearby, a small goaf area behind the longwall was presented, and the low roof collapsed rapidly with fracture angles between 64° and 72°, and no obvious fractures were detected in the far-field extremely thick strata, which only caused minor surface subsidence. In the retreating period along with goafs, the roof breaking height expanded to the extremely thick strata with fracture angles increased, and the fractures emerged in the extremely thick strata, which caused a stepwise increase of the surface subsidence. Due to the large thickness and far-field strength of the extremely thick cretaceous sandstone strata, it is deduced that when the longwall chainage was about 324.3 m, the extremely thick strata met the requirement of initial fracture with a fracture step of 83.7 m, and the strong mine earthquakes started to be detected. The triggering mechanism of mine earthquake induced by breakage of extremely thick strata is that: with the increase of goaf area, the fracture height of the low roof gradually extends to the far-field extremely thick strata, and it is easy to trigger mine earthquake during the vertical “O-X” type initial fracture, fracture adjustment and periodic fracture of the extremely thick strata. The outcome of this study can provide reference on the prevention and control of mine earthquakes occurred in the far-field extremely thick strata.