9. Microseismic activity associated with hydraulic mining

Microseismic activity induced by the hydraulic mining of steeply inclined coal seams has been monitored in order to clarify the strata mechanics of hydraulic mining. A mine-wide seismometer network detected 652 microseismic events, with local magnitudes greater than −2.5, during mining of the panel which is situated at 1000 m below the surface. The largest microseismic event with a local magnitude of 0.1 was felt both on the surface and underground. The magnitude-frequency relation of seismic events followed Gutenberg-Richter's formula, in which the b-value was estimated as 0.92. The seismic energy release rate varied with face advance due to changes in the mining geometry and configuration. A majority of hypocentres concentrated on the coal pillar which had been left in the old workings and was bounded by the active mining panel. Another group of seismic foci clustered on a pre-existing fault which runs through the border of the panel. The other events were distributed around the coal face and moved with the face advance. The spatial distribution of the seismic energy suggests that microseismic activity associated with hydraulic mining is mainly induced by collapse of coal pillars and fracture of the front abutment of coal face, and also partly caused by the roof caving into the goaf and by floor heave.