Mining-induced seismicity of a seam located in rock mass made of thick sandstone layers with very low strength and deformation parameters

The paper presents the results of calculations and analyses aimed at explaining the cause and mechanism of strong seismic events E≥10⁵ J (local magnitude, M_l≥1.7). Their source are thick and at the same time weak layers of sandstone, theoretically unable to accumulate elastic energy during the exploitation of the seam 209 in hard coal mine Ziemowit in the edge area of seams 206/1, 206/1-2 and 207. In the developed method of forecasting the pressure/stresses in the areas of the rock mass, which are affected by mining exploitation, the formulas linking the results of geophysical measurements of the longitudinal wave anomaly to the pressure values in its impact zone are used. This method was used to locate areas where elevated pressure occurred compared to gravitational pressure, which had a decisive impact on confinement of layers affected by mining exploitation. In such areas the level of confinement/stiffening of the layers is very high, that is why their deformation in the direction of the gobs is limited. In the case of a formation of a large area of gobs, large-size rock blocks are formed in the rock mass, whose mining-induced separation causes their the impact on the ground. Seismic energy accompanying this event is a function of the impact energy reduced by damping resulting from the physiomechanical properties of the cracked substrate.