f Monitoring mining seismicity and hazard analysis, examples from Poland

Many examples have shown that the activity accompanying underground mining works can be hazardous both to underground staff and mining installations, as well as to ground structures. Magnitude of the strongest seismic events in copper mines of Legnica-Glogow Copper District (LGCD) in Poland exceeds 4.0. Furthermore, due to shallow focal depth, events of this size can give rise to peak ground acceleration of more than 2 m/s2. An accurate assessment of seismic hazard posed by mining-induced events is thus a problem of primary importance. Monitoring mining seismicity in LGCD is carried on both underground as well as from the surface. The underground seismic network is supposed to provide information about seismic sources. Yearly several thousands events are recorded and parameterized. The surface accelerometric ground motion monitoring aims at assessing seismic impacts to buildings and other man-made structures in this considerably urbanized area. The mining seismic events are weak earthquakes but the seismicity in mines significantly differs from the earthquake process. Firstly, mining event occurrence is predominantly controlled by time-varying mining works, therefore the active zones in mines are, by their nature, transient. Moreover, even during their lifetime, the activity of these zones changes considerably. In our approach to seismic hazard assessment in mines, locations and times of activity of the zones that will be active in the future are deduced from programs of mining operations. Characteristics of fracturing process in these zones are inferred from characteristics of the zones that were active in the past and can be considered as models for the future activity. The model zones are selected by expert judgment based on anticipated similarity between mining and geologic conditions of the past and future zones. The epistemic uncertainty of such selections is considered within the logic tree scheme. Secondly, due to the heterogeneity of the rockmass fracturing process, the magnitude distribution of seismic events induced by exploitation is often non-Gutenberg-Richter's, complex and multimodal. As a remedy the model-free approach with the non-parametric kernel estimator of magnitude density is applied. This approach ensures reliable estimates of probability functions of event size regardless the actual complexity of the underlying distribution of magnitude. The mentioned ways of analyzing the seismic hazard posed by mining-induced seismicity are illustrated by a practical example in which we predict future seismic activity and estimate limiting values for ground motion in the 20 years horizon.