Recent increase of induced seismicity and high rock stresses in the Pyhäsalmi mine have proven the need for an analysis of the seismic data and prediction of hazard. Seismic events and other influencing factors are analysed in order to forecast and quantify the level of induced seismicity risk in the Pyhäsalmi mine. The expected maximum event size is found by clustering seismic data using the Quality Threshold and Single Linkage clustering algorithms, and applying the Gutenberg-Richter's frequency-magnitude relationship. Induced seismicity risk is assessed using the Quantitative Seismic Hazard and Risk Assessment Framework. The potential tor induced seismicity is calculated as a function of the maximum local seismic magnitude, rock stresses and strength, local geology, excavation geometry, and ground support capacity. The exposure to hazard is taken into account in order to quantify the level of risk. The assessment is performed on three mining levels at about 1.2 km depth, which are divided into assessment zones. Most of the assessment zones have low seismic risk and only few have an elevated potential of rockburst. The highest risk is found along the northern contact zone of ore and waste rock, adjacent to important mine infrastructure. The critical areas are identified and installation of ground support with increased dynamic capacity is recommended as a risk mitigation strategy.
The Pyhäsalmi mine is an underground copper, zinc and pyrite mine located in central Finland and owned by First Quantum Minerals Ltd. It is the oldest Finnish metal mine in operation and one of the oldest in Europe. Pyhäsalmi is 1440 m deep, what makes it the deepest metal mine in Europe. Over the last years mining has progressed to depths over 1000 m, what resulted in an increase in rock stresses and induced seismicity. The Pyhäsalmi mine is characterized by high horizontal stresses. The average major principal stress at the -1125 level was measured to be 65 MPa dipping at 5° towards 310°E. Results of measured and estimated in-situ stresses are presented in Table 1.