Modelling Seismic Hazard for Mines

Unlike earthquakes, seismic rock mass response to mining can be controlled. Seismic hazard in mines is driven by the volume and the spatial and temporal distribution of rock extraction. To forecast seismic hazard for a given mine layout we need to extrapolate its parameters in the volume mined domain rather than in time. It is difficult to reconcile the far-field point source model and ground motion recorded away from seismic sources, where details of the rupture process are hidden, with the violent nature of damage resulting from major rockbursts. From linear elastic considerations the rate of stress release with an increase in slip velocity must, at all times, be less than the acoustic impedance, v S. The maximum possible ground velocity is then limited by the bulk shear strength of the rock, bss, and may be roughly estimated as _ umax = 0.5 bss/v S. However if the rate of loading exceeds the rate at which energy can be removed by elastic waves the large strains may have to travel faster than small ones generating extreme ground motion that extends further away from the source. Seismic networks in mines are not suitable to measure strong ground motion close to excavations. Therefore to assess damage potential to mine infrastructure one needs to model ground motion produced by large seismic events expected in a given area. Preliminary results of 3D finite-difference kinematic modelling applied to complex source mechanisms in the presence of underground excavations are presented. This includes two-fault systems proposed by Ortlepp, 1984 and 1997, as well as simulations of surface ground motions in an urban environment. A rotated staggered grid is used as basis for the forward model, which simplifies the implementation of free surface conditions and is more applicable to the highly heterogeneous medium model in mining situations. Even when the static source parameters are kept fixed, there are great differences in resolved ground motions at points of interest under different rupture velocities and slip distributions.

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