MOMENT TENSOR INVERSION OF INDUCED MICROSEISMIC EVENTS - EVIDENCE OF NONSHEAR FAILURES IN THE -4 LESS-THAN M LESS-THAN -2 MOMENT MAGNITUDE RANGE

To assess objectively the source mechanism of mining-induced seismic events, we developed an algorithm to invert for the moment tensor components. From this inversion, the ratio of isotropic/deviatoric component in the source is obtained. We are applying this method to microseismic events (-4<M<-2) recorded at the Underground Research Laboratory (URL) in Pinawa, Manitoba, where a tunnel is being excavated at 420 m depth. To monitor the seismic activity induced by this opening an array of 16 triaxial accelerometers was installed around the planned tunnel. Results from the moment tensor inversion on a first data set provide evidence of non-shear failure mechanisms. Furthermore, the location of events displaying a tensile component corresponds to a breakout observed underground in the roof of the tunnel.