Abstract
The mechanism generating the micro-fractures in the mine was investigated.
The micro-fractures occurring immediately after an explosion were observed for about an hour at the Nakatatsu mine in Fukui prefecture in 1982. The depth of the explosion point was about 700m. The velocity of P-wave near the observation site was 6.32km/s. Three accerelation-type seismometers and ten velocity-type seismometers were placed three-dimensionally surrounding the explosion point. The average span between seismometers was about 50m. The accerelation response of the accerelation-type seiemomenter system is flat up to 10kHz and the velocity response of the velocity-type seismometer system is flat up to about 1kHz.
The relative foci to the explosion point were solved from the travel time residuals between the explosion and the micro-fractures. The hypocenters concentrated near one of the corners of the newly excavated face.
The mechanism solution of the micro-fractures is normal-fault type, which indicates that the micro-fractures were generated by the compressional stress in the vertical direction.
The micro-fractures occuring immediately after an explosion are generated by the stress concentration, that the load of the upper mass which had been supported by the rock which was excavated with the explosion concentrates near the corner of the newly excavated face.
