A study on seismic source process in short- term and imminent stage before destructive mining shock

Three destructive mining shocks successively occurred nearby the seismic Station No.6 located in a shaft of the Fangshan Coal Mine of Beijing Mining Service in 4 minutes at 19 o′clock of May 15, 1993. The largest shock is of M=2.3 (M0=1.5×1011 N·m). Analysis of synthetic seismogram provides that the three shocks exhibited predominantly a dip-slip movement mode, which is consistent with the collapse of coal mass from the coal bed at high dip-angle, as observed by those who were present at the site. The near-field records of the main shock and a series of events prior to it made at the Station No.6 are different from normal records, it had not only high-frequency vibration, but also low-frequency vibration. By using elastic wave theory and nucleation theory of seismic fracture during recent years, analysis of the data indicates that the low-frequency vibration maybe long-period wave as the subcritical extension is pushing forward. It is an unrecovered deformation. The high-frequency vibration is just the mining shock event, exhibiting a wave field of brittle fracture radiation. From the dominant frequency of low-frequency vibration in the records of M=2.3 event and the records of foreshocks at 5.4 s before it, it is inferred that the volume of dilatation zone at the termination of shock-generating fracture has rapidly enlarged during the occurrence of the main shock. In 20 or more days before the main shock, the source process was of the following characteristics: the subcritical extension occurred for many times; during this period the volume of dilatation zone at the termination of shock-generating fracture little changed. The subcritical extension not only exits long-period waves, but also induced small events at the same time. The dominant orientation of subcritical extension is basically consistent with the direction of slip movement during main shock.