Local seismicity due to rockbursts and near-field attenuation of ground motion in the Kolar gold mining region, India

摘要

The following conclusions emerge from the present study:(a) Even though the mining operation in some of the mines at KGF has been discontinued for quite some time now, the rockbursts in the old as well as recent underground workings are frequently occurring at shallow depths up to a kilometre level where a lot of rain water has gradually accumulated over time and spread through the excavated area. The present local seismicity in the inundated mining region seems due mainly to strata failure induced by the seepage of water through the weak zones prestressed to critical limits. This is consistent with the current picture of mine tectonoseismicity where rockburst seismic sources are found to be clustering in close proximity of NW-SE striking set of geologic faults of which Mysore North Fault (MNF) in most prominent. In particular, the two seismically most active segments of MNF lie at present centred around the north-western block in Nundydroog Mine and the south-eastern block in Mysore Mine.(b) Although almost all of the rockbursts, at least those that occurred during the period of two years (1997-1999) covered in the present study, are equivalent to microearthquakes in terms of their local seismic magnitudes estimated to be in the range -1.65<=M_L<=3.14, the ground motion produced by some of the larger ones of them (felt appreciably), is strong enough to cause some minor damage to surface structures situated in the vicinity. The present investigation of seismic wave attenuation in the near field in the hard rock region of KGF needs to be followed up with more extensive data to map the seismic hazard in the region in greater detail.(c) Attenuation modelling of peak particle velocity (PPV) obtained from the digital seismograms of 609 clearly detected and identified rockburst events indicates R_S~(-0.461) dependence For P-wave maxima up to at least 5 km from the source, R_S being the scaled hypocentral distance.(d) In comparison with the attenuation of PPV corresponding to P-wave maxima, the PPV representing the largest seismic signal of any given rockburst decays faster according to R_S~(0.672) in the monitored distance range up to nearly 5 km from the source.(e) It is remarkable to note that the magnitude (M_L) dependence of PPV in the cases of both maximum P signal as well as the largest signal due to a given rockburst obeys nearly √ M_L power law independent of the distance from the source within the local area.