Energy budgets of mining-induced earthquakes and their interactions with nearby stopes

摘要

In the early 1960's. N.G.W. Cook, using anunderground network of geophones, demonstrated thatmost Witwatersrand tremors are closely associatedwith deep level gold mining operations. He also showedthat the energy released by the closure of the tabularstopes at depths of the order of 2 km was more thansufficient to account for the mining-inducedearthquakes. I report here updated versions of thesetwo results based on more modern underground dataacquired in the Witwatersrand gold fields. Firstly,an extensive suite of in situ stress data indicate thatthe ambient state of crustal stress here is close tothe failure state in the absence of mining even thoughthe tectonic setting is thoroughly stable. Mininginitially stabilizes the rock mass by reducing thepore fluid pressure from its initial hydrostatic stateto nearly zero. The extensive mine excavations, asCook showed, concentrate the deviatoric stresses, inlocalized regions of the abutments, back into afailure state resulting in seismicity. Secondly,there appears to be two distinct types of mining-induced earthquakes: the first is strongly coupled tothe mining and involves shear failure plus a coseismicvolume reduction: the second type is not evidentlycoupled to any particular mine face, shows purelydeviatoric failure, and is presumably caused by moreregional changes in the state of stress due to mining.Thirdly, energy budgets for mining inducedearthquakes of both types indicate that, of theavailable released energy, only a few per cent isradiated by the seismic waves with the majority beingconsumed in overcoming fault friction.