MOMENT TENSORS OF MINING TREMORS-DETECTION TOOL OF THE MODE OF ROCK-MASS FRACTURING: MECHANISM OF DEEP EVENTS ON OCTOBER 24 AND OCTOBER 12,2005 IN MOAB KHOTSONG MINE, S.AFRICA

摘要

Contrary to natural earthquakes, where the features of the geological environ without void spaces and the character of loading naturally favor shear slip, in foci of seismic events induced by man-made activities there is a physically based reason for occurrence of non-shear processes. Especially in mines, the existence of mined-out cavities enables implosive events which are improbable among natural earthquakes. In particular, great depth of excavation typically occurring in gold ore mining results in extreme loading of mine structures underground. High stress concentrations cause collapsing of excavated cavities, which is reflected by specific non-double-couple (non-DC) mechanisms. Apart from mining tremors, natural earthquakes of a tectonic origin may occur in the area where mining activity takes place. The mechanism in the moment tensor (MT) representation, in particular the DC vs. non-DC issue, is an important discrimination instrument. To develop it into a reliable methodological tool it is however vital to assess properly errors in the MT retrieval and especially errors in the non-DC component determination, which is vulnerable to possible deterioration of the quality of records, station distribution, event location and velocity structure modelling. We demonstrate the approach on data from Moab Khotsong mine, S.Africa: we determine complete moment tensors of two tremors, discuss the reliability of the retrieval, interpret their decomposition into equivalent forces, check the significance of the dominance of the DC component by an F-test and hypothesize on this basis about their tectonic origin.