Source mechanism of mining induced seismic events - Resolution of double couple and non double couple models

Traditional assumption about a seismic source as a double couple (DC) can represent a serious limitation for describing mechanisms of mining induced seismic events. In mining conditions more general process than a shear slip can occur due to stress concentration on mined-out areas. We assume both non-DC components within the dipole description of the source - the moment tensor, and a single force. We processed five seismic events recorded at Driefontein gold mine in South Africa to demonstrate that waveform modeling is not successful enough due to poor information about the medium. Inversion of amplitudes of direct P- and S-waves is more prospective due to their smaller sensitivity to detail of the medium and possibility to correct them for ray bending and shear wave splitting prior to the inversion. Moreover, we are able to estimate the error of the resulting mechanism due to neglect of ray bending. All five events processed display non-DC mechanisms which are significant at 95% considering the mismodelling of the rock mass by a homogeneous isotropic medium. The source mechanisms are in agreement with the underground mining and geological structures. One event has a sub-vertical pressure single dipole complemented by an implosion, which is the body force equivalent of a collapsing horizontal crack or a burst of a pillar supporting the hangingwall of a mined-out area. Two events have mechanisms close to a pressure single couple. All three events are located on pillars. Two out of these three events can be assigned with a downward single force as an alternative mechanism, which matches the amplitude data successfully. It is the force equivalent of a mined-out area collapse, when a rock mass falls from the hangingwall to the footwall. The events are however rather strong, which would require too large rock mass fallen. One of the remaining events is a nearly horizontal tensile single couple, corresponding to a vertical tensional fault. It could be originated by a break of the hangingwall of a mined-out area, however the hypocenter of this event is deep into the footwall were there is no mining taking place. The last event is mostly DC accompanied by a compensated compressive dipole, which may be associated to nearly vertical dip-slip along a fault in the hangingwall. This event is located on the margin of a wide pillar which supports the interpretation. © 2008.