Twenty thousand aftershocks of a very small (M 2) earthquake and their relation to the mainshock rupture and geological structures

We have determined the locations of more than 20,000 aftershocks (as small as moment magnitude M_w -4:4 or even smaller) following an M 2 event in a South African gold mine, using manually picked arrival times. Spatial clustering into five groups was clearly discerned. Amajority of the aftershocks formed a planar cluster (~4 m in apparent thickness, ~100 × 80 m in areal extent). This cluster is thought to delineate the rupture area of the mainshock because its orientation and spatial extent were consistent with the nodal plane of the centroid moment tensor (CMT) solution and with the corner frequency of the mainshock, respectively. The cluster's attitude suggests that the mainshock was a Mohr-Coulomb failure (or formation of a shear rupture surface in intact rock at an angle that obeys the Coulomb failure criterion) that took place in a vertical compression stress field that is indicated by borehole breakout patterns. The aftershock distribution also shows that the mainshock rupture was largely confined to the interior of a 25-m-thick vertical dike, although there are indications of interactions taking place between the rupture and the dike's material boundary with the host rock.