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 Mw 4:4 or even smaller) following an M 2 event in a South African gold mine, using manually picked arrival times. Spatial clustering into fivegroupswasclearlydiscerned.Amajorityoftheaftershocksformedaplanarcluster (∼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 wereconsistent with thenodalplaneofthecentroid moment tensor (CMT)solution and withthecornerfrequencyofthemainshock,respectively.Thecluster'sattitudesuggests that the mainshock was a Mohr-Coulomb failure (or formation of a shear rupture sur- faceinintactrockatananglethatobeystheCoulombfailurecriterion)thattookplacein 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.

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