Fracture coalescence as a mechanism for earthquakes: Observations based on mining induced microseismicity

Microseismic events, with magnitudes less than zero, recorded within a volume at depth in an underground mine are used to evaluate failure conditions associated with a magnitude 2.9 event (rockburst). The approach incorporated several independent methods of analysis, including the principal component analysis (PCA) of seismicity, focal mechanisms, stress inversion, underground in situ stress measurements and structural mapping, and three-dimensional numerical modelling derived stresses. A good correlation was found between stress inversion and PCA derived microseismic failure planes and structural mapping, and between the in situ principal stress orientations and those obtained through stress inversion and numerical modelling. A few days prior to the large event and during the aftershock sequence, a change in microseismic failure plane orientation to a shallower dip angle than mapped underground was found td coincide with the numerical modelling derived plane of maximum slip potential. This orientation, along with measured fracture characteristics (persistence and spacing) and an increase in event clustering into the plane, suggests that the formation of the large event plane of rupture was likely related to the coalescence of mapped fractures. An observed reduction in stress release accompanying the increased tendency towards planarity prior to the large event indicates that the development of a large rupture surface occurred under an increased effective shear stress build up as part of the earthquake generation process, acid, the process of coalescence can be considered as the breakage of barriers (new fracture growth) accompanied by an increase in the effective shear stress on the remaining asperities (pre-existing fractures).