Precursory microcracking and brittle failure of Latur basalt and migmatite gneiss under compressive loading

Microcracking activity which occurs in a phased manner in deforming rock under stress has been monitored successfully in the laboratory using acoustic emission (AE) as a tool. The rock samples tested include a massive basalt and a migmatite gneiss that are obtained from the top and bottom portions respectively, of a very deep borehole (KLR-1) drilled at Latur near the surface rupture zone of the Latur–Killari earthquake. The rock samples were subjected to fracture at a constant stress rate under uniaxial compression. The AE statistical parameters used for the analysis include event and energy release rate, amplitude distribution (b-value), cumulative energy and ring down count. The results show that dilatancy occurs early in migmatite gneiss and the rock suffers more damage on account of early crack growth during the pre-peak stress regime compared to the massive basalt which is stronger and showed extensive damage only at the peak stress. However, both the rocks show three distinct phases of microcracking activity, namely primary, secondary and nucleation prior to the extensile brittle failure. The AE statistical behaviour of each individual phase is controlled by the size and density distribution of microcracks in rock as inferred from the AE signatures.