Application of the shear-tensile source model to acoustic emissions in Westerly granite

Abstract

Traditionally in seismology and in acoustic emission (AE), full moment tensor (MT) is applied as a default model of the mechanism. We present an alternative - an application of shear tensile crack (STC) source model to AEs generated by uniaxial compression loading of Westerly granite. The advantages of STC over the conventional MT are as follows: (i) contrary to the MT, the STC is physical source since it describes straight and simple fracture modes anticipated inside a loaded sample, namely the shear-slip and both of opening and closing tensile cracks; and (ii) the STC is simpler because it is described by fewer parameters (five instead of six required for an unconstrained MT), an essential feature for stabilizing the inverse problem.Better suitability of STC over MT is demonstrated by three exemplary AEs (tensile, shear, and combined). The obtained results were confirmed using a statistical analysis of 1630 reliably determined source mechanisms. The STC, as compared to the MT, provides smaller confidence regions for orientation and even smaller regions for decomposition parameters. Thus, the STC solution appeared to be substantially more useful than the MT, namely for mechanisms with a high content of non-double-couple (non-DC) component, as it allowed better distinction between tensile and shear AEs.Grain scale cracks of tension and combined source type, localized within the middle-height circumferential portion of the specimen, dominated fracturing. Azimuthal distribution of fault planes was found to be approximately uniform for all three source types. The fault dip increased with increasing content of the non-DC component. The average values were 16°, 21°, and 26° for the tensile, combined and shear source types, respectively. The specimen failed by flaking in areas of a high AE activity. AE locations and failure mechanisms indicate a perfect confinement end-boundary conditions between the tested specimen and the loading platens.