The ‘shear-transformation zone’ (STZ), which may be referred to as a weak domain in granular material, is the main source of plastic deformations in non-cohesive soils such as sand or gravel. To theoretically investigate the vibration-induced shear resistance reduction (ViSRR) of granular materials, in this paper an extended STZ model is proposed that considers the coupling effect between vibration and quasi-static loadings. The framework of the model consists of three components: (a) the motion of STZs including the transition, creation and destruction of STZs; (b) the relation between the motion of small-scale STZs and the observable, macroscopic plastic strain; and (c) the evolution law of a ‘configurational temperature’ that reflects the energy that drives the motion of STZs. The conventional STZ model developed for amorphous materials is enhanced to accommodate both volumetric and shear deformations in the spatial stress state. Specifically, in addition to considering plastic shear strains induced by the change in STZ orientation as the result of the transition, as in conventional STZ models, the extended STZ model correlates plastic volumetric strains with the change in STZ amount resulted from creation and destruction.