Homogeneous stress hypothesis and actual fault slip: a distinct element analysis

Abstract

The different inverse methods used in slip data analysis depend on the general validity of the mechanical model adopted by Carey and Brunier, and others. The mechanical scheme is based on the Wallace-Bott relationship assuming a faulted rock mass as a system of rigid blocks interacting mechanically without friction. Until now the validity of this conceptual model was supported by internally consistent results obtained by applying numerical inversion techniques to fault slip data. This paper presents the first results of a numerical modelling investigation of this simple mechanical model by a direct approach to the problem. The numerical method used here is a three-dimensional Distinct Element Method which is suitable to study discontinuous media. First, by modelling the behaviour of one fault, we propose an extrinsic verification of the Wallace-Bott relation in three dimensions and considering rock linear elasticity and friction effects on faults. According to the small discrepancy obtained (i.e. the deviation between the modelling results and the theoretical results), we conclude that the assumptions of the basic model are justified in the range of stress values that we study. Second, using a two-fault model, the effects of overlap in stress perturbations around the faults results in particular fault slip interactions. The existence of such phenomena invalidates, in a generally minor way taking into account practical uncertainties, the assumption of fault slip independence in the basic model. Finally, we discuss limits to inverse methodologies and the necessity for interactions between data collection, interpretation of these data and the ability of the basic model to be used in microtectonic analyses.