Numerical modeling of fault formation and the dynamics of existing faults.

Abstract

This research is an investigation into two different aspects of the faulting process. The first part of the study focuses on the initial stages of fault formation, while the second analyzes the deformation produced by an existing fault. The section on fault formation is an attempt to determine whether slip on an existing fault has a significant effect on the formation of subsequent faults. A two-dimensional elastic finite element technique is used to examine the system of stresses produced by slip on an initial fault, assuming that deformation occurs either elastically or by brittle failure. A Mohr-Coulomb failure criterion is used to determine the most likely region of secondary fault initiation. A strain energy criterion is then used to find the preferred direction of fault propagation. The study on fault formation is subdivided into two sections representing two idealized tectonic environments: purely extensional and purely compressional. The section on extensional fault formation explains the prevalence of grabens in extensional tectonic regimes as a consequence of the stress perturbations due to slip on an initial normal fault. Slip on the initial fault produces a region of high proximity to failure at the surface of the downthrown block. A secondary fault would be expected to initiate in this region. The direction of propagation of this fault that most effectively relieves the shear stress (and therefore minimizes the total strain energy) is toward the initial fault, resulting in an antithetic orientation, or graben. The width of the graben is found to be controlled by the depth of the initial normal fault, rather than the depth to a change in material properties. The study of compressional fault formation indicates that, except for steeply-dipping faults, the presence of an initial thrust fault tends to suppress the formation of other faults in its vicinity. However, if a secondary fault initiates near an initial thrust fault, the direction in which it propagates will be influenced by the presence of the initial fault. The way in which it is influenced is dependent on the fault dip. The final part of this study examines the deformation produced by repeated earthquake cycles on the San Andreas fault in southern California. A three-dimensional, time-dependent kinematic finite element model is used to investigate the influence of slip distribution and rheological parameters on the predicted horizontal and vertical deformation. The models include depth-varying rheological properties and power-law viscoelastic behavior. The predicted deformation patterns are fairly sensitive to the parameters used in this study. Of particular importance is the calculation of vertical uplift rate since, in many cases, models that cannot be distinguished from each other on the basis of horizontal deformation may produce distinctive vertical uplift patterns.