Mechanical controls on fault geometry

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Faults inevitably become non-planar because of how they grow and how they are affected during slip by mechanical heterogeneities inherent in the earth. Some faults acquire a non-planar geometry because of non-uniform tectonic deformation or because they grow by the linkage of originally discontinuous structures. However, even faults that are originally planar are unlikely to remain so. Elastic analyses show that as a fault slips, it rotates with the surrounding rock. It rotates uniformly and remains planar if: (a) the shear stress drop along the fault is uniform, (b) the rock surrounding the fault is uniform and isotropic, and (c) the far-field stress state is uniform. Variation in stress drop or in fault strength, heterogeneity in host-rock stiffness, and interaction with other faults cause non-uniform rotation along a fault that slips, and the fault geometry deforms. Mechanical and geometrical heterogeneities are inherent in the earth, so all natural faults will tend to become non-planar to some degree as they slip, even if they were initially planar. Information on fault shape can illuminate the mechanics of faulting, and, in conjunction with slip data, help locate contacts between rock bodies of different elastic moduli. In uniform isotropic rocks, fault curvature is proportional to the rate at which the stress drop varies as a function of position along a fault, whereas the slip profile reflects a weighted average of the stress drop.