No abstract

Abstract. Traces of many normal faults form an array of closely spaced overstepping segments.In three dimensions, fault segments may either be unconnected or link vertically or laterally into a single continuous fault surface. The slip distribution along segmented faults is complex and asymmetric, and the point of maximum slip generally is not located at the center of a segment. In relay zones between segments, slip gradients may be gentler or steeper, depending on the spatial fault arrangement. Branch points are characterized by steep slip gradients. One explanation for these observations is mechanical interaction between neighboring faults which occurs through local perturbation of the stress field. Three-dimensional (3-D) boundary element models show that the degree of fault interaction and hence the degree of asymmetry in the slip distribution increases with increasing fault overlap and downdip fault height and with decreasing fault spacing and Poisson's ratio. Interaction is strongest for faults with uniform shear strength and decreases if there exists a zone of greater shear strength near the tip line. This analysis provides a mechanical rationale for more frequent occurrence of overlapping segments relative to underlapping segments and for the limited range of the ratio between segment overlap and spacing along natural faults. Echelon segment configurations promote interaction, maximize the capacity to accommodate slip, and do not necessarily require a strike-slip movement component. Model idealizations of some outcropping fault arrays and of branching/merging faults capture a wide variety of common field observations. Consistent, mechanically based 3-D normal fault models can be obtained by combining different types of field data such as fault slip-to-length ratios, location of maximum slip, segment overlap-to-spacing ratios, and footwall uplift/hanging wall subsidence. By capitalizing on these data one can understand the mechanics of faulting, constrain the boundary conditions that govern the formation and growth of faults, and provide a rationale for interpreting normal faults in seismic surveys.

Abstract. Sliding along a preexisting flaw can result in the formation of tensile cracks where stresses concentrate near the flaw tips. These tensile cracks are referred to as wing cracks and are generally oriented oblique to the preexisting flaw. Previous studies based on linear elastic fracture mechanics (LEFM) showed that the kink angle depends on the ratio of normal to shear loading on the flaw. We present analytical solutions for cohesive end zone (CEZ) flaw models and find that the relationship between kink angle and load differs significantly from that for LEFM flaws. Furthermore, the remote flaw-parallel normal stress may significantly reduce or increase the kink angle, especially for CEZ flaws with large end zones. These results suggest that multiple interpretations are possible for some measured kink angles. In some materials, solution surfaces may form at the tip of the sliding flaw. By considering the angle between wing cracks and solution surfaces it is possible to determine whether the LEFM or CEZ model is more appropriate and thus to provide a better constrained interpretation of the boundary conditions that accompanied sliding. For some CEZ flaws the stress state in the cohesive end zone is nearly homogeneous, possibly promoting formation of arrays of opening mode cracks and solution surfaces that together form a shear zone. The CEZ flaw model can explain some orientations and patterns of cracks and solution surfaces commonly observed along natural faults that cannot be accounted for with the LEFM model.

No abstract

No abstract