Segmented normal faults: Correspondence between three‐dimensional mechanical models and field data

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.

Evolution of vertical faults at an extensional plate boundary, southwest Iceland

“…The program has been used in several previous studies of geological problems involving faulting (Willemse et al, 1996;Willemse, 1997;Crider and Pollard, 1998;Pollard, 1999, 2001;Kattenhorn et al, 2000;Maerten, 2000;Maerten et al, 2000Maerten et al, , 2002Crider, 2001). The program is based on the governing equations of linear elastic fracture mechanics for homogeneous and isotropic solids.…”

“…Mechanical interaction between horizontally overlapping normal fault segments promotes lateral propagation of the segments which can result in composite faults with aspect ratios . 1 (Willemse, 1997;Kattenhorn and Pollard, 2001). By using square faults in our models, we make no assumptions about final fault configurations that may result from linkages between such fault segments.…”

Slip tapers at the tips of faults and earthquake ruptures

“…The displacement profiles in such cases become asymmetric: the peak slip region is shifted towards the interacting tip and the FTT at that end becomes steeper than that of the distal end [Peacock and Sanderson, 1991;Contreras et al, 2000]. This occurs because the FTT at the interacting tip must overcome both the rupture resistance and the stressdrop of the adjacent fault [Willemse, 1997;Scholz, 2002, pp. 127 -129].…”

Fault linkage: Three‐dimensional mechanical interaction between echelon normal faults

“…All three serve to make the models more realistic and thus provide important, if limited, innovations in the context of numerical modeling of geologic structures. Previous work on the mechanical interaction between segmented normal faults prescribed a uniform stress drop applied directly to the fault plane [Willemse, 1997] or specified a slip distribution [Nostro et al, 1997]. Here, by applying loads remotely, we simulate tectonic forces acting on faults and permit more complex displacement discontinuities across the faults.…”