Abstract Faults have complicated shapes. Non-planarity of faults can be caused by variations in failure modes, which in turn are dictated by mechanical stratigraphy interacting with the ambient stress field, as well as by linkage of fault segments. Different portions of a fault or fault zone may experience volume gain, volume conservation and volume loss simultaneously depending on the position along a fault's surface, the stresses resolved on the fault and the associated deformation mechanisms. This variation in deformation style and associated volume change has a profound effect on the ability of a fault to transmit (or impede) fluid both along and across the fault. In this paper we explore interrelated concepts of failure mode and resolved stress analysis, and provide examples of fault geometry in normal faulting and reverse faulting stress regimes that illustrate the effects of fault geometry on failure behaviour and related importance to fluid transmission. In particular, we emphasize the utility of using relative dilation tendency v. slip tendency on fault patches as a predictor of deformation behaviour, and suggest this parameter space as a new tool for evaluating conduit v. seal behaviour of faults.

Abstract A valid structural geologic interpretation should simultaneously honor available surface and subsurface data (e.g., well and seismic) to constrain structural geometry; ideally be restorable to an original unstrained condition – taking into account the possibility of three-dimensional (3-D) movement, volume loss, or volume gain; and incorporate structural styles known or expected for the mechanical stratigraphy and deformation conditions in the region. Incorporating what is known about the mechanical stratigraphy can provide crucial constraints on viable structural styles, for example, where faults are likely to cut across stratigraphy vs. where fault displacement is likely to be accommodated by alternative mechanisms (e.g., ductile flow or folding). Conversely, the structural style can often help to understand the mechanical stratigraphy, including the recognition of dominant competent or incompetent mechanical stratigraphic units. Using this approach provides the interpreter another set of constraints toward improving interpretations, testing hypotheses, and developing valid structural interpretations. Outcrop characterization provides insights into the influence of mechanical stratigraphy and structural position on seismic- and subseismic-scale deformation in the layers. Examples of extensional deformation in Cretaceous carbonate strata in central and west Texas illustrate the utility of considering how mechanical stratigraphy influences the development of different deformation styles, even where deformation conditions are otherwise similar.

Abstract The Newberry Springs Fault Zone experienced slip associated with the 1992 Landers earthquake in the Mojave Desert of California, USA. Detailed analysis of scaling relationships from single-event ground ruptures in the Newberry Springs Fault Zone mapped in the field shows an average maximum displacement to length ( D max / L ) relationship for fault segments (rupture lengths in the range of 100–1000 m) of 8×10 −5 –consistent with previously published D max / L ratios for normal fault earthquake ground ruptures (rupture lengths in the range of 1–100 km) of 7×10 −5 . To explore the ability of remote sensing (interferometric synthetic aperture radar or InSAR) to map small-displacement single-event fault ruptures and add constraints on segment displacements, we applied established interferometry methods with phase unwrapping to produce maps of line-of-sight displacement and displacement gradient. These maps highlight fault traces that experienced displacement during the time between collection of the synthetic aperture radar images. Comparison of published 1992 single-event ground rupture maps with mapping based on photogeologic interpretation of 1950s vintage aerial photographs indicates that most of the 1992 ruptures occurred as reactivation of pre-existing slip surfaces. In general, D max / L for total fault displacement is approximately 100 times D max / L for single-event ruptures. Evidence from the Newberry Springs Fault Zone indicates that, since the Pleistocene, at least 10–20 Landers-like slip events have occurred, reactivating the Newberry Springs Fault Zone. Evidence of wide damage zones and reactivation of individual segments developed in alluvial floodplain deposits, at relatively small (order of metres) fault displacements, supports a conceptual model of fault damage zone width being established early, during fault propagation. With continued displacement by accumulation of additional slip events, fault zone damage intensifies. The fault zone width may remain relatively stable, although the active portion of the fault zone will likely narrow as faulting continues and a throughgoing slip surface develops and accumulates the bulk of displacement.