Relatively simple through-going fault planes at large-earthquake depth may be concealed by the surface complexity of strike-slip faults

At the surface, strike-slip fault stepovers, including abrupt fault bends, are typically regions of complex, often disconnected faults. This complexity has traditionally led geologists studying the hazard of active faults to consider such stepovers as important fault segment boundaries, and to give lower weight to earthquake scenarios that involve rupture through the stepover zone. However, recent geological and geophysical studies of several stepover zones along the San Andreas fault system in California have revealed that the complex nature of the fault zone at the surface masks a much simpler and direct connection at depths associated with large earthquakes (greater than 5 km). In turn, the simplicity of the connection suggests that a stepover zone would provide less of an impediment to through-going rupture in a large earthquake, so that the role of stepovers as segment boundaries has probably been overemphasized. However, counter-examples of fault complexity at depth associated with surface stepovers are known, so the role of stepovers in fault rupture behaviour must be carefully established in each case. Major active strike-slip faults are often characterized by major (kilometre-scale) discontinuities, or stepovers. The surface fault expression within these stepover zones can be exceptionally complex. Primary fault strands or primary displacement zones (PDZs) may not directly connect, or connect through abrupt bends, and through-going fault strain may be accommodated by multiple oblique faults and (or) folds. In many analyses of seismic hazards, these stepover zones are considered to be important fault segment boundaries (for example, King & Nabelek 1985; Sibson 1985, 1986). Because of the complex and disrupted nature of the stepover zones, segment-based rupture probability studies usually assign lower weight to scenarios that require rupture through a stepover (for example, the Working Group on California Earthquake Probabilities 1999, 2003). Because of their potential importance for earthquake dynamics, many quantitative studies of stepovers have been made (see Harris & Day 1993 for a review of the literature). Recent numerical modelling studies (for example, Harris et al. 1991; Harris & Day 1993) suggest that stepover width plays an important role in the potential for through-going rupture. However, recent studies of the dominantly rightlateral strike-slip San Andreas fault system (Fig. 1) in central and southern California, USA, have revealed that at depths associated with major earthquakes (greater than 5 km), the connection through some stepovers may be simpler and more direct than that expressed at the surface. Fault rupture through such stepovers may be more likely than previously suggested, perhaps resulting in lessfrequent, but larger earthquakes. Below we outline the geological and geophysical evidence for simplicity at depth associated with several restraining and releasing stepovers in the San Andreas fault system. We also discuss a counter-example revealed by geophysical studies in southern California. Lastly, we look at the palaeoseismic and historic earthquake evidence for faults associated with simple-at-depth stepovers in order to examine the possibility of an earthquake that has ruptured through a stepover. Throughout this paper we make extensive use of hypocentre locations relocated using the double difference technique to define the active fault surface at depth. In a recent application of this technique, Schaff et al. (2002) state that location errors for the relocated hypocentres are typically one to two orders of magnitude smaller than the 1.2 km horizontal and 2.5 km vertical average 95% confidence estimates for the standard catalogue data in northern California. However, the relative location of closely spaced relocated hypocentres is likely to be better than absolute location of relocated hypocentres, so the analysis that follows is based on the shape of the fault at depth as defined by the relative location of the hypocentres, rather than details of the position of the fault surface at depth, relative to surficial features, as defined by the absolute location of the hypocentres. From: CUNNINGHAM, W. D. & MANN, P. (eds) Tectonics of Strike-Slip Restraining and Releasing Bends. Geological Society, London, Special Publications, 290, 189–201. DOI: 10.1144/SP290.5 0305-8719/07/$15.00 # The Geological Society of London 2007. Restraining stepovers – Hayward– Calaveras Fault stepover In the San Francisco Bay region, the San Andreas fault system includes several dominantly rightlateral strands, including the San Andreas, Calaveras and Hayward faults (Fig. 1). At the Earth’s surface, the Hayward Fault splays from the southern Calaveras Fault, and about two-thirds of the total Calaveras slip is transferred on to the Hayward Fault, through a 25-km-long, 5-km-wide left step in the dominantly right-lateral faults (Fig. 2). This stepover is characterized by multiple oblique reverse faults (Fig. 2), with no single, simple direct surface connection between the main fault strands or PDZs. However, at fault depths associated with large earthquakes (greater than 5 km), double-difference relocated microseismicity (Figs 2 & 3; Waldhauser & Ellsworth 2002; Hardebeck et al. 2004) reveals that the fault connections are much simpler, with a single continuous east-dipping fault at depth (Simpson et al. 2003, 2004; Manaker et al. 2005). Cross-sectional views of seismicity and geology suggest that the surface complexity is probably limited to the upper 3 to 5 km in the stepover Fig. 1. Index map of the San Andreas Fault system in California, showing the location of the areas discussed in the text. R. W. GRAYMER ET AL. 190