Geometry and slip distribution in normal fault systems: Implications for mechanics and fault‐related hazards

Abstract

Normal faults are important sources of large, destructive earthquakes in continental rift provinces. We investigate the mechanical characteristics of multiple, linked master and subsidiary faults using two‐dimensional boundary element models. The master fault system is a Coulomb‐frictional fault that dips 50° from the surface to a depth of 15 km, where it is linked to a subjacent shear zone that creeps under low resolved shear stress. We consider two basic configurations, one in which the shear zone and master fault are coplanar, and the other in which the shear zone dips 20°, forming a bent or crudely listric fault system. Antithetic and splay faults are added to the master fault system in each case to model failure of multiple fault systems that are typical of half‐grabens. We focus on the mechanical interactions between the various faults in order to identify points of initial failure, spatial variations in the amount of fault slip, and the patterns of surface deformation that may precede catastrophic failure of the master fault system. In all cases, failure on the master frictional fault initiates at the base, where it joins the creeping shear zone, and at the Earth's surface during the same stress increment. The listric fault system fails at lower regional differential stress than the planar fault system. Failure of a large antithetic fault that dips at 70° from the surface to the base of the frictional master fault initiates at lower regional differential stress for the listric than for the planar case. Several repeated failure episodes may be required to cause surface offset on the antithetic fault if the master fault system is planar, but antithetic surface faulting may occur during each master fault failure episode in the listric case. Modeled splay faults dip from the surface at either greater (70°) or lower (50°) angles than the frictional master fault to a depth of 5 km, where they intersect the master fault. In both cases, surface slip is greater on the master fault and lesser on the splay fault. Surface deformation prior to catastrophic failure of the frictional master fault is generated by creep on the deep‐seated shear zone and during the initial stages of master fault rupture. Footwall uplift is predicted above a steeply dipping, locked Coulomb fault and creeping shear zone, but the footwall uplift is absent in the listric fault ‐ shear zone case. A localized zone of increased extensional strain is centered on the surface trace of the master fault as the result of near‐surface extensional and shear fracturing during the initial stages of fault slip. These geodetic anomalies are of interest because they are a near‐surface phenomenon which may also alter fluid flow and gas flux and be detectable by hydrologic and geochemical instrumentation as well as geodetic measurements.