Active normal faulting and crustal extension

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Abstract
Summary: A world-wide review of fault-plane solutions and focal depths for large normal-faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6–15 km on faults dipping between 30 and 60°. In the few cases where levelling or seismic data are good enough, these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust. There is evidence that, in some cases, as a result of the enormous transitory increase in strain rate during large earthquakes, rupture continues into the normally ‘ductile’ lower crust on surfaces with substantially gentler dips. These low-angle surfaces may be analogous to some of the ‘detachments’ seen in metamorphic core complexes of the western USA, but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the ‘detachments’ and the blocks on either side. Such contrasts, however, are unlikely to introduce substantial curvature to an originally planar shear zone.If the observed spread in active normal-fault dips is caused by rotation of the faults and the blocks they bound during extension, then a maximum β value of 1.7 can be accommodated byseismicactivity on a single generation of normal faults. With continued extension, either a new generation of steeper faults, cutting the rotated first faults, is likely to form, or the deformation will continue aseismically on faults dipping at less than 30°.Large earthquakes have not been observed to nucleate on very low-angle (<20°) normal faults within the continental crust anywhere in the world. Such faults can of course move aseismically, but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active. Thus extensional models that require concentrated simple shear on large sub-horizontal faults within the brittle upper crust will also require a spatial separation between aseismic, very low-angle faulting and seismic high-angle faulting. Since seismogenic high-angle faults dominate the topography of extending regions, upper-crustal very-low-angle faulting, if it occurs on a large scale, presumably does so in areas that are relatively flat.