Faulting and hazard in low seismicity areas

The USEPA (United States Environmental Protection Agency) standard definition of 1981 defines an active fault as 'any fault that can be shown to have produced an earthquake in the last 10 000 years'. The inherent difficulty with respect to this definition might be the issue to associate an earthquake to a given 'active' fault. Thus, R.M.W. Musson, BGS, United Kingdom argued in favour of a new classification of faults as 'inert', 'reactivatable' and 'controlling' faults. Inert faults are defined as faults that cannot be reactivated, at least in present stress regime. Reactivatable faults can be, or have been reactivated. Controlling faults are the preferential loci for current deformation. The assessment of faulting matters for two types of hazards: rupture and shaking hazards. The rupture hazard is clearly important as it can disrupt the waste area. Rupture features may induce secondary induced hazards such as changes in groundwater flow. The first defence against rupture hazard is obviously siting as a vast majority of earthquakes takes place along pre-existing structures (reactivatable faults). Rupture hazards in argillaceous media have been understudied. In such environments, earthquakes usually occur in the underlying crystalline rock and do not reach the surface. In some cases, argillaceous sediment may inhibit the rupture propagation but may not prevent deformation after the event. (author)