Mechanical analyses of listric normal faulting with emphasis on seismicity assessment

Abstract

Mechanical analyses of listric normal faults analogous to a model of Yucca Mountain (Nevada) faults were conducted to examine the possibility and consequences of slip on such faults. Specific consequences such as distribution of ground-motion amplitudes and possibility of triggered slip on other faults and fractures within the zone of influence associated with slip on listric faults were investigated. Slip on listric faults is likely to initiate at depth on steep fault segments and propagate upwards towards the ground surface, and downward and laterally along the low-angle detachment. We present argument that low-angle normal faults are credible sources of seismicity but the seismic risk presented by such faults is smaller than the risk associated with steeply dipping faults because of reduced rupture propagation rates on low-angle faults. Slip on a listric fault is likely to trigger slip on steeply dipping faults within its hangingwall. Such triggered slip tends to originate at shallow depths (less than 1 km). The shape of listric faults has considerable effect on the distribution of ground-motion amplitudes associated with fault slip, because of the tendency for energy focusing due to the curvature of the fault. As a result, ground-motion amplitudes at a distant location may exceed those at locations closer to the fault; furthermore, although ground-motion amplitudes generally decrease with increasing depth below the ground surface, there are several situations for which the amplitude at depth exceeds the amplitude on the ground surface at the same map location.