MECHANICAL ANALYSES OF LISTRIC NORMAL FAULTING WITH EMPHASIS ON SEISMICITY ASSESSMENT

Mechanical analyses of listric normal faults analogous to a model of Y ucca Mountain (Nevada) faults were conducted to examine the possibilit y and consequences of slip on such faults. Specific consequences such as distribution of ground-motion amplitudes and possibility of trigger ed slip on other faults and fractures within the zone of influence ass ociated with slip on listric faults were investigated. Slip on listric faults is likely to initiate at depth on steep fault segments and pro pagate upwards towards the ground surface, and downward and laterally along the low-angle detachment. We present argument that low-angle nor mal faults are credible sources of seismicity but the seismic risk pre sented by such faults is smaller than the risk associated with steeply dipping faults because of reduced rupture propagation rates on low-an gle faults. Slip on a listric fault is likely to trigger slip on steep ly dipping faults within its hangingwall. Such triggered slip tends to originate at shallow depths (less than 1 km). The shape of listric fa ults has considerable effect on the distribution of ground-motion ampl itudes 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 generall y decrease with increasing depth below the ground surface, there are s everal situations for which the amplitude at depth exceeds the amplitu de on the ground surface at the same map location. (C) 1998 Elsevier S cience B.V. All rights reserved.