Abstract

The Ms 5.4 Little Skull Mountain earthquake of 29 June 1992 occurred within 21 km of Yucca Mountain, Nevada, the site of a proposed high-level nuclear-waste repository. Data from a dense portable seismic network have enabled us to examine the faulting process in detail. Standard linearized least-squares procedures were used to determine hypocenter estimates for all the events. A grid-search technique was applied to test the robustness of estimates of the hypocenters of the mainshock and six of the largest aftershocks. Projection of the mainshock fault plane to the surface, delineated by its focal mechanism and the distribution of aftershocks, defines a strike subparallel to a north-northeast structural trend within Jackass Flats and collinear with the Mine Mountain fault system. This result implies reactivation of mapped faults with no known Quaternary movement, and growth or reactivation of buried faults having no clear surface manifestation. The aftershock distribution suggests that postseismic strain release occurred up-dip from the mainshock on the mainshock fault plane and on several secondary faults. The mainshock focal mechanism and the aftershock distribution indicate a normal-slip rupture on a plane with a strike of N55°E, a dip of 56°, and a rake of −72°. Results of the grid-search analysis show that the mainshock and three of the largest aftershocks probably occurred on the same fault plane and that the other three occurred off the mainshock fault plane; two on a steeply dipping, N200°E-striking plane (with dextral strike-slip displacements), and the other possibly on a plane conjugate to the mainshock rupture plane (focal mechanism unknown).

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