The Little Skull Mountain, Nevada, earthquake of 29 June 1992: Aftershock focal mechanisms and tectonic stress field implications

Abstract

The ML = 5.6 29 June 1992 Little Skull Mountain earthquake occurred in the southern Nevada Test Site, Nye County, Nevada, within 20 km of a potential national nuclear waste repository at Yucca Mountain, Nevada. While microseismicity in the immediate epicentral vicinity occurred for years prior to the mainshock, foreshock activity ( ML 2) were also sufficiently well recorded to provide local network P -wave focal-mechanism solutions. The mainshock and the majority of the aftershocks through September 1992 display predominantly normal-slip focal mechanisms, but unambiguous strike-slip and oblique-slip focal-mechanism solutions are also present in the aftershock series. The mainshock and immediate foreshock depths were 10 ± 1 km below sea level, and depths of focus of aftershocks range from about 6 to 12 km. A large subset of the aftershock hypocenters define a southeast-dipping structure, probably the mainshock fault plane. No variation of mechanism style with depth is apparent. Two mainshock and 84 aftershock focal-mechanism solutions were input into Gephart's (1990) stressfield inversion program, and principal compressional stress directions, σ j , j = 1, 2, 3, were determined, as well as the stress difference ratio R = (σ1 − σ2)/(σ1 − σ3). The results indicate a uniform stress field in which σ3 (minimum stress) is oriented 116°/5° (azi, plunge) and σ1 (maximum stress) is oriented 219°/69° with R = 0.35. The inversion's selection of preferred nodal planes invokes Bott's (1959) criterion, which states that slip occurs in the direction of resolved shear traction on preexisting fault surfaces. For the predominantly normal-slip Little Skull Mountain earthquake mechanisms having ∼northeast-striking planes, the southeast, moderately to steeply dipping (dip > 55°) nodal planes are selected. However, for the dip-slip earthquake mechanisms exhibiting more northerly trending strikes, shallowly to moderately west-dipping (25° < dip < 65°) nodal planes are selected. When a Coulomb-Mohr analysis was applied to the focal mechanisms using the above principal stress directions and an assumed ratio σ1/σ3, the ratio of resolved shear stress to normal stress, τ/σ n , on the southeast-dipping nodal plane of the mainshock exhibited a near-maximum level, suggesting a high fault strength. In contrast, a wide range of fault strengths, or apparent coefficients of friction, was suggested by the τ/σ n ratios for Little Skull Mountain aftershock preferred nodal planes. Seismicity on misoriented faults, i.e., those on which shear stress was relatively low, may have been the result of a temporarily elevated fluid pore pressure following the mainshock.