Precarious Rock Evidence for Seismic Shaking during and Prior to the 1992 ML 5.6 Little Skull Mountain, Nevada, Earthquake

Abstract

The 29 June 1992 Richter local magnitude ( ML) 5.6 Little Skull Mountain earthquake, 20 km southeast of Yucca Mountain, caused rockfalls on steep slopes of Little Skull Mountain, while some semi-precarious rocks remained in place. The observation that ground motion was sufficient to cause rockfalls and yet leave semi-precarious rocks untoppled offers both upper- and lower-bound estimates of the history of ground motion at Little Skull Mountain. A ground-motion synthesis code, used in Yucca Mountain probabilistic seismic hazard analysis (PSHA), was applied to assess the spatial distribution of peak ground acceleration around Little Skull Mountain with respect to the surface attenuation and evidence for ground shaking during the earthquake. Instrumental records for the earthquake, rockfalls, remaining precarious rocks, intensity values estimated from damage to local facilities, and scaling of ground motions based on a recent ML 4.4 Little Skull Mountain aftershock were compared with model predictions. Predicted peak accelerations based on a kappa value of ∼20 ms (used in the Yucca Mountain PSHA) are too high to be consistent with this evidence of ground shaking. This suggests some ground-motion predictions conducted for the Yucca Mountain PSHA may need to be re-examined, and may be too conservative.