Analysis of rock falls in the Yosemite Valley, California

The volume-frequency distribution of rock falls and rock slides in the Yosemite Valley are well described by a simple power-law relationship, where log N(Vol) = 3.48 - 0.57(log Vol). This relationship, based on 214 documented rock-fall and rock-slide events that occurred from 1900 to 1992, allows determination of estimated return periods and probabilities for rock-fall events of different sizes. Based on this relationship, the largest prehistoric rock fall in the Yosemite Valley at Mirror Lake has an estimated return period of 325 years. On 10 March 1987 two massive rock falls from Middle Brother, with a combined volume of 600,000 m3, spread across the talus cone and covered Northside Drive, blocking the primary exit from the Yosemite Valley. Brittle fracture indicated by rock popping noises and suggesting release of horizontal residual stress accompanied small (<50 m3) rock falls that preceded these two massive rock falls from Middle Brother. Removal of “key blocks” by the smaller rock falls may have released the interlocked geometry of closely jointed and fissured rock of the face of Middle Brother and permitted the failure of the much larger rock mass. During the subsequent months the number of continuing small rock falls at Middle Brother exhibited an inverse power law decay with time.