Statistical Analyses of Historical Rock Falls in Yosemite National Park

Abstract:

The steep cliffs of Yosemite Valley produce dozens of rock falls each year that pose a hazard to the four million annual visitors to Yosemite National Park. To better understand rock-fall processes, we use 156 years of rock fall data to examine temporal and spatial correlations between rock falls and seasonality, environmental conditions, and rock type. We also investigate the complexity of rock fall triggers, the most notably precipitation-related triggers (precipitation, snowmelt, rain-on-snow), earthquakes, thermal stress, and freeze-thaw. Comparing rock fall occurrences and cumulative precipitation plots for 16 years between 1983 and 2011 demonstrates a temporal correlation between precipitation and rock falls; this is corroborated by the observation of 17 rock falls that occurred within a one-week period of significant precipitation in late winter 2014. Yet there are many rock falls that do not coincide with precipitation; we attribute these to other triggers when clear temporal correlations exist, or, lacking clear temporal correlations, we investigate possible factors involved in “unrecognized” triggers. The large number of rock falls in the database (925) affords the opportunity to establish a volume-frequency relation similar to that of earthquakes. We also investigate both the frequency and volume of rock falls for each significant lithologic unit exposed in Yosemite Valley and find that two units in particular, the granodiorite of Arch Rock and the granodiorite of Kuna Crest, produce higher rates of rock fall relative to the other lithologies. The aim of these analyses is to better understand conditions that contribute to rock fall and to increase understanding of rock-fall hazard within Yosemite National Park.