Near-surface rock strength estimates from the South Napa earthquake relative to coseismic landslide hazards

Secondary effects of earthquakes sometimes include widespread landsliding where strong ground motions exceed 0.2g PGA, although many earthquakes that generate sufficient ground motions do not have landsliding. For example, the South Napa earthquake of August 24, 2014 failed to produce appreciable landsliding in the presence of steep topography despite PGA estimates in excess of 0.5g. We calculate minimum strength estimates for near surface rock units using a Newmark analysis that predicts slope stability during an earthquake based on local topography, peak ground acceleration, and Mohr-Coulomb strength parameters. When considering other earthquakes with large landslide events, we determine that the minimum strength of Napa surficial rocks are comparable to total strength estimates from southern California during the 1994 Northridge and Taiwan during the 1999 Chi Chi earthquakes, but are substantially weaker than estimates from western China during the 2008 Wenchuan earthquake. These comparisons suggest that minimum estimates from the South Napa earthquake may be representative of the average strength. If correct, then we would predict widespread landsliding in even a slightly larger event or under wet conditions. Because rock strength at the spatial scale calculated here integrates the effects of climate, tectonic history and lithology, short-term variability due to recent rainfall and drought conditions also should be considered. Hillslope strength estimates from different earthquake events underscores the need to better quantify hillslope strength in various settings and under a range of conditions, in order to more accurately predict landslide hazards associated with large earthquakes.