Quantifying Near‐Surface Rock Strength on a Regional Scale From Hillslope Stability Models

Rock strength is a fundamental property of earth materials that influences the morphology of landscapes and modulates feedbacks between surface processes, tectonics, and climate. However, rock strength remains challenging to quantify over the broad spatial scales necessary for geomorphic investigations. Consequently, the factors that control rock strength in the near‐surface environment (i.e., the critical zone) remain poorly understood. Here we quantify near‐surface rock strength on a regional scale by exploiting two hillslope‐stability models, which explicitly relate the balance of forces within a hillslope to Mohr‐Coulomb strength parameters. We first use the Culmann finite‐slope stability model to back‐calculate static rock strength with high‐density measurements of ridge‐to‐channel hillslope height and gradient. Second, we invert the Newmark infinite‐slope stability model for strength using an earthquake peak ground acceleration model and coseismic landslide inventory. We apply these two model approaches to a recently inverted sedimentary basin in the eastern Topatopa Mountains of southern California, USA, where a tectonic gradient has exposed stratigraphic units with variable burial histories. Results show similar trends in strength with respect to stratigraphic position and have comparable strength estimates to the lowest values of published direct‐shear test data. Cohesion estimates are low, with Culmann results ranging from 3 to 60 kPa and Newmark results from 6 to 30 kPa, while friction angle estimates range from 24° to 44° from the Culmann model. We find that maximum burial depth exerts the strongest control on the strength of these young sedimentary rocks, likely through diagenetic changes in porosity, cementation, and ultimately, lithification.Key PointsWe quantify hillslope‐scale, near‐surface rock strength on a regional scale using the Culmann and Newmark hillslope‐stability modelsStrength estimates are generally low, with cohesion model predictions less than 60 kPa for friction angle predictions of 24° to 44°The cohesion of young sedimentary rocks in the shallow subsurface increases with increasing maximum burial depthPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156196/3/jgrf21208.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156196/2/2020JF005665-sup-0001-Data_Set_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156196/1/jgrf21208_am.pd