title: Anthropocene rockfalls travel farther than prehistoric predecessors.
abstract: Human modification of natural landscapes has influenced surface processes in many settings on Earth. Quantitative data comparing the distribution and behavior of geologic phenomena before and after human arrival are sparse but urgently required to evaluate possible anthropogenic influences on geologic hazards. We conduct field and imagery-based mapping, statistical analysis, and numerical modeling of rockfall boulders triggered by the fatal 2011 Christchurch earthquakes (n = 285) and newly identified prehistoric (Holocene and Pleistocene) boulders (n = 1049). Prehistoric and modern boulders are lithologically equivalent, derived from the same source cliff, and yield consistent power-law frequency-volume distributions. However, a significant population of modern boulders (n = 26) traveled farther downslope (>150 m) than their most-traveled prehistoric counterparts, causing extensive damage to residential dwellings at the foot of the hillslope. Replication of prehistoric boulder distributions using three-dimensional rigid-body numerical models that incorporate lidar-derived digital topography and realistic boulder trajectories and volumes requires the application of a drag coefficient, attributed to moderate to dense slope vegetation, to account for their spatial distribution. Incorporating a spatially variable native forest into the models successfully predicts prehistoric rockfall distributions. Radiocarbon dating provides evidence for 17th to early 20th century deforestation at the study site during Polynesian and European colonization and after emplacement of prehistoric rockfall. Anthropocene deforestation enabled modern rockfalls to exceed the limits of their prehistoric predecessors, highlighting a shift in the geologic expression of rockfalls due to anthropogenic activity. Reforestation of hillslopes by mature native vegetation could help reduce future rockfall hazard.
authors.affiliation: Department of Geological Sciences, University of Canterbury, Christchurch, Christchurch 8011, New Zealand.
authors.forename: Josh Walter
authors.initials: JW
authors.lastname: Borella
authors.affiliation: Department of Geological Sciences, University of Canterbury, Christchurch, Christchurch 8011, New Zealand.; School of Earth Sciences, The University of Melbourne, Parkville, Victoria 3010, Australia.
authors.forename: Mark
authors.initials: M
authors.lastname: Quigley
authors.affiliation: Department of Geological Sciences, University of Canterbury, Christchurch, Christchurch 8011, New Zealand.
authors.forename: Louise
authors.identifier: 0000-0001-9159-071X
authors.initials: L
authors.lastname: Vick
mesh_terms: D000818:Animals; D003247:Conservation of Natural Resources / Q000639:trends; D018534:Earth, Planet; D055866:Earthquakes; D017753:Ecosystem; D065928:Forests; D055593:Geological Phenomena; D006801:Humans
keywords: Anthropocene; New Zealand; Prehistoric rockfall; South Island; anthropogenic deforestation; earthquakes; landscape evolution; modern rockfall; radiocarbon dating; spatial and volumetric rockfall distributions