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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/nhess-2019-178
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2019-178
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Jun 2019

Submitted as: research article | 11 Jun 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

Geologic and geomorphic controls on rockfall hazard: how well do past rockfalls predict future distributions?

Josh Borella1,2, Mark Quigley3,2, Zoe Krauss4,1, Krystina Lincoln5,1, Januka Attanayake3, Laura Stamp5,1, Henry Lanman6,1, Stephanie Levine7,1, Sam Hampton1,2, and Darren Gravley1,2 Josh Borella et al.
  • 1Frontiers Abroad, 3 Harbour View Terrace, Christchurch, 8082, New Zealand
  • 2Department of Geological Sciences, University of Canterbury, Christchurch, 8041, New Zealand
  • 3School of Earth Sciences, The University of Melbourne, Victoria, 3010, Australia
  • 4Department of Geology, Colorado College, Colorado Springs, CO, 80903, USA
  • 5Department of Geosciences, Williams College, Williamstown, MA, 01267, USA
  • 6Department of Geology, Whitman College, Walla Walla, WA, 99362, USA
  • 7Department of Geology, Carleton College, Northfield, MN, 55057, USA

Abstract. To evaluate the geospatial hazard relationships between recent (contemporary) rockfalls and their prehistoric predecessors, we compare the locations, physical characteristics, and lithologies of rockfall boulders deposited during the 2010–2011 Canterbury earthquake sequence (CES) (n = 185) with those deposited prior to the CES (n = 1093). Population ratios of pre-CES to CES boulders at two study sites vary spatially from 5:1 to 8.5:1. This is interpreted to reflect (i) variations in CES rockfall flux due to intra- and inter-event spatial differences in ground motions (e.g. directionality) and associated variations in source cliff responses, (ii) possible variations in the triggering mechanism(s), frequency, flux, record duration, boulder size distributions, and post-depositional mobilization of pre-CES rockfalls relative to CES rockfalls, and (iii) geological variations in the source cliffs of CES and pre-CES rockfalls. On interfluves, CES boulders traveled approximately 100 to 250 m further downslope than prehistoric (pre-CES) boulders, interpreted to reflect reduced resistance to CES rockfall transport due to preceding anthropogenic hillslope de-vegetation. Volcanic breccia boulders are more dimensionally equant, rounded, larger, and traveled further downslope than coherent lava boulders, illustrating clear geological control on rockfall hazard. In valley bottoms, the furthest-traveled pre-CES boulders are situated further downslope than CES boulders due to (i) remobilization of pre-CES boulders by post-depositional processes such as debris flows, and (ii) reduction of CES boulder velocities and travel distances by collisional impacts with pre-CES boulders. A considered earth-systems approach is required when using preserved distributions of rockfall deposits to predict the severity and extents of future rockfall events.

Josh Borella et al.
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Short summary
In this paper we evaluate geologic, geomorphic, and anthropogenic controls on rockfall hazard and highlight the complexity of interpreting future rockfall hazard based on former boulder distributions. To evaluate how past rockfall deposits relate to contemporary rockfall hazard, we mapped then compared the locations, physical characteristics, and lithologies of rockfall boulders deposited during the 2010–2011 Canterbury earthquake sequence (n = 185) with their prehistoric counterparts (n = 1093).
In this paper we evaluate geologic, geomorphic, and anthropogenic controls on rockfall hazard...
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