Introduction

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These forecasts are derived from an empirical relationship between the volumes of rock that fell from the selected Port Hills rock slopes during the 2010/11 Canterbury Earthquake Sequence (CES), New Zealand, in response to earthquake shaking.

RoARS is underpinned by the assumption that if a slope is strongly shaken by an earthquake, the volume of debris that falls from it is a function of the magnitude of the shaking (e.g., Peak Ground Acceleration) and the geometry of the slope (e.g., slope, height, and area). The method and results are presented in Massey et al. (2022).

Overview method

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The methodology described below is split into several steps. In summary, these are: Step 1) estimate the rockfall volumes that fell from a selected number of rock slopes during the CES; Step 2) estimate the earthquake-induced ground motions for the main earthquakes in the CES at slopes of interest; Step 3) carry out a regional-scale statistical analysis of the factors influencing rockfall susceptibility – in this case the volume of debris that could fall from each slope – using the results from Steps 1) and 2).

The data, results, and analyses relate to the ‘larger’ slopes in the Port Hills of Christchurch. These slopes all meet the criteria of being: (1) coastal or relict coastal in origin; (2) formed in rock; (3) greater than 10 m in vertical height (referring to the rock-dominated portion of the slope); (4) steeper than 45° (referring to the mean angle of the rock-dominated portion of the slope); and (5) greater than 50 m² in surface area. Such slopes were found to be susceptible to failure during the CES. Slopes less than 10 m in height and/or flatter than 45° did not typically fail during the CES. Slopes less than 50 m² in surface area were not included as they are too small and numerous for this analysis. Slopes modified by anthropogenic cutting were also not included unless they were originally abandoned coastal slopes and satisfied the aforementioned slope criteria.

Limitations and Disclaimers

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The data and model on which this Tool is based, and the creation of the tool, were prepared by the Institute of Geological & Nuclear Sciences Limited (GNS Science) as part of a New Zealand Government funded research project (MBIE Endeavour - C05X1709). The information is derived from multiple data sources, including third party data, which are at various scales and resolutions. The tool in this context is defined as the data, model and process that allows the user to forecast the volume of the material that might fall from a slope of given height, angle and area, at a given level of earthquake shaking. The algorithm (model) underpinning the tool is based on the one published in Massey et al., (2022).

As there is always uncertainty associated with the data used and the models developed from it, GNS Science gives no warranties of any kind concerning its assessment and estimates, including accuracy, completeness, timeliness or fitness for purpose, and accepts no responsibility for any actions taken based on or reliance placed on the forecasts by any person or organisation. GNS Science excludes to the full extent permitted by law any liability to any person or organisation for any loss, damage or expense, direct or indirect, and however caused, whether through negligence or otherwise, resulting from any person or organisation's use of or reliance on the Tool, the results from it and the data it is based on.

As much of the analysis and assessment relies on inferences from slopes within a particular geology and physiographical setting that were shaken by a particular suite of earthquakes, the applicability of the tool to a specific slope(s) should be considered. It is recommended that on-site verification be carried out to assess whether the tool is applicable for use at a particular site of interest. The Tool is intended to be used by suitably qualified people (e.g., Professional engineering geologists and geotechnical engineers) as part of their regional-scale ‘desktop’ assessment. This Tool should not be used as a replacement or substitute for site- specific analyses and assessments.

References

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Keefer, D. K. (1984), Landslides caused by earthquakes. Geological Society of America Bulletin. 95(4), 406–421.

Macfarlane, D. F., & Yetton, M. D. (2013), Management and documentation of geotechnical hazards in the Port Hills, Christchurch, Following the Canterbury earthquakes. Hanging by a thread. Proceedings of the New Zealand Geotechnical Society Inc. 19th Symposium, 20-23 November 2013, Queenstown, New Zealand.

Massey C.I., McSaveney M. J., Taig, T., Richards, L., Litchfield, N. J., Rhoades, D. A., McVerry, G. H., Lukovic, B., Heron, D. W., Ries, W., & Van Dissen, R. J. (2014), Determining rockfall risk in Christchurch using rockfalls triggered by the 2010–2011 Canterbury earthquake sequence. Earthquake Spectra. 30(1): 155-181.

Massey, C.I.; Olsen, M.J.; Wartman, A.; Senogles, A.; Lukovic, B.; Leshchinsky, A.; Archibald, G.C.; Litchfield, N.J.; Van Dissen, R.J.; de Vilder, S.J.; Holden, C. 2022 Rockfall activity rates before, during and after the 2010/11 Canterbury Earthquake Sequence. Journal of Geophysical Research. Earth Surface, 127(3): e2021JF006400; doi: 10.1029/2021JF006400