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Since 2003, land-use planning for active faults in Aotearoa New Zealand under the Resource Management Act 1991 (RMA) has generally been guided by the ‘Planning for development of land on or close to active faults’ guidelines published by the Ministry for the Environment (hereafter the ‘MfE Active Fault Guidelines’ or ‘the guidelines’) (Kerr et al. 2003). In that time, Aotearoa New Zealand has experienced a number of major earthquakes, including those during the 2010/11 Canterbury Earthquake Sequence and the Kaikōura earthquake in 2016. Land-use planning for natural hazards has also evolved, with a risk-based approach now considered to be best practise. However, while land-use planning for natural hazards has progressed significantly over recent decades, it remains challenging to manage the potential impacts of hazards such as fault rupture, which typically have high consequences but long return periods or recurrence intervals. In the absence of legislation that mandates for active faults to be considered in planning documents, it can prove difficult to restrict development rights in response to a hazard that is perceived as only occurring every 1000 or more years. With increasing need for housing in our cities, there is also increasing pressure to allow for the development of land above and adjacent to active faults. As such, this ‘proof of concept’ project sought to consider whether the guidelines are still fit for purpose or if there are potential amendments that might achieve improved land-use planning outcomes for active fault rupture. In particular, the authors were interested to test whether the assigning of a probability to surface fault rupture (as is done for many other natural hazards) had the potential to better convey the risk associated with low-likelihood, high-consequence hazards such as fault rupture to decision-makers and the public, resulting in improved visibility and land-use planning decisions. This was done by: Considering the lessons learnt from the Canterbury and Kaikōura earthquakes and any implications of these lessons on the framework provided in the guidelines. Undertaking a stocktake of the implementation of the existing guidelines, particularly in the Wellington region, and any limitations or challenges experienced. Examining international land-use planning approaches to manage active fault hazards. Exploring the use of rupture probabilities as a potential pathway to increase the political and community understanding of the risk associated with these low-likelihood but high-impact events. Inserting rupture probabilities into a risk-based framework using a case study of selected active faults in the Wellington region to test for perverse outcomes. What was found is that there is merit in assigning a probability of rupture to align with planning timeframes employed for other natural hazards and to allow decision-makers a clearer comparison of the risk posed with that of other hazards. However, initial findings suggest that, while this approach shows potential for well-known faults where conditional probability is based on extensive scientific investigation, it may not work as well for lesser-known faults. For the purposes of this study, Recurrence Interval Class (RIC) boundaries were re-cast as 100-year likelihoods and Annual Exceedance Probabilities (AEP) in lieu of conditional probabilities for lesser-known faults. The testing indicated that this approach can result in a less-nuanced planning outcome than provided for under the MfE Active Fault Guidelines. Further testing of the use of rupture probabilities based on RIC is needed for these lesser-known faults. One option that was considered was a hybrid approach, where the risk is assessed based on both the RIC and probability of rupture and the most onerous outcome from the two assessments applied. In this way, the risk posed by faults with a long recurrence interval but a high 100-year conditional probability of rupture would be better captured in land-use planning decisions. However, care needs to be taken to not create the perverse outcome whereby, once the fault has ruptured, the rupture probability reduces to such a low likelihood that the planning framework allows for structures to be built across active faults. Past planning decisions have demonstrated that, once buildings are permitted to be constructed on hazard-prone land, it is difficult to reverse, so a precautionary approach needs to be applied. It is recognised that significant advances are being made in seismic engineering, such that constructing buildings over faults may become acceptable provided appropriate mitigation measures are employed. However, at this time, defining fault avoidance and awareness zones is still considered to be the best method for ensuring the safety of people and structures. (The authors) |
