Modelling fire following earthquake for multiple scenarios affecting Wellington City

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Scheele FR, Lukovic B, Moratalla J, Dunant A, Horspool NA. 2020. Modelling fire following earthquake for multiple scenarios affecting Wellington City. Lower Hutt (NZ): GNS Science. 26 p. (GNS Science report; 2020/12). doi:10.21420/H7PX-XD46.

Abstract
Fires are a common secondary hazard following earthquakes and, on rare occasions, can develop into major events with severe consequences. Wellington City has many characteristics that make it susceptible to fire following earthquake (FFE), including the potential for conflagrations and significant losses (e.g. property, infrastructure and casualties). Through the ‘It’s Our Fault’ (IOF) research programme, we are revisiting FFE modelling for Wellington City, building on the previous models developed by Cousins et al. (2002). Previous reports detailing work on FFE under IOF are a literature review of globally available models (Scheele et al. 2018) and the development and application of ignition and spread modelling for the Wellington Fault (Scheele et al. 2019). This report details ignition and fire spread modelling for multiple fault sources affecting Wellington City, including a newly developed model for estimating fire suppression. The ignition and spread modelling has been performed for five potential earthquake scenarios that could affect Wellington City. In each scenario, we account for uncertainty in the resulting ground motion by running hundreds of simulations to sample the distributions of the peak ground acceleration (PGA) from each event. The fault sources used for the scenarios are the Wellington, Wairarapa and Wairau faults and the Hikurangi subduction zone’s interface fault. Wind speed and direction is sampled from wind rose data for each simulation run to account for important weather conditions affecting fire spread. A suppression model has been developed that accounts for ignitions over time, suppression by residents and emergency services, water availability and road access. Loss estimates accounting for suppression are calculated for Wellington City in order to indicate the relative impact of earthquake scenarios affecting Wellington City. The Hikurangi subduction zone interface fault source results in the highest mean losses at around $3b from fires alone. The earthquake scenarios on the Wellington and Wairarapa faults have similar mean losses from fire of around $2b. The earthquake scenario on the Wairau Fault produces lower PGAs than the other scenarios for Wellington City, with mean losses from fire of around $0.3b. Across all scenarios, the mean loss for burn zones (the extent of fire spread from an ignition) is $46m, indicating the potential savings of suppressing an ignition. The mean population within burn zones is 324. Wind speed is shown to have a very strong effect on the loss estimates, and the number of ignitions also has an influence. By considering the weather conditions following an earthquake event, a rapid assessment of the threat from FFE could be undertaken. Future work will involve further development of the suppression model for assessing spatial and temporal components of firefighting resources and infrastructure disruption, utilising techniques such as network modelling. The suppression model is a key tool for evaluation of mitigation strategies, targeted toward supporting risk reduction initiatives for FFE in Wellington City. (auth)