Abstract
The two most devastating earthquakes of the 2010-2011 Canterbury sequence, the September 4, 2010 Darfield and February 22, 2011 Christchurch earthquakes occurred on previously unknown faults. The 2010 event was associated with rupture of the previously unmapped Greendale Fault, 30 km away from Christchurch. The 2011 Christchurch earthquake event occurred under the city and along a fault that did not rupture to the surface. A pressing question was whether a similar scenario to the September 4, 2010 Darfield event could occur in Dunedin city. Dunedin has a similar built environment to the one Christchurch had prior to 2010, and is known to have active faults nearby (Akatore and Titri faults). In this project we assessed whether these faults, or other active faults, could extend into the city. Our efforts were motivated by the Canterbury Earthquakes Royal Commission (CERC) recommendation that "Research continues into the location of active faults near Christchurch and other population centres in New Zealand, to build as complete a picture as possible for cities and major towns". The 2010-2011 Canterbury events demonstrated that for areas of low seismicity, it is essential to understand whether active faults may lie under or close to major cities in New Zealand. Various lines of evidence indicate that the Akatore Fault does not extend into Dunedin city (Figure E1). We also discovered that the Titri Fault extends northwards and closer to the city than previously mapped, but it but it has buckled rather than broken the ground surface there. A newly mapped and possibly active fault, the Kaikorai Fault, lies in the western part of the city. Our interpretation is that the Kaikorai Fault most likely would rupture together with the Green Island Fault, located offshore. In contrast, our study was non-conclusive regarding the presence of potentially active faults within Otago Harbour. In low to moderate seismic areas, such the wider Dunedin area active faults can be quiescent time periods greater than 100,000 years. While the Akatore and Titri faults seem to be in an active period (from studies parallel to this one), the Kaikorai is either inactive or in a phase of inactivity (or dormant). The research team for this project consisted of 18 researchers from GNS Science, Otago University and the Geological Survey of Spain, and included several students. The work was co-funded by the Natural Hazards Research Platform 2015 Contestable Fund, GNS Science Core Funds, University of Otago and EQC. A variety of scientific techniques were implemented to assess whether active faults extend beneath Dunedin city. Geological assessments helped find signs of sediments and rocks displaced by faults, and produce detailed maps of landscape surfaces displaced by faults. Geophysical imaging of the geological layers under the seabed and ground surface was used to determine subsurface locations where layers have been broken by faults. Measures of the Earth's gravity discriminated whether some areas may have subsided as a result of long term fault displacements. Seismographs were installed in order to identify subtle signals of fault activity at depth. Satellite images and measurements of ground movement with GPS instruments also provided a regional perspective of active Earth's deformation due to tectonic forces. In terms of the seismic hazard to Dunedin city, this study does not substantially change the levels of hazards that were estimated previously. The new faults sources we have defined have been assigned very low levels of activity. The activity of the Akatore and Titri faults (from recent studies and used here), sum up to the same value that was previously assigned to the Akatore Fault. Therefore, our hazard estimations are similar to those obtained in the National Seismic Hazard Model that informs the New Zealand Building Code. We have also assessed possible strong shaking scenarios in Dunedin from rupture of the modelled faults. Although these values have large uncertainties, it is possible that Dunedin could experience Modified Mercalli Intensity 6, and perhaps up to 8 in some parts of the city. MMIs of 6 to 8 will cause damage to unreinforced masonry buildings and could produce landslides and liquefaction in some areas. However, it is important to bear in mind that the likelihoods of rupture of these faults is low, although no lower than those of faults around Christchurch prior to 2010. A better understanding of the presence and levels of active faults beneath, or close to, New Zealand cities is essential for appropriate and effective preparation and implementation of emergency response, more resilient land use planning and informed risk management. In low seismicity areas, presence of active faults and their level of activity are difficult to assess. Multidisciplinary projects like this help arrive to conclusions that one single technique will not be able to reach. However, we have only scratched the surface of the understanding of fault activity in Dunedin and wider region, as more data and analysis are required to reduce the uncertainty on our knowledge. With this study, we have advanced the knowledge, tested techniques that have proven useful for the purpose of finding faults under cities, and set a base line for Dunedin city that should be revisited as new data is available. Ideally, multi-discipline, multi-agency (researchers, regional and local authorities, infrastructure owners, etc.) and multi-year (5+) programs should be established for each New Zealand City (e.g., IOF in Wellington and DEVORA in Auckland) to slowly build our knowledge around potential active faults close to our cities.