A West Coast community will need to consider drastic changes to its town to prepare for an inevitable Alpine Fault earthquake. The fault runs diagonally through Franz Josef Township and geologists have recommended a “fault avoidance zone” 100 meters either side to protect buildings. GNS Science sent four of its geologists to Franz Josef to talk about what will happen when the Alpine Fault ruptures. The 600-kilometre fault has ruptured five times in the past 1100 years. Each time it produced an earthquake of magnitude 7.0 to 8.0, the last being in 1717. Dr Ursula Cochran said studying the fault was complicated because different sections had ruptured at different times. While researching a section of the fault in South Westland, Cochran and her team found evidence for 24 earthquakes over 8000 years. Based on this and similar research, scientists calculated the average interval between earthquakes was about 300 years in South Westland.
From that information, scientists conclude there is a 30 per cent chance of a large earthquake occurring on the southern section of the fault in the next 50 years. That is considered reasonably high by global standards. Fellow geologist Dr Jamie Howarth said an Alpine Fault earthquake would be accompanied by dramatic changes in the Southern Alps in the following decades. Scientists had found evidence of an influx of erosion material flowing into lakes and rivers following a large earthquake. Howarth said that could be seen as a “considerable delayed hazard” with implications for restoration of infrastructure such as road and rail links. Dr Rob Langridge has worked to map the fault and determine how much it will slip in the next large earthquake. He said there would likely be instantaneous movement between 7 to 8 meters horizontally and 1 to 2m vertically. Langridge has worked on the concept of a “fault avoidance zone” which is being considered for the Franz Josef Township. It would mean a 100m-wide zone along the fault, which runs diagonally through the township, with special considerations for buildings. The Westland District Council has proposed changes to its district plan to account for the earthquake risk in Franz Josef. It will hear public submissions on the changes on March 30 and 31. –Stuff NZ