Officials tell public, it’s time to prepare for an Alpine Fault earthquake

Wednesday, July 24, 2013
By Paul Martin
July 24, 2013

NEW ZEALAND – The fault-line behind the swarm of quakes in central New Zealand may be much longer than previously realized and therefore capable of larger quakes. The National Institute of Water and Atmospheric Research is heading into Cook Strait tomorrow to map the area around the swarm of earthquakes that has been rattling Wellington and Marlborough since Friday. It is hoped the work will identify the fault-line from which the quakes have been generated. GNS Science seismologist John Ristau said that, because the quakes appeared to be happening between the known Vernon and Needles fault-lines, it seemed increasingly likely that the London Hill fault was to blame. In 2003 a Marlborough District Council geotech report described London Hill as a one of “several relatively small faults near the east coast between Seddon and the Waima River.” Dr. Ristau said it now appeared the fault-line might be much longer than originally thought, which meant it could be capable of creating much larger earthquakes. “That means it’s actually considerably longer than initially thought … the larger a fault-line is, the larger the earthquake.” It was important scientists were able to determine exactly which fault the earthquake had occurred on, as it would allow them to establish whether other nearby faults could now produce large earthquakes as a result. It appeared the level of stress had not greatly affected the other Wellington faults, but there were a few lines in the Marlborough region that were capable of creating quakes of magnitude 7 or greater. “If they increase stress, it could trigger a similar-sized earthquake or even a much larger earthquake … hopefully by the end of the week we’ll be able to be a lot more definitive.”

Yesterday, GNS Science seismologist Stephen Bannister said scientists could not rule out the possibility that the quakes could stir up other faults and “kick off” the Alpine Fault. He said the biggest misconception the public had when it came to earthquakes was that small ones minimized or took “the edge off” the possibility of a large one occurring. However, GNS Science spokesman John Callan said the recent swarm of quakes east of Seddon was “not increasing or decreasing the risk of a quake on the Alpine Fault.” He said the quakes were too far away to affect the Alpine Fault, which stretches 600 kilometers from Milford Sound along the western Southern Alps to Marlborough and is the on-land boundary between the Pacific and Australian tectonic plates. Recent research led by GNS Science found it last ruptured 296 years ago, and it predicted a 30 per cent chance of a big quake along it in the next 50 years. The average interval between large quakes on the fault was 330 years. In the past 8000 years it had ruptured 24 times and caused magnitude-8 quakes, including four in the past millennium. The longest gap between major quakes was 510 years and the shortest 140 years. “An earthquake on the Alpine Fault in the near future would not be a big surprise. Equally, it could be many decades away, based on its past behavior,” Callan said. “There is no better time than the present to prepare for the next quake on the Alpine Fault. The more thoroughly we prepare, the lower the eventual impact will be.” He said work was under way to prepare to drill a deep borehole into the fault early next year to study processes taking place at depth inside a major plate boundary fault. GNS Science is jointly leading the Deep Fault Drilling Project with Otago and Victoria universities, and it involved 22 organizations from eight countries. –Stuff

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