NZ quake zone may face extended aftershock "cluster"

The Canterbury region in the South Island of New Zealand may be subject to an extended aftershock "cluster" and the probability of another strong earthquake has risen, according to a new modeling method by the GNS Science institute. Based on a combination of two different statistical forecasting models, GNS scientists on Friday predicted that in the period to Oct. 15 next year, the probability of a magnitude 6 to 6.4 quake would increase to 15 percent. In September they predicted it was 10 percent. There was also a 46 percent chance of another tremor between 5. 5 and 5.9, up from 37 percent in September. "Nothing has changed inside the earth to increase the risk of an earthquake - it's just that there is a change in the way the probabilities are calculated," said Dr Kelvin Berryman, manager of the Natural Hazards Research Platform, in a statement Friday. Until recently, the earthquake probability has been dominated by a short-term clustering model. However, in October the medium-term clustering model started to exert a bigger influence on the earthquake probabilities, said the statement. The scientists believed the aftershocks are occurring as expected. They were declining in frequency quite quickly, but the possibility of triggered earthquakes at greater distances was becoming a prominent component of the total hazard, so that was being reflected in the more complex forecast model. An important aspect of the modeling was that these figures were for the entire aftershock zone, and not just for Christchurch city. The combined earthquake forecast model for the region would continue to be used. The devastating earthquake measuring 6.3 on the Richter scale killed at least 181 people in Christchurch, the largest city in the South Island, in February.