Seismologists looking at 12 months of data have found the frequency and size of the aftershocks are similar to the seismic aftermath observed in eastern Californian and Tasmanian events, where the tremors lasted decades but were increasingly smaller in scale.
Since the original rumble in September 2010, 10,000 quakes have rattled Canterbury. More than 5500 of these have occurred since the deadly February quake re-energised the sequence, and 214 of these tremors were magnitude 4 or larger.
Natural hazards research platform manager Kelvin Berryman said the Darfield aftershock sequence was expected to last for 30 years, with tremors tailing off gradually until they were unnoticeable.
Cantabrians repeatedly asked whether they were experiencing a particularly vicious sequence, in particular when a magnitude 5.8 earthquake occurred two days before Christmas.
Seismologists say the Darfield sequence was not violent compared with historical records. In contrast, Sumatra is still experiencing moderate-size quakes eight years after the magnitude 9.1 quake which caused the Boxing Day tsunami in 2004.
But unlike the Sumatran sequence, the Canterbury quakes were limited to a smaller geographic area, and have been particularly hard on Christchurch's central city.
GNS Science's latest computer modelling shows there is an 8% chance of a 5.5 to 5.9 magnitude tremor in the next year and 2% chance of a magnitude 6 to 6.4 quake.
That means, every day for a year, there is a one-in-50 chance of a tremor equivalent in magnitude to the February 2011 quake occurring in the wider aftershock zone.
A paper released last month said a week before the February 22, 2011, magnitude 6.3 quake, computer modelling had shown a 25% chance of "a magnitude 6 or greater earthquake occurring in the general aftershock zone of the Darfield earthquake in the next year".
GNS Science stood by its decision not to publicly announce this figure, because the projected earthquake could have occurred in six or seven different places, and an announcement could have needlessly alarmed residents.
- The New Zealand Herald
