GNS Science is pointing out to Christchurch people that the time-gap between the June earthquakes and the latest spate of quakes is almost exactly the same as the gap in time between September 2010 and February 2011.

GNS says its  scientists had been reminding the public of this in recent weeks because of the perception that the earthquakes were over.

“The current earthquake activity will drive upward the statistical likelihood of further earthquake activity over the coming months as this aftershock sequence plays out,” says a statement on the GNS website

In terms of what happens next, GNS says the earthquakes have the same characteristics of the February and June sequences – very strong and violent shaking with more than average rates of aftershocks.

“This (latest) cluster is most likely going to follow the same path. Frequent jolts over the next few weeks, gradually decaying away over a period of several months (just like the period from June to December this year) can be expected.

“While it is always possible there could be a yet larger earthquake in the future this is judged to be unlikely based on geological interpretation. The current earthquakes appear to have resulted from movement on the eastern part of the fault zone that had activity in February and June.

“The marine surveys undertaken by NIWA in April and May 2011 did not find any evidence of active faulting extending any further offshore than the current earthquake activity.”

On the subject of what will happen long-term, GNS says: It may be some time, possibly many years, before the high levels of stress in the earth’s crust are fully dissipated, and some heightened level of earthquake activity will accompany this.

“The (latest) earthquakes are very disturbing but there has been little damage and services are being restored very quickly. Some further liquefaction has occurred in eastern suburbs as expected, and this reinforces the correctness of decisions not to rebuild in these areas.”

The earthquakes are a consequence of release of accumulated stress on geological faults deep (about 6 - 8 km) in the earth’s crust that have been perturbed by the initial earthquake in September 2010, and the more recent sequences. It is most likely that the current earthquakes were triggered by stress readjustment driven by the February and June 2011 earthquakes.

The earthquake forecasts that have been developed by GNS Science since the February 2011 earthquake were suggesting, in the most-recently released forecast on 16 December 2011, that for the coming year there was a high likelihood of earthquakes in the magnitude 5-5.4 range, about a 46 per cent probability of a magnitude 5.5 - 5.9 earthquake, and a 15 per cent chance of a magnitude 6 - 6.4 earthquake. Thus, the current sequence is not unexpected, although their timing and location are very unfortunate.