Last update Monday 9 January, 9:30 am
How did the sequence start off?
A magnitude 5.8 earthquake at 1:58 pm struck east of Christchurch just off the coast of New Brighton. As with other earthquakes of this shaking intensity, liquefaction occurred in the eastern suburbs of Christchurch. This new sequence of earthquakes is further east again from the June 13 set of quakes. Being further from people, and coupled with the slightly lower magnitudes of the biggest shakes, the effects have been less damaging to structures than on previous occasions.
Aftershocks rolled on throughout the afternoon and overnight, several over magnitude 5. The strongest was a magnitude 6.0 earthquake at 3:18 pm.
We sincerely apologise for the delays in letting you know what's been going on with the earlier aftershocks. The timeline of the earthquakes greater than magnitude 4.0 for the first three hours was:
- 1:58 pm, Magnitude: 5.8
- 2:06 pm, Magnitude: 5.3
- 2:14 pm, Magnitude: 4.2
- 2:30 pm, Magnitude: 4.8
- 2:41 pm, Magnitude: 4.4
- 2:50 pm, Magnitude: 4.2
- 3:18 pm, Magnitude: 6.0
- 4:14 pm, Magnitude: 4.6
- 4:24 pm, Magnitude: 4.4
- 4:50 pm, Magnitude: 5.0
- 5:08 pm, Magnitude: 4.0
What is happening in Christchurch?
The earthquakes that started mid-afternoon yesterday are similar in character to the June 13 sequence. The earthquakes are mostly a few kilometres offshore from Brighton Beach.
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.
Were the current earthquakes expected?
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% probability of a magnitude 5.5 - 5.9 earthquake, and a 15% chance of a magnitude 6 - 6.4 earthquake. Thus, the current sequence is not unexpected, although their timing and location are very unfortunate. The gap in time since the June earthquakes is almost exactly the same as the gap in time between September 2010 and February 2011, and scientists have 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.
What is going to happen next?
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 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.
Will there be tsunami since the earthquakes are offshore?
It is possible the magnitude 6 earthquake produced a very small tsunami that might have been detected on accurate gauges at Lyttleton, but this information is not available yet. Modelling undertaken by GNS Science after the February 2011 earthquake indicated that an earthquake of more than magnitude 7 would be needed to produce a hazardous tsunami in Pegasus Bay, and this is unlikely. However, everyone should always take standard precautions so that if an earthquake is felt that goes on for longer than was experienced in the recent magnitude 6, then if you are near the beach or an estuary you should immediately go to higher ground (a few metres above the high tide mark).
When will the region settle down to its former seismically quiet state?
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 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.