A briefing Friday for media and Councillors in Christchurch was aimed at what the recent spate of earthquakes meant. Appropriately there was a magnitude 5 earthquake in the middle of it.
The new lot of earthquakes are east of the city, mostly at sea, but currently don't present any risk of tsunami.If they move as far north as the Kaiapoi Fault the risk of a 7 increases , and localised tsunami would be possible.
The initial earthquakes were located along the Greendale Fault and within Christchurch city, but since February 2011 the location of the earthquakes has moved southward (June events) and east December and January events. GNS Science advise that this migration pattern is expected and they predict that the location of the earthquakes may continue to move further east and out to sea.
The probability of a magnitude 5.0 – 5.9 earthquake is likely to have increased slightly after the earthquakes of December 2011 and January 2012. GNS Science will provide details of the updated modelling by the end of January and will release this information to the public. GNS Science predict that the probability of a magnitude 7.0 earthquake or above is low. GNS Science advise that this migration pattern is expected and that the location of the earthquakes may move further east and will be located out to sea.
GNS Science predict that the likelihood of a tsunami as a result of an earthquake in the Canterbury region has not changed since modelling was completed in April 2011. A tsunami is unlikely unless an earthquake is located offshore, close to Canterbury and measures 7.0 magnitude or above. Any tsunami would result in the maximum of a one to two metre high wave, except in bays on Banks Peninsula where some increase can be expected, and where residents have been individually advised.
Most of the recent earthquakes seem to be located at a depth of eight to 10 km. What does that mean?
In the Canterbury region, Greywacke rock is located at a depth of eight to 10 km and is over 100 million years old. This rock is very old and fractured and this is where most of the earthquakes are located.
The earthquake on 23 December felt different to the earthquakes in February and June 2011. Why is this?
There are a number of factors which affect how an earthquake feels, but one key change was the decrease in ground shaking levels. This decrease is expected because they were deeper than earlier events, were smaller in magnitude than February in particular, and were at greater distance from populated places. There are also some preliminary indications that the recent earthquakes have contained less energy at their source. GNS Science expect that the levels of ground shaking will continue to decrease, especially if the earthquakes are more distant and have similar 8-12 km depth..
Are we likely to have another ‘big one’ – an earthquake over 6.0 magnitude?
GNS Science predict that we are most likely to experience further earthquakes of 5.0 to 5.9 magnitude, but there is a low probability that we will experience further earthquakes of magnitude 6.0 and above.
GNS Science advise that the energy which has been released to date is now in line with what would be expected at this time so that ‘we have caught up’ on the level of energy which was expected to be released following the Darfield earthquake of September 2010.
However GNS Science predicts that we will have an ongoing but slowly decreasing level of seismicity for several decades. While the earthquakes will not suddenly stop, the events felt by residents will reduce in frequency progressively from the levels experienced over the last 12 months. They will become undetectable over time.
GNS Science predict that events will continue for some years and if there is a magnitude 6.0 earthquake, then it is unlikely to be located within Christchurch city due to the trends to date.
What is the local tsunami risk in Christchurch and Pegasus Bay?
The risk is low in Christchurch and Pegasus Bay and has not changed as a result of the recent earthquakes.
From the information that GNS Science has, a hazardous local source tsunami from an earthquake fault in Pegasus Bay is thought to be unlikely.
However it cannot ruled it out completely. If you are near the coast and feel strong ground shaking that lasts for a minute or more and it is hard to stand up, then an excellent precaution is to move inland or to higher ground as quickly as you can, using the safest route that you can.Do not wait for an official warning, or for any sirens to sound, to evacuate. There will not be time to give an official warning or to sound any warning sirens for a local source tsunami.