Roger Hanson: A good reason to be ready for the next big quake
The magnitude 7.8 Kaikoura earthquake is one of four earthquakes of more than magnitude 7 in New Zealand in the last seven years.
According to Kathryn Ryan on Radio New Zealand (RNZ), a similar period was 1929-1942 during which one third of all of the magnitude 7 earthquakes to shake New Zealand in the last 160 years, occurred.
Only five earthquakes greater than 7.8 have ever been recorded in New Zealand.
Based on these figures it looks like we are entering a period of increased seismic activity. Does a big shake mean the stress has been transferred to other faults and the risk of big quakes occurring nearby has increased?
Yes, in a RNZ interview, says Dr Kelvin Berryman, Principal Scientist at GNS, but it is more complex than we previously thought.
Two types of stress are associated with earthquakes. Firstly, there is the stress that is relieved by the intense energy pulse at the time of the earthquake and, the stress associated with the slow long term pushing of the plates.
In the North Island, the Pacific Plate dives below the Australian Plate (the opposite occurs at the southern end of the South Island). The process is called subduction and at the deepest part of the subduction zone the plates are slipping over each other continuously.
Further up, 15-50kms from the surface, the plates start to stick and slip, this region is where slow-slip earthquakes take place.
It is only since 2009 that GNS has had the instruments to monitor these slow-slip earthquakes which have been recorded off the East Coast and off the Kapiti Coast.
Slow-slip earthquakes, which we can't feel, involve periods of rock movement lasting from hours to months. The stress built-up by the plates pushing into each other is most dramatically relieved by the first type of earthquake, a short duration big energy pulse.
But as Dr Berryman points out this is not the whole story.
Being at the boundary of two tectonic plates, New Zealand is going to have earthquakes and it could be argued that the location and timing of any earthquake is entirely random.
It just happens to be that between 2009 and 2016 there have been several big earthquakes – in Fiordland, Canterbury, off East Cape and Kaikoura. However, scientists, by drilling at the big faults have been able to examine the earthquake record not only of the past few decades but over thousands of years.
These show there have been many earthquake episodes – that is many of these relatively short time intervals during which several big earthquakes have occurred.
It is not known why these big earthquakes occur in clusters and why the earthquakes in each cluster are widely distributed across the country.
Based on the experience of numerous earthquakes and also from evidence in the geological record, a pattern is revealed of the declining frequency of aftershocks after a big quake.
These also show that big aftershocks can occur many months after the initial earthquake.
Much work has been done to assess the change in the stress imposed by a big earthquake on nearby faults. Dr Berryman says there is no doubt that the stress on the geological formations in and around the earthquake epicentre has changed in the Kaikoura region, although assessing exactly how that affects earthquake probability in the region needs more work.
For example, in March 1929 there was a 7.1 magnitude earthquake at Arthur's Pass and a few months later a magnitude 7.8 at Murchison. The Alpine Fault is close to the epicentres of both of these earthquakes but scientist do not know why these two big energy pulses had no effect on the Alpine Fault.
Based on the size and timing of the Kaikoura earthquake and on data from New Zealand's seismic history, Dr Berryman says the chance of a big earthquake of up to magnitude 8 and above, that is, part of this current earthquake episode, has increased, and is now put at one in 20 for the next year.
But importantly this could occur anywhere in the country – a good reason not to be complacent and to ensure that we are sufficiently prepared.