One of the causes of unusually violent shaking in the
Canterbury earthquakes could be present as far south as
Dunedin, a GNS seismologist says.
Dr Martin Reyners, of Wellington, received a $717,391 grant
through this year's Marsden Fund for research to find the
southern boundary of the Hikurangi Plateau and whether the
plateau affected Otago tectonics.
Dr Reyners said finding the southern boundary of the plateau
was important because it had an effect on how violent shaking
was in earthquakes.
''Research we have done on the Canterbury quakes shows that
one of the reasons the shaking of the earthquakes was so
strong ... was that this plateau ... underlies that area at
about 10km depth,'' he said.
If the plateau was also underneath Otago, a large quake in
the region could cause more damage than would normally be
''What we would like to do for the engineers, say for
engineers working in Dunedin for example, is to tell them if
we had an earthquake what type of shaking we would expect.
''At the end of the day, we try to inform better building
codes,'' he said.
The study involved installing 20 portable seismographs in
locations from Dunedin to South Canterbury.
The seismographs would be used to record activity for a year
and GNS would then spend the next two years analysing what
Dr Reyners said there was already evidence the plateau went
as far south as Oamaru.
''In 1876, there was a sequence of two earthquakes near
Oamaru ... and they were both just under magnitude 6. They
have a very uncanny resemblance ... to what happened in
Canterbury,'' he said.
For their magnitudes, the earthquakes caused an unusual
amount of damage to buildings.
The plateau resulted in more violent earthquakes because it
was made up of strong rock.
''Earthquakes are all about strain building up in the crust
and then being released in an earthquake, and because these
rocks are very strong they can actually store more strain
before they break,'' Dr Reyners said.
This meant earthquakes in areas where the plateau was present
were likely to be less frequent, but when they did happen
they had ''a lot more energy'' than in places where the crust
Dr Reyners said the research would also reveal more about the
geological history of Otago, including why gold was abundant
in parts of the region.