- Category: Education
- Created: Tuesday, 19 May 2015 11:53
Scientists have found evidence that two large ‘subduction’ earthquakes occurred under central New Zealand in the past 1000 years.
This is the first time that direct geological evidence has been found of subduction quakes occurring under central New Zealand. Scientists have previously found evidence of these quakes occurring under Hawke Bay.
The quakes under central New Zealand – the Cook Strait-Marlborough area - occurred 520 to 470 years ago and 880 to 800 years ago.
Scientists identified the quakes from sediment cores extracted from Big Lagoon, a large coastal lake east of Blenheim. Organic material from various levels in the cores was radiocarbon dated to provide estimates of when the quakes occurred.
The cores showed evidence of two sudden subsidence events during the past 1000 years where the land dropped by up to half a metre. Sudden large drops of this nature can only be caused by moderate-to-large earthquakes, and these two events did not match any known large earthquakes on nearby faults in the upper (Australian) plate.
The research is outlined in a paper in the Bulletin of the Seismological Society of America, published this week.
Lead author Kate Clark, of GNS Science, said the geological evidence pointed towards the two quakes occurring on the dipping subduction zone about 10km to 30km beneath the seabed in Cook Strait.
The current national seismic hazard model takes these types of earthquakes into account as it has always been assumed they could occur, but geological evidence was previously lacking.
“The findings are significant in terms of understanding earthquake and tsunami hazards in the lower North Island and upper South Island,” Dr Clark said.
Subduction earthquakes had the potential to be larger in magnitude than ‘upper plate fault ruptures’. They also affected a larger area and were more likely to trigger a tsunami.
Subduction quakes differ from normal quakes in that they occur on the under surface of the upper plate, where two plates meet, instead of on faults within the upper plate.
They are responsible for some of the biggest quakes – and tsunamis – in the world. Recent examples include the magnitude 9.0 Tohoku earthquake and tsunami in Japan in March 2011 and the magnitude 9.3 Indian Ocean earthquake and tsunami in December 2004.
The older of the two earthquakes identified at Big Lagoon was accompanied by a 3m-high tsunami that travelled inland about 360m at the study site. There was no evidence of a tsunami with the more recent of the two quakes.
Dr Clark said the more recent of the two quakes possibly correlated with a quake already identified further north on the central section of the Hikurangi margin (under Hawke Bay).
This raised the possibility that both central and southern sections of the margin may have ruptured in the same quake.
The evidence did not allow scientists to estimate the size of the two quakes, but quakes with similar impacts in comparable geological settings were in excess of magnitude 7.5.
Dr Clark said she and other scientists were investigating other locations in the lower North Island to find further evidence of subduction quakes. This could help to provide a better picture of how big these quakes might have been and how they impacted the region.
Subduction quakes were difficult to study because there were not many places in the landscape where records of their occurrence were preserved.
At a glance
- For many years scientists have strongly suspected that the southern part of the Hikurangi Margin could rupture in an earthquake
- This is the first time they have found direct geological evidence that this is the case
- This type of quake (a subduction quake) is not a 'new' hazard. It is already accounted for in the National Seismic Hazard Model, which feeds into the building code and informs engineering standards
- The evidence did not enable scientists to estimate magnitudes for the two quakes that were identified
- Data from just two earthquakes is not enough for scientists to work out a robust recurrence interval for this type of quake
- Subduction quakes are difficult to study because there are no many places in the landscape where their records are preserved.
The National Seismic Hazard Model is like a ‘black box’ of earthquake activity in New Zealand. It is a computer model that estimates the likely strength of earthquake ground-shaking an area can expect over a defined period, typically periods such as 50 years or 500 years. The two main components of the model are knowledge of all of New Zealand’s active faults and the record of New Zealand’s earthquakes for the past 160 years. Data from the NSHM feeds into the Building Code. In simple terms, the ground-shaking that subduction earthquakes can generate is accounted for in modern engineering standards.
In 2006 scientists studying sediment cores from Ahuriri Lagoon in Napier found evidence of seven earthquakes in the past 7000 years, one of which at 600-400 years ago, was probably a subduction earthquake.