Scientists uncover big quakes

A graphic illustrating the study area. Photo: Supplied
Scientists have discovered evidence that large ‘subduction’ earthquakes similar to the Sumatran quake that generated the Boxing Day tsunami in 2004 and the Japan quake in 2011 have happened under central New Zealand in the past 1000 years.
 
Both the Sumatran quake and the Japan quake registered nine on the Richter scale, and GNS scientists have found geological evidence of similar subduction quakes occurring under central New Zealand.
Subduction earthquakes have the potential to be larger in magnitude than ‘upper plate fault ruptures’. They also affect a larger area and are more likely to trigger a tsunami.
 
Subduction quakes differ from normal quakes in that they occur on the underside 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.
 
The older of the two earthquakes identified in sediment cores, taken at Big Lagoon east of Blenheim, 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.
 
Sediment cores from the salt marshes at Big Lagoon 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.
 
Organic material from various levels in the cores was radiocarbon dated to provide estimates of when the quakes occurred. 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, says the geological evidence points towards the two quakes occurring on the dipping subduction zone about 10km to 30km beneath the seabed in Cook Strait.
 
“The findings are significant in terms of understanding earthquake and tsunami hazards in the lower North Island and upper South Island,” says Kate.
 
The evidence doesn’t enable scientists to estimate magnitudes for the two identified quakes, and data on the two quakes isn’t enough for scientists to work out a robust recurrence interval for this type of quake.
 
Dr Clark says the more recent of the two quakes possibly correlates with a quake already identified further north on the central section of the Hikurangi margin under Hawke’s Bay.
 
It raises the possibility that both central and southern sections of the margin may have ruptured in the same quake.
 
Scientists are investigating other locations in the lower North Island to find further evidence of subduction quakes to help provide a better picture of how big these quakes might have been and how they impacted the region.
 
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.
Courtesy of sunlive.co.nz

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