Research has found evidence that at least ten huge earthquakes have impacted Hawkes Bay in the last 7500 years. Research leader, Dr Bruce Hayward of Geomarine Research, Auckland, says that at least four, and possibly more, of these earthquakes were likely megathrust earthquakes produced by massive ruptures on the tectonic plate boundary that dips beneath the east coast of the North Island.
Megathrust earthquakes are the largest earthquakes known to occur on Earth, but New Zealand has not experienced any since European colonisation, even though a sloping plate boundary exists beneath the east coast of the North Island as far south as Marlborough.
All the massive earthquakes around the world have been of the megathrust type and include the 2004 Sumatra-Andaman and 2011 Tohoku earthquakes.
"It has been known for many decades that New Zealand has the kind of subducting plate boundary that can produce megathrust earthquakes, but the big question has been whether it has a history of such huge ruptures,” says Hayward.
The devastating 1931 Napier Earthquake uplifted a 50 km-long coastal stretch by up to 1.5 m and was produced by rupture on a shallow fault within the Australian crustal plate. Eight of the large earthquake displacements recognised in this new study resulted in coastal subsidence totalling 8-9 m in the last 7500 years. These are consistent with modelled megathrust ruptures occurring on the plate boundary that is located just 20-25 km below the Hawke’s Bay coast.
Four of the earthquakes can be correlated with geological evidence of large seismic events onshore along 250-600 km of the east coast and with dated submarine landslides offshore. These four earthquakes, dated at approximately 500, 950, 1700 and 7100 years ago, have all the characteristics of modelled megathrust earthquakes for New Zealand.
“This research indicates that another megathrust earthquake can be expected to occur sometime in the future with severe shaking, land displacement and possibly tsunami over many hundreds of kilometres along a significant length of the east coast between East Cape and Marlborough. Low-lying coastal land, such as that uplifted out of the sea around Napier in 1931, can be expected to subside back below sea level again,” says Hayward.
The research was part of a government-funded programme undertaken by Geomarine Research in collaboration with scientists from GNS Science addressing the question of whether New Zealand has had a history of megathrust earthquakes or not. The research involved multiple coring of young sediment in salt marshes and coastal lagoons between Poverty Bay and Blenheim.
These cores were studied in great detail using the tiny fossil shells of amoeba-like organisms called foraminifera, which were used to identify and date sudden changes in elevation produced by major earthquakes. These land displacements were correlated between cores and then compared with records from other study sites further along the coast to determine how widespread they were.
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