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The science behind the 7.1 quake and its 'unknown' risk zone

4 years ago by John Edens


1472780944897

Republished from Stuff

The region to the northeast of New Zealand where the Australia plate meets the Pacific plate has a history of generating large magnitude quakes and tsunami.

But it's not a region with the same amount of public understanding as the Alpine Fault and represents an "unknown" risk zone, according to GNS Science.

To the east of the North Island is the 3,000-kilometre long boundary between the Australia and Pacific plates.

As the Pacific plate moves westwards it subducts beneath the Australia plate at the Kermadec and Hikurangi trenches at a rate of around 4.7cm a year.

The United States Geological Survey (USGS) and GNS Science say the M7.1 quake was likely within the Pacific plate, as opposed to a "thrust" earthquake, in which part of the plate slips and moves upwards. 

Plate tectonics of the region are segmented, as the boundary snakes along the west of the South Island, through the central North Island and the volcanic plateau to the northeast of the North Island and the Kermadec Trench.

GNS Science has previously described the trench system as the biggest "unknown" seismic risk to New Zealand.

By mid-morning on Friday, GNS recorded more than 100 aftershocks since the big one at 4.38am.

Deep earthquakes can occur in the Pacific plate as it grinds downwards while shallow quakes tend to to occur within the Australia plate. The Pacific plate begins its westward thrust - at a rate of around 4.7cm per year - beneath the Australian plate at the Kermadec Trench and the Hikurangi trough, which run together.

Friday's big earthquake was 65km to the east of the plate boundary.

Directly to the east of the country is the Hikurangi trough, largely responsible for the main tsunami risk to eastern New Zealand. The plate tectonics have the potential for a so-called megathrust quake, M8 or above.

GNS seismologist John Ristau​ said the M7.1 was 150km off the coast but it was difficult to know its exact depth. Exact readings and locations of earthquakes would become clear in the days ahead.

There were at least 100 aftershocks, including a M6 and an M6.2, since the big one at 4.38am.

"As [the Pacific plate] pushes the boundary it bends and cracks as it pushes down beneath the plate.

"This area is part of the entire Pacific ring of fire. It's one of the most seismically active areas. There are other places that have more than that such as Japan and South America. The last time we had one in this area [of this magnitude] was 1995.

"[Data] show over 100 quakes in the offshore area near where the 7.1 happened but also a whole bunch in the East Cape but those aren't located very well [by instruments]. It's reasonable to say at least 100 offshore earthquakes."

GNS Science, in a study of the Hikurangi trough tsunami risk, said large tsunami such as the 2004 Boxing Day tsunami are more often associated with subduction. The trough reaches depths of 3,750 metres and is shallow compared to its neighbour.

At its northern end, the Hikurangi drops 1,000 metres into the Kermadec Trench, the world's second deepest oceanic feature which drops to 10km below the surface.

GNS say the Hikurangi trough is best described as three segments, the lower North Island, the Hawke's Bay and the Raukumara peninsula, and each segment has earthquake characteristics and tsunami potential.

"The lower North Island segment is found to be storing elastic energy over a wide area, which has the potential to be released in large tsunami-causing earthquakes.

"In the Hawke's Bay region, the region in which the plates are storing energy is considerably narrower; however we find that [a fault], which rises from the plate interface under Hawke's Bay, is a likely source of hazardous tsunamis."

The report said extinct volcanoes attached to the Pacific plate are subducted under the Raukumara peninsula and the East Cape. This region appears to be responsible for earthquakes capable of producing large tsunami, and the average interval between such events could be 70 years. 

USGS says the region is one of the most active areas in world as the 3,000-kilometre long plate boundary extends from south of the Macquarie Island to the south of the Kermadec island chain.

At the other end of the country, the subduction zone to the west and southwest of the South Island moves eastwards, thrusting the Southern Alps upwards at a rate of four millimetres a year.

North of New Zealand, the Australia-Pacific boundary runs east of Tonga and Fiji to the south of Samoa.

 

NZ'S HISTORY OF BIG ONES

Since 1900, there have been 15 quakes of M7.5 or above. An M8.2 occurred along the Macquarie ridge in 1989.

The largest recorded earthquake in New Zealand was the 1931 M7.8 Hawke's Bay quake, which killed 256 people.

Earlier on Friday, GNS had received 5,000 felt reports and by 7.10am, there had been 57 aftershocks.

GeoNet says the aftershocks can continue for some time, but there was no way to tell whether the M5.7 shake on Thursday was a "foreshock".

Within a 250km radius of the M7.1 shake, there have been 28 earthquakes of M6 or larger during the 20th century, according to the USGS and, in 1995, the M7.2 was followed by a M6.5 aftershock five days later.