Why Is New Zealand Experiencing Powerful Earthquakes At The Moment?

The epicenter of the 7.8M quake, along with the intensity of the seismic waves radiating outwards. USGS

New Zealand has been rocked by a series of powerful earthquakes and a small tsunami over the last few days. The most powerful event registered as a 7.8M, taking place on November 13, and was felt across both the North and South Islands.

Although it’s lucky just two people have been killed, the country is still tense as aftershocks more powerful than most will ever experience in their lifetime – including a 6.3M – continue to hit the country. So what’s happening beneath the Land of the Long White Cloud to cause all this?

The nation is no stranger to earthquakes. Situated on a collisional zone between the Indo-Australian and Pacific tectonic plates, it sits on the Ring of Fire, one of the boundaries most prone to both earthquakes and volcanic eruptions.

Every year, New Zealand experiences around 14,000 quakes, and about one in 10 are powerful enough to be felt.

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In the north of the country, there is a subduction zone, where the Pacific Plate is sliding under the Indo-Australian Plate. This generates both volcanic activity and deep-seated earthquakes whenever stress is built up and released by a sudden jutting forwards.

Along most of the South Island, the angle between the two plates is skewed at a strange angle, so they push up against and slide across each other instead of along the so-called Alpine Fault. This causes far more shallow earthquakes than those that take place on the North Island.

The primary earthquake in this sequence, the 7.8M, occurred at a depth of 23 kilometers (14.3 miles) in the north of the South Island – right along the section where the subduction zone of the North Island turns into the strange high-angle fault network running along the South Island.

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It’s safe to say that it took place in a very enigmatic region. The United States Geological Survey (USGS) are somewhat bemused, as the depth and power of the quake suggests it was a subduction zone event.

The subduction zone was not thought to extend this far south. However, the northern location of the aftershocks suggests that there was indeed a sudden slipping forwards of the descending Pacific Plate, which is still continuing sporadically.

Either way, this event makes it clear that there is a lot about the seismology of New Zealand that is yet to be understood.

A subduction zone-style earthquake is what was thought to cause the tsunami that struck the eastern coastline, as only this type of fault – where one slides down over another – can move so much water as to cause a tsunami.

Waves reached a height of 2 meters (6.6 feet). For comparison, the tsunami that hit Japan’s eastern seaboard in 2011 were up to 39 meters (128 feet), so things could have been far worse.

The event took place across an area of about 12,000 square kilometers (4,633 square miles), and was felt particularly strongly in Christchurch – still recovering from the devastating 2011 event – and Wellington.

“I was on the third level of my flat when I felt the earthquake,” Dr Manda Safavi, a scientific advisor to the Environmental Protection Authority in Wellington, told IFLScience. Describing the “continuous aftershocks,” she said that “this has been the longest twenty-four hours of my life.”

Some unfortunate cows were stranded after the primary earthquake caused a few landslides. Associated Press via YouTube

Destroying plenty of infrastructure, and even cutting off the town of Kaikoura through landslides, it was the most powerful quake in the region since 1929. Disconcertingly, the location of the earthquake may mean that the danger will not be over for some time.

“Because of the complexity of this plate boundary region, strain is being accommodated on many different structures of varying orientations, making it possible that more than one fault may be activated in this earthquake sequence,” the USGS said in a statement.

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