What caused the Levin earthquakes this week?

The earthquake struck off the northwest coast of Levin on Monday and was followed by a series of aftershocks.
The earthquake struck off the northwest coast of Levin on Monday and was followed by a series of aftershocks. Photo credit: Getty

Since Monday New Zealand has had over 148 earthquakes and aftershocks off the west coast of Levin, and over 26,000 Kiwis have reported feeling them.

The main 5.8 magnitude earthquake occurred on Monday morning and caught Jacinda Ardern in the middle of a live interview with The AM Show

It was then followed by aftershocks from a magnitude of 1.7 to a 5.2 at 12:30pm on Tuesday.

GNS Science duty seismologist Dr Jonathan Hanson says the earthquakes are caused by New Zealand sitting on top of the boundary between the Pacific and Australian tectonic plates, which are colliding.

"This creates stresses in the Earth's upper crust, manifested by the many active faults in New Zealand," he says.

This graphic shows the relative positions of the two tectonic plates under the lower North Island.
This graphic shows the relative positions of the two tectonic plates under the lower North Island. Photo credit: Supplied

"The Pacific plate is moving westward by about 45mm a year, and this builds up stresses at the interface between the two plates. The resulting stresses are relieved by occasional quakes in the many faults in the upper crust. "

He says the movement of the Pacific Plate is the equivalent of the rate of fingernails growing, meaning New Zealand experiences between 20,000- 30,000 earthquakes a year.

This graphic shows 10 years' worth of quakes in NZ - it illustrates that nearly all of NZ is prone to earthquakes.
This graphic shows 10 years' worth of quakes in NZ - it illustrates that nearly all of NZ is prone to earthquakes. Photo credit: Supplied

While most are too small to be felt, between 1 and 2 percent are big enough to be reported by the public.

Hanson says where the most recent earthquakes have come from, off the west coast of the lower North Island, is an active belt of faults.

"The faults off the Kapiti Coast are known to be reasonably active and commonly produce a number of earthquakes each year. Mostly they are between magnitude 2 and magnitude 5.

"This week's quakes were slightly larger than normal, but still within the known range of the offshore Kapiti fault system."

He says the recent earthquakes have occurred 30-40km below the earth's surface where the Pacific plate begins to dip more sharply under the Australian plate.

This flexes the plate, building up stresses which can be released as an earthquake.

"Monday's M5.8 quake has been followed by hundreds of aftershocks and again this is normal. It looks to be a main shock followed by an aftershock sequence, with aftershocks decreasing in magnitude and frequency with time. 

"Though nature can be messy and it's not necessarily a perfect straight line decline. With what we know about the main shock and about statistical probabilities, seismologists at GNS Science believe the most likely outcome is that the aftershock sequence will continue and eventually die away."

He says the earthquake was widely felt, including by people in Dunedin, Auckland and Christchurch, because the seismic energy between the plates travelled up the plate boundary.

Hansen says Kiwis registering when they feel an earthquake - a 'felt report' - is "very helpful" for them to gain an understanding of the impact of the earthquakes in New Zealand.

The felt reports for the 25 May 2020 earthquake.
The felt reports for the 25 May 2020 earthquake. Photo credit: Supplied