New Zealand earthquake: large aftershocks less likely due to depth, GNS says

Tuesday, 30 October 2018

Parliament was in session when the quake struck, and was subsequently suspended.

Large aftershocks from the 6.2-magnitude earthquake near Taumarunui on Tuesday are less likely to be felt due to its depth, GNS Science says.

Seismologist Dr John Ristau said deep earthquakes - this one was 207km below the surface, according to GeoNet - 'tend not to have aftershock sequences'.

'Even if you have a few aftershocks, they tend to be much smaller than the main shock, and the energy from such depth dissipates as it moves towards the surface.'

Thousands of people around the country felt the earthquake.

The depth was 'not that unusual' for a moderate to large quake in that area.

**READ MORE:

The Pacific Plate is 'subducting' - basically meaning its being jammed under the North Island. The dark brown area in the graphic is the rigid rock in the upper portion of the Pacific Plate. The yellow area is a mushy mix of rigid and molten rock.

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'Under the central and western part of the North Island, the plates are subducting down and are deep. Below Taranaki they [quakes] can even be 500 to 600km deep.'

While the epicentre was close to Taumarunui, the nature of such deep quakes meant their effects spread far more widely, he said.

'So if you look at the 'felt reports' around that epicentre, there's hardly any. Instead they are farther away and most of them were in Wellington and Christchurch.

'In these deep quakes, it takes a while for all the energy released to get up to the surface. By the time it does, it has already spread a fair distance from the quake's location.

'You often get a lot more felt reports out to the east, because the subducting plate is very effective at getting the energy up to the surface. It's a bit like if you hold your ear up to a long piece of wood and someone scratches it right at the other end - you can hear that like it's right in your ear.'

Ristau said the quake was not directly linked with the North Island volcanoes.

'It's much deeper and a bit further to the west - about halfway between the Taupo volcanoes and Taranaki. There is no link.'

Geonet said large, deep earthquakes usually cause fewer aftershocks than shallower ones.

It said: As this was a very deep quake, you can see from the pattern of felt reports that the energy travelled along the rigid subducting slab on the east of the North Island.

Rigid rock is better at transmitting earthquake waves than mushy rock. So when an earthquake happens in or near this slab of subducting plate, most of the quake's energy (and therefore shaking) travels up and along the slab to the surface.

The energy that travels straight up to the surface has to travel through the softer, semi-molten rock, which 'dampens' the waves. This means that fewer people around the epicentre will feel these types of earthquakes.

Victoria University geophysics professor John Townend said the type of rock affected how the earthquake was felt around the country.

Volcanic areas like the Bay of Plenty had a lot of ash and weak rock, which soaked up energy so did not experience much shaking.

'But if the seismic waves travel through hard rock they preserve a lot of that energy.

'That means they can shake strongly at a greater distance - so in Wellington, Wairarapa and Hawke's Bay, that's what's happening.'