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ScienceOctober 30, 2018

The subducting slab: Why the large, deep #eqnz sent shakes far from the epicentre


Today’s 6.2 earthquake was centred near Taumarunui and transmitted via a rigid subducting slab on the east of the North Island 

A magnitude 6.2 earthquake struck at 3.13pm today, centred 25 kilometres south-west of Taumarunui. Fire and Emergency said there were no immediate reports of damage. Classified as large and deep by GNS, it led to a suspension of parliament, but they returned to normal business after a brief break.

The quake, which was initially reported as 6.7 but revised down, was widely felt through the country. Geonet – the quake monitoring site run by GNS Science and the Earthquake Commission – registered more than 15,000 “felt reports”.

“Large, deep earthquakes like this usually cause fewer aftershocks than shallower ones,” explained GeoNet.

“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.”

It was a phenomenon similar to that experienced in a 4.3 event in July near Rotorua, when felt reports were again located some distance from the epicentre, on the east and south of the North Island.

At the time GeoNet reported: “Deep North Island quakes … often produce little shaking near the epicentre. The North Island sits on the Australian Plate but these deep earthquakes occur in the Pacific Plate, which is subducting or diving under the North Island.

“The ‘slab’ of subducting rock in the upper portion of the Pacific Plate is rigid (the dark brown area in the graphic below), and the surrounding rock (shown in yellow) is more ‘mushy’ – a mixture of rigid and molten rock.

North Island Subduction Zone

“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 – closer to the East Coast of the North Island.

“The energy that travels straight up to the surface from 150km depth 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.”

The biggest sigh of relief, however, is this:

Keep going!