Cheviot earthquake: Tracing the source of the 7.5 magnitude quake   14 Nov 2016

Looking back over hundreds of thousands of years, the Hope Fault was considered to have one of the highest slip rates among those faults. It was an important structure in accommodating plate movement in that part of the country, Kaiser said.

"This earthquake occurred right along that line of very active deformation."

The Marlborough fault system was a kind of transition zone between the Alpine Fault and the subduction interface between the tectonic plates under the North Island.

"So far, it doesn't look to be the Hope Fault, rather (and perhaps more concerning) it seems to have ruptured across the Hope Fault," USGS seismologist Professor Kevin Furlong said.

"Most/many aftershocks extend to Cook Strait, implying that the rupture may have extended quite a bit to the north.

"One concern is whether this has involved any of the subduction plate boundary that starts up at Kaikoura and extends along the [east coast of the] North island.

"At a minimum, it may have changed the stress conditions on the main megathrust interface – we are exploring that possibility right now.

"The main event is quite complex and may explain some of what you felt (and the different magnitude reports).

"It appears to have initiated for about 40 seconds with something that might be seen to be about a magnitude 6.6 or 7.0 event, and then it had a big pulse of rupture lasting for the 100 seconds or so (and constant with a Mw 7.9), which is what the USGS now has it as."

At the location of this earthquake, the Pacific plate moved to the west-southwest with respect to the Australia plate at a rate of approximately 40 mm/yr, USGS said.

The epicentre of the earthquake was about 30-45km south-southeast of the Hope Fault - the main surface expression of the plate boundary in the region.

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