It was two earthquakes - not one.

That appears to be the explanation for this morning's long, unrelenting shaking. It explains the phenomena which left many expressing astonishment at how long the earth shook.

GeoNet's Sara McBride said quakes hit one after the other and were each a different type of seismic shift.

"Our reports indicate that the combination of these two quakes lasted two minutes, with the most severe shaking around 50 seconds."

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One of the quakes appeared to have been a "strike-slip" event, a vertical fault which moves horizontally.

The other was a "thrust fault", where older parts of the earth's crust are shoved up through the earth on top of newer layers.

The only surety now was that there were going to be further quakes, McBride said. The aftershock sequence meant the impact would diminish over time.

GeoNet had created three scenarios following the quakes.

"There are very different probabilities for each scenario. Some of these are more concerning than others. We recognise that while these scenarios may increase anxiety the best thing is to be prepared.

"Remember: If you feel a long or strong earthquake and you are on the coast, evacuate immediately."

Scenario One - 80 per cent and greater

"A normal aftershock sequence that is spread over the next few months. Felt aftershocks (eg a magnitude of greater than 5) would occur from the 7.5-magnitude epicentre near Culverden, right up along the Kaikoura coastline to Cape Campbell over the next few weeks and months. This is the most likely scenario."

Scenario Two - 60 per cent and greater

"In the next month, it would be likely that rupture of earthquakes of about an 6 magnitude in the North Canterbury and Marlborough regions will occur, as well as potentially offshore in Southern Cook Strait and offshore Kaikoura."

Scenario Three - less than 40 per cent

"The least likely scenario is that in the next month, (it is unlikely but still possible) there would be rupture of longer known faults (with earthquakes of about 7 magnitude), in the Marlborough and Cook Strait regions."

It meant aftershocks would most likely hit from Christchurch north to Wellington and out to Golden Bay.

"It is important to understand that earthquakes can and do happen outside this box but the box represents the most likely area related to this sequence."