NZ- East Coast rocked by more than 700 tremors after 7.1 earthquake

More than 700 aftershocks have rocked the East Coast of the North Island since Friday's big quake and seismologists are warning tremors could last for months.

In the past three days there have been 744 quakes since the region was jolted by a strong magnitude 7.1 quake which happened at sea early on Friday.

It included 86 quakes measuring over magnitude 4.

GNS duty seismologist Caroline Little said the actual number of aftershocks was likely to be far higher but, because of the offshore location, land-based earthquake drums had not detected every tremor.

"We know we're missing lots of [magnitude] twos offshore," she said.

Teams of international scientists were now examining data from Friday's quake to work out where on the Australian or Pacific plates the quake occurred and if this indicated any new developments in seismic activity.

Little said a quake of this strength would mean the region was likely to be affected by ongoing tremors for weeks and even months.

But given the majority of quakes were happening on the sea floor most were so weak they would go unnoticed by people living in the East Coast.

"Most likely it's going to decrease in frequency as time goes on but that's still months away."

Little said the level of activity followed a scientific formula where for every magnitude 4 quake there would be 10 tremors of magnitude 3 and 100 of magnitude 2.

The 2010 Darfield quake, which had the same strong magnitude, had 885 aftershocks over the first three days.

She said in the latest Te Araroa quake a magnitude 5.7 foreshock rocked the region the day before, recorded at the same deep-sea location as the strong early morning quake that sparked a small tsunami along the east coast.

At this stage there was nothing to show this latest quake was triggered by any other seismic activity happening in the region.

It was located well away from an earlier slow slip quake which happened off the Gisborne coast some months ago and there had been no recent activity detected near the epicentre,130km northwest of Te Araroa.