Hundreds of aftershocks could rattle the East Cape for several years following Friday earthquake, says GNS Science seismologist Dr John Ristau.

Statistical seismologists are working on detailed aftershock forecasts following the 7.1 magnitude quake which struck 130km north-east of Te Araroa at a depth of 55km at 4.37am on Friday.

GNS Science seismologist Dr John Ristau says aftershocks could continue rattling the East Cape for several years following Friday's 7.1 magnitude quake. File Photo.

John says the area along the east coast of the North Island and north of East Cape is one of the more seismically active regions of New Zealand and large earthquakes (over 6 magnitude) occur regularly.

These include a 7.2 magnitude quake off the east coast of East Cape in 1995, a 6.7 magnitude quake in Gisborne in 2007, and a 6.8 magnitude quake in 2014.

'The aftershocks for the 1995 magnitude 7.2 earthquake continued for around two years, and since this earthquake is similar in size it will likely have a similar aftershock sequence”, explains John.

'The largest aftershock for earthquakes like this are typically about 1 magnitude unit smaller than the mainshock, and we have had a magnitude 6.2 aftershock. There have also been several with magnitude 5–6 which is what we would expect, and a large number with magnitude 4–5.”

John adds there's likely to be hundreds of aftershocks with magnitudes under 4, but given the earthquake and the aftershocks are offshore it's unlikely they will be widely felt.

Just under an hour after the earthquake struck on Friday, the Ministry of Civil Defence and Emergency Management issued for the North Island's East Coast which included the Bay of Plenty of Waikato.

To produce a tsunami an earthquake must cause a large displacement in the sea floor, which in turn displaces a large amount of water. But without these conditions, a tsunami does not occur, meaning the earthquake must be large and shallow the 2011 Japanese earthquake and tsunami, says John.

'[Friday's] earthquake was much smaller than this earthquake, and occurred below the boundary of the Australian and Pacific Plates, but did generate enough displacement of the seafloor to cause a small tsunami of around 25 cm at East Cape.

'In 1947 there were two earthquakes off the coast of Gisborne with magnitudes of about 7 which produced tsunamis with heights of several metres. These were a special type of earthquake called slow-rupture tsunami earthquakes.

'With these earthquakes the speed at which the fault ruptures is much slower than is typical for an earthquake, and as a result it causes a large displacement in the seafloor but does not produce significant high-frequency energy.

'The high-frequency energy is what people normally feel, which means the earthquakes were only weakly felt and no one was aware that a large tsunami was approaching,” says John.