Bay of Plenty: 280 aftershocks recorded following magnitude 5.2 quake offshore

Roughly 280 aftershocks have been recorded since a magnitude 5.2 earthquake rattled the Bay of Plenty on Saturday, with seismologists urging locals to always be prepared for the possibility of a more damaging tremor.

The magnitude 5.2 shake hit about 120 kilometres off the coast of Whangamata at 3:47am last Saturday, providing an early wake-up call for people across the Bay of Plenty and even in Auckland. 

More than 2500 people from Auckland to Whakatane submitted a 'felt report' on GeoNet, indicating they had experienced the quake.

Several of the aftershocks have had magnitudes of 4 and over, GNS Science seismologist John Ristau told Newshub, and they are ongoing.'

Significant aftershocks include a magnitude 4.6 quake on Saturday evening and a magnitude 4.7 tremor on Sunday morning, both of which were felt in the Bay of Plenty and areas of Auckland.

Ristau said regular "earthquake swarms" in the region are to blame for the ongoing activity.

"The Bay of Plenty region and the immediate onshore region to the south - e.g. Tauranga and Whakatane - regularly have what we call earthquake swarms."

Typically when there is a large earthquake, the tremor produces aftershocks which are smaller than the 'mainshock'. The largest aftershock is about one magnitude unit smaller than the main earthquake, Ristau explained.

However, in a swarm, several larger earthquakes with a similar magnitude will occur, rather than a mainshock.

"The largest earthquakes in an earthquake swarm are normally magnitude 6 and under," he said. 

"Earthquake swarms can produce several of these moderate-sized earthquakes and many smaller ones, and they can last for days, weeks or even months."

Although Ristau urges Bay of Plenty residents to always remain prepared, he said the quakes are typical of seismic activity in the region and there is "no need to be alarmed".

"However, as the Bay of Plenty is quite seismically active, it's a good reminder to residents to always be prepared for the possibility of a damaging earthquake," he advised, referencing the similar 1987 magnitude 5.2 Edgecumbe quake that caused widespread damage. However, that shake occurred on-shore. 

"As we approach the 10-year anniversary of the magnitude 7.1 Darfield earthquake in Canterbury, it's a reminder that no place in New Zealand is truly immune to large earthquakes," Ristau said.

"This includes the Auckland and Northland regions as damaging earthquakes can occur there, albeit rarely."